4. 使用 GHCi

GHCi [1] 是 GHC 的交互式环境，我们可以在 GHCi 中直接输入 Haskell 表达式和程序， 代码即刻就会被解释执行并返回结果。如果你接触过 Hugs ， 就会发现 GHCi 很容易上手。此外，GHCi 还支持加载编译后的代码，同时也支持 几乎全部 [2] 的 GHC 的语言扩展。不仅如此，GHCi 中还包含了一个交互式调试器 (参见 The GHCi Debugger)

[1]	“i” 代表交互式 (”Interactive”)

[2]	当前并不包括 foreign export

4.1. GHCi 简介

让我们从运行 GHCi 开始。你可以通过 ghci 命令来打开 GHCi。

$ ghci
GHCi, version 8.0.1: http://www.haskell.org/ghc/  :? for help
Prelude>

由于要加载 prelude 和一些标准库，我们需要短暂等待 GHCi 完成启动并显示出提示符， 然后就像头一行提示语上写的, 输入 :? ，你就可以看到所有可用命令， 并且每个命令都会有半行的描述。后续我们会介绍其中的大部分命令，而如需查看完整 全部命令，则可翻阅 GHCi commands 一节。

可以在提示符后直接输入 Haskell 表达式：

Prelude> 1+2
3
Prelude> let x = 42 in x / 9
4.666666666666667
Prelude>

GHCi 将整行代码作为表达式进行解释并计算结果。不允许多行的表达式，只要 按下回车，GHCi 就会直接执行计算

在 Haskell 中，let 表达式后面得跟着 in ，但在 GHCi 中，表达式也可以被 当作实在 IO Monad 中执行，因此 let 后面也可以不跟 in ，而得到的反馈 就是一个空行，亦如上例。（译者注：上例中并无 let 后不跟 in 的情况）

4.2. 加载源文件

假设我们在 Main.hs 文件中保存了如下源代码：:

main = print (fac 20)

fac 0 = 1
fac n = n * fac (n-1)

我们可以把 Main.hs 存在任何地方，不过，如果并没有存在当前目录下 [3] ， 那就需要先在 GHCi 中切换到正确的目录。

Prelude> :cd dir

这里的 (dir) 就是你保存 Main.hs 的那个目录。

然后使用 :load 来把 Haskell 源文件加载进 GHCi。

Prelude> :load Main
Compiling Main             ( Main.hs, interpreted )
Ok, modules loaded: Main.
*Main>

GHCi 加载完 Main 模块，提示符变成 *Main>> ， 表示后续输入的表达式 都将在刚刚载入的 Main 模块的环境下运行（我们会在后面 What’s really in scope at the prompt? 一章中解释这里的 * 是什么意思）。然后我们就可以使用 Main.hs 中 定义的函数来写点表达式了：

*Main> fac 17
355687428096000

加载包含多模块的程序也是一样的，直接用 :load 命令载入“最顶层”的 模块即可（提示：:load 可以被简写为 :l ）。通常最顶层的模块 就是 Main ，但也可以是其它的。GHCi 会通过最顶层的模块来判断，哪些模块 被引用了，直接的还是间接的，并把它们按照依赖顺序加载进来。

[3]	如果是从命令行启动 GHCi，GHCi 的当前目录就是 shell 所在的目录。 如果是从 Windows 的开始菜单启动 GHCi，那当前目录就可能是类似这样的 C:\Documents and Settings\user name.

4.2.1. 模块 vs. 文件名

问：GHC 是如何找到包含模块 (M) 的文件的？ 答：GHC 会去找名为 M.hs 或 M.lhs 的文件。所以对大部分模块来说， 你的模块名需要和文件名保持一致。否则，GHC 就没法找到它。

此通用规则有一个例外：当你通过 :load 加载一个程序，或直接在 启动 ghci 时就指定一个待加载程序时，你也可以不用模块名，而是直接使用 文件名。只要这个文件名存在，那无论里面的模块名是什么，都会被加载。这一点 对于同一文件夹里包含多个 Main 模块的情况，就会很方便，因为肯定不能把 这些文件都命名为 Main.hs 。

可以在 GHCi 命令行中通过 -i 来指定查找源文件的搜索路径，比如：

ghci -idir1:...:dirn

也可以在 GHCi 中通过 :set 命令来设置 （参见 Setting GHC command-line options in GHCi ）[4]_

GHCi 通过依赖关系来确定要加载的模块，其结果之一就是需要满足一个模块一个文件。 唯一的例外是来那些自于包 (package) 的模块，其中包括 Prelude 和一些标准库， 如比 IO 和 Complex 。如果 GHCi 找不到你想要加载的模块的源文件，即使 存在该模块的目标文件和接口文件，你也会得到一个错误信息，

[4]	注意，在GHCi 中，如果开启了 --make 模式，那 -i 就是 用来指定*源*文件的搜索路径的，而在标准的分批编译模式下， -i 则是 用来指定接口文件的搜索路径的，参见 The search path 。

4.2.2. 文件修改与重新编译

如果你修改了源文件并希望 GHCi 能够重新编译程序，此时就可以使用 :reload 命令。 程序会按需要进行重新编译，而实际上，GHCi 会尽量避免在外部依赖没有变化的情况下重新编译模块。 在分批编译的模式下，也有同样的机制来避免没有必要的重新编译（参见 The recompilation checker ）。

4.3. 加载已编译代码

当你把一个 Haskell 源文件载入 GHCi，它通常会被转成字节码，然后通过解释器运行。 不过，解释后的代码也可以和编译后的代码一起在 GHCi 中运行；实际一般情况下， GHCi 在启动时就会去加载已经编译完了的 base 包的代码，其中就包含了 Prelude 。

那我们有什么理由要去使用编译后的代码呢？原因很简单，编译后的代码，其运行速度大概比 解释后的代码要快上10倍。不过缺点是编译所消耗的时间大概是解释的2倍（如果开启优化，时间可能更长）。 所以值得考虑的做法是，编译那些很少会改动的代码，然后用解释器去运行开发中的代码。

当我们通过 :load 去加载代码模块时， GHCi 通常会先看一下有没有对应编译后的 目标文件，如果有就尽量不去解释源文件。举例来说，假设我们有一段程序，包含4个模块，分别是 A、 B、C 和 D。模块 B 和模块 C 同时都只引用了模块 D ，而模块 A 引用了模块 B 和模块 C 。

  A
 / \
B   C
 \ /
  D

我们可以先编译 D，然后再加载整个程序，如下：

Prelude> :! ghc -c -dynamic D.hs
Prelude> :load A
Compiling B                ( B.hs, interpreted )
Compiling C                ( C.hs, interpreted )
Compiling A                ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D (D.o).
*Main>

从编译器给出的信息我们可以看出，这里并没有提到 D。原因是自从上次编译之后，它的源代码及 它所依赖的代码都没有变化，所以就没有必要再去重新编译 D 了。

注意这里的 GHC -dynamic 开关：GHCi 使用动态链接的目标代码 （如果你所在的系统支持的 话），所以如果要让 GHCi 使用编译后的代码，就需要开启动态链接的开关来进行编译。

你可以随时使用 :show modules 命令来查看当前已加载进 GHCi 的模块：

*Main> :show modules
D                ( D.hs, D.o )
C                ( C.hs, interpreted )
B                ( B.hs, interpreted )
A                ( A.hs, interpreted )
*Main>

如果我们现在修改模块 D 的源代码（也可以用 Unix 命令在源文件上 touch 一下来方便地 假装修改），编译器就没办法再去使用那个已过期的目标文件了。

*Main> :! touch D.hs
*Main> :reload
Compiling D                ( D.hs, interpreted )
Ok, modules loaded: A, B, C, D.
*Main>

注意，虽然模块 D 被重新编译了，但 touch 之后源代码并没有改变，生成的接口文件还 和之前一样，所以模块 A、模块 B``和模块 ``C 就不用再重新编译了，这个判断由重新编译的 检查器来下。

然后让我们试着挑一个其它模块去编译：

*Main> :! ghc -c C.hs
*Main> :load A
Compiling D                ( D.hs, interpreted )
Compiling B                ( B.hs, interpreted )
Compiling C                ( C.hs, interpreted )
Compiling A                ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.

竟然并没有使用模块 C 的编译后代码！这是怎么回事？原来在 GHCi 中，一个编译后的模块只能 依赖于其他编译后的模块，而在我们上面的场景中，模块 C 依赖于模块 D，但 D 并没有 对应的目标文件，所以 GHCi 就顺势不去使用 C 的目标文件了。好了，那就让我们再来编译 D。

*Main> :! ghc -c D.hs
*Main> :reload
Ok, modules loaded: A, B, C, D.

什么都没发生！这又是另一课：:reload 并不会使用新编译出来的模块，只有 :load 会去使用它们：

*Main> :load A
Compiling B                ( B.hs, interpreted )
Compiling A                ( A.hs, interpreted )
Ok, modules loaded: A, B, C (C.o), D (D.o).

目标文件的自动加载，有时候也会造成混乱，这是因为只有在解释一个模块的代码时，我们才能在 GHCi 提 示符下输入表达式，使用模块中未输出的那部分定义。所以，有时候你可能会希望强制 GHCi 使用解释器去 加载一个模块。使用 :load 加载模块时，在模块名或文件名前加上 * 前缀就可以 做到这一点，例如：

Prelude> :load *A
Compiling A                ( A.hs, interpreted )
*A>

用了 * 之后，GHCi 就会忽略那些预编译的目标代码，并去解释执行这个模块。如果你已经加载了 一堆模块的目标代码，然后想要去解释执行其中一个模块，你并不用重新加载所有模块，而是可以用 :add *M 来指定说，我就想解释执行 M 这一个模块（注意，这一步也可能导致其他模块被解释 执行，因为编译后的模块不能依赖于解释后的模块）。

如果想要把所有东西都编译成目标代码，而不去使用解释器，那就要使用 -fobject-code 开关（参见 Compiling to object code inside GHCi）。

提示

只有在 GHCi 确定编译后的版本没有过期时，才会去使用编译后的目标文件。在一个大型程序的开发 中，有一个很好的技巧，即时不时地运行 ghc --make 去编译整个项目（比如在你去吃午饭 前），然后接着用解释器去跑你的代码。每次你修改代码，对应的模块就会被解释执行，而项目中 的其他模块还是用的编译后代码。

4.4. Interactive evaluation at the prompt

When you type an expression at the prompt, GHCi immediately evaluates and prints the result:

Prelude> reverse "hello"
"olleh"
Prelude> 5+5
10

4.4.1. I/O actions at the prompt

GHCi does more than simple expression evaluation at the prompt. If you enter an expression of type IO a for some a, then GHCi executes it as an IO-computation.

Prelude> "hello"
"hello"
Prelude> putStrLn "hello"
hello

This works even if the type of the expression is more general, provided it can be instantiated to IO a. For example

Prelude> return True
True

Furthermore, GHCi will print the result of the I/O action if (and only if):

• The result type is an instance of Show.
• The result type is not ().

For example, remembering that putStrLn :: String -> IO ():

Prelude> putStrLn "hello"
hello
Prelude> do { putStrLn "hello"; return "yes" }
hello
"yes"

4.4.2. Using do notation at the prompt

GHCi actually accepts statements rather than just expressions at the prompt. This means you can bind values and functions to names, and use them in future expressions or statements.

The syntax of a statement accepted at the GHCi prompt is exactly the same as the syntax of a statement in a Haskell do expression. However, there’s no monad overloading here: statements typed at the prompt must be in the IO monad.

Prelude> x <- return 42
Prelude> print x
42
Prelude>

The statement x <- return 42 means “execute return 42 in the IO monad, and bind the result to x”. We can then use x in future statements, for example to print it as we did above.

-fprint-bind-result

If -fprint-bind-result is set then GHCi will print the result of a statement if and only if:

• The statement is not a binding, or it is a monadic binding (p <- e) that binds exactly one variable.
• The variable’s type is not polymorphic, is not (), and is an instance of Show.

Of course, you can also bind normal non-IO expressions using the let-statement:

Prelude> let x = 42
Prelude> x
42
Prelude>

Another important difference between the two types of binding is that the monadic bind (p <- e) is strict (it evaluates e), whereas with the let form, the expression isn’t evaluated immediately:

Prelude> let x = error "help!"
Prelude> print x
*** Exception: help!
Prelude>

Note that let bindings do not automatically print the value bound, unlike monadic bindings.

You can also define functions at the prompt:

Prelude> add a b = a + b
Prelude> add 1 2
3
Prelude>

However, this quickly gets tedious when defining functions with multiple clauses, or groups of mutually recursive functions, because the complete definition has to be given on a single line, using explicit semicolons instead of layout:

Prelude> f op n [] = n ; f op n (h:t) = h `op` f op n t
Prelude> f (+) 0 [1..3]
6
Prelude>

To alleviate this issue, GHCi commands can be split over multiple lines, by wrapping them in :{ and :} (each on a single line of its own):

Prelude> :{
Prelude| g op n [] = n
Prelude| g op n (h:t) = h `op` g op n t
Prelude| :}
Prelude> g (*) 1 [1..3]
6

Such multiline commands can be used with any GHCi command, and note that the layout rule is in effect. The main purpose of multiline commands is not to replace module loading but to make definitions in .ghci-files (see The .ghci and .haskeline files) more readable and maintainable.

Any exceptions raised during the evaluation or execution of the statement are caught and printed by the GHCi command line interface (for more information on exceptions, see the module Control.Exception in the libraries documentation).

Every new binding shadows any existing bindings of the same name, including entities that are in scope in the current module context.

警告

Temporary bindings introduced at the prompt only last until the next :load or :reload command, at which time they will be simply lost. However, they do survive a change of context with :module: the temporary bindings just move to the new location.

提示

To get a list of the bindings currently in scope, use the :show bindings command:

Prelude> :show bindings
x :: Int
Prelude>

提示

If you turn on the +t option, GHCi will show the type of each variable bound by a statement. For example:

Prelude> :set +t
Prelude> let (x:xs) = [1..]
x :: Integer
xs :: [Integer]

4.4.3. Multiline input

Apart from the :{ ... :} syntax for multi-line input mentioned above, GHCi also has a multiline mode, enabled by :set +m, :set +m in which GHCi detects automatically when the current statement is unfinished and allows further lines to be added. A multi-line input is terminated with an empty line. For example:

Prelude> :set +m
Prelude> let x = 42
Prelude|

Further bindings can be added to this let statement, so GHCi indicates that the next line continues the previous one by changing the prompt. Note that layout is in effect, so to add more bindings to this let we have to line them up:

Prelude> :set +m
Prelude> let x = 42
Prelude|     y = 3
Prelude|
Prelude>

Explicit braces and semicolons can be used instead of layout:

Prelude> do {
Prelude| putStrLn "hello"
Prelude| ;putStrLn "world"
Prelude| }
hello
world
Prelude>

Note that after the closing brace, GHCi knows that the current statement is finished, so no empty line is required.

Multiline mode is useful when entering monadic do statements:

Control.Monad.State> flip evalStateT 0 $ do
Control.Monad.State| i <- get
Control.Monad.State| lift $ do
Control.Monad.State|   putStrLn "Hello World!"
Control.Monad.State|   print i
Control.Monad.State|
"Hello World!"
0
Control.Monad.State>

During a multiline interaction, the user can interrupt and return to the top-level prompt.

Prelude> do
Prelude| putStrLn "Hello, World!"
Prelude| ^C
Prelude>

4.4.4. Type, class and other declarations

At the GHCi prompt you can also enter any top-level Haskell declaration, including data, type, newtype, class, instance, deriving, and foreign declarations. For example:

Prelude> data T = A | B | C deriving (Eq, Ord, Show, Enum)
Prelude> [A ..]
[A,B,C]
Prelude> :i T
data T = A | B | C      -- Defined at <interactive>:2:6
instance Enum T -- Defined at <interactive>:2:45
instance Eq T -- Defined at <interactive>:2:30
instance Ord T -- Defined at <interactive>:2:34
instance Show T -- Defined at <interactive>:2:39

As with ordinary variable bindings, later definitions shadow earlier ones, so you can re-enter a declaration to fix a problem with it or extend it. But there’s a gotcha: when a new type declaration shadows an older one, there might be other declarations that refer to the old type. The thing to remember is that the old type still exists, and these other declarations still refer to the old type. However, while the old and the new type have the same name, GHCi will treat them as distinct. For example:

Prelude> data T = A | B
Prelude> let f A = True; f B = False
Prelude> data T = A | B | C
Prelude> f A

<interactive>:2:3:
    Couldn't match expected type `main::Interactive.T'
                with actual type `T'
    In the first argument of `f', namely `A'
    In the expression: f A
    In an equation for `it': it = f A
Prelude>

The old, shadowed, version of T is displayed as main::Interactive.T by GHCi in an attempt to distinguish it from the new T, which is displayed as simply T.

Class and type-family instance declarations are simply added to the list of available instances, with one exception. Since you might want to re-define one, a class or type-family instance replaces any earlier instance with an identical head or left hand side (respectively). (See Type families.)

4.4.5. What’s really in scope at the prompt?

When you type an expression at the prompt, what identifiers and types are in scope? GHCi provides a flexible way to control exactly how the context for an expression is constructed:

• The :load, :add, and :reload commands (The effect of :load on what is in scope).
• The import declaration (Controlling what is in scope with import).
• The :module command (Controlling what is in scope with the :module command).

The command :show imports will show a summary of which modules contribute to the top-level scope.

提示

GHCi will tab-complete names that are in scope; for example, if you run GHCi and type J<tab> then GHCi will expand it to Just.

4.4.5.1. The effect of :load on what is in scope

The :load, :add, and :reload commands (加载源文件 and 加载已编译代码) affect the top-level scope. Let’s start with the simple cases; when you start GHCi the prompt looks like this:

Prelude>

which indicates that everything from the module Prelude is currently in scope; the visible identifiers are exactly those that would be visible in a Haskell source file with no import declarations.

If we now load a file into GHCi, the prompt will change:

Prelude> :load Main.hs
Compiling Main             ( Main.hs, interpreted )
*Main>

The new prompt is *Main, which indicates that we are typing expressions in the context of the top-level of the Main module. Everything that is in scope at the top-level in the module Main we just loaded is also in scope at the prompt (probably including Prelude, as long as Main doesn’t explicitly hide it).

The syntax in the prompt *module indicates that it is the full top-level scope of ⟨module⟩ that is contributing to the scope for expressions typed at the prompt. Without the *, just the exports of the module are visible.

注解

For technical reasons, GHCi can only support the *-form for modules that are interpreted. Compiled modules and package modules can only contribute their exports to the current scope. To ensure that GHCi loads the interpreted version of a module, add the * when loading the module, e.g. :load *M.

In general, after a :load command, an automatic import is added to the scope for the most recently loaded “target” module, in a *-form if possible. For example, if you say :load foo.hs bar.hs and bar.hs contains module Bar, then the scope will be set to *Bar if Bar is interpreted, or if Bar is compiled it will be set to Prelude Bar (GHCi automatically adds Prelude if it isn’t present and there aren’t any *-form modules). These automatically-added imports can be seen with :show imports:

Prelude> :load hello.hs
[1 of 1] Compiling Main             ( hello.hs, interpreted )
Ok, modules loaded: Main.
*Main> :show imports
:module +*Main -- added automatically
*Main>

and the automatically-added import is replaced the next time you use :load, :add, or :reload. It can also be removed by :module as with normal imports.

4.4.5.2. Controlling what is in scope with import

We are not limited to a single module: GHCi can combine scopes from multiple modules, in any mixture of * and non-* forms. GHCi combines the scopes from all of these modules to form the scope that is in effect at the prompt.

To add modules to the scope, use ordinary Haskell import syntax:

Prelude> import System.IO
Prelude System.IO> hPutStrLn stdout "hello\n"
hello
Prelude System.IO>

The full Haskell import syntax is supported, including hiding and as clauses. The prompt shows the modules that are currently imported, but it omits details about hiding, as, and so on. To see the full story, use :show imports:

Prelude> import System.IO
Prelude System.IO> import Data.Map as Map
Prelude System.IO Map> :show imports
import Prelude -- implicit
import System.IO
import Data.Map as Map
Prelude System.IO Map>

Note that the Prelude import is marked as implicit. It can be overridden with an explicit Prelude import, just like in a Haskell module.

With multiple modules in scope, especially multiple *-form modules, it is likely that name clashes will occur. Haskell specifies that name clashes are only reported when an ambiguous identifier is used, and GHCi behaves in the same way for expressions typed at the prompt.

4.4.5.3. Controlling what is in scope with the :module command

Another way to manipulate the scope is to use the :module command, whose syntax is this:

:module +|- *mod1 ... *modn

Using the + form of the module commands adds modules to the current scope, and - removes them. Without either + or -, the current scope is replaced by the set of modules specified. Note that if you use this form and leave out Prelude, an implicit Prelude import will be added automatically.

The :module command provides a way to do two things that cannot be done with ordinary import declarations:

• :module supports the * modifier on modules, which opens the full top-level scope of a module, rather than just its exports.
• Imports can be removed from the context, using the syntax :module -M. The import syntax is cumulative (as in a Haskell module), so this is the only way to subtract from the scope.

4.4.5.4. Qualified names

To make life slightly easier, the GHCi prompt also behaves as if there is an implicit import qualified declaration for every module in every package, and every module currently loaded into GHCi. This behaviour can be disabled with the -fno-implicit-import-qualified flag.

4.4.5.5. :module and :load

It might seem that :module/import and :load/:add/:reload do similar things: you can use both to bring a module into scope. However, there is a very important difference. GHCi is concerned with two sets of modules:

• The set of modules that are currently loaded. This set is modified by :load, :add and :reload, and can be shown with :show modules.
• The set of modules that are currently in scope at the prompt. This set is modified by import and :module, and it is also modified automatically after :load, :add, and :reload, as described above. The set of modules in scope can be shown with :show imports.

You can add a module to the scope (via :module or import) only if either (a) it is loaded, or (b) it is a module from a package that GHCi knows about. Using :module or import to try bring into scope a non-loaded module may result in the message module M is not loaded.

4.4.6. The :main and :run commands

When a program is compiled and executed, it can use the getArgs function to access the command-line arguments. However, we cannot simply pass the arguments to the main function while we are testing in ghci, as the main function doesn’t take its directly.

Instead, we can use the :main command. This runs whatever main is in scope, with any arguments being treated the same as command-line arguments, e.g.:

Prelude> main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]

We can also quote arguments which contains characters like spaces, and they are treated like Haskell strings, or we can just use Haskell list syntax:

Prelude> :main foo "bar baz"
["foo","bar baz"]
Prelude> :main ["foo", "bar baz"]
["foo","bar baz"]

Finally, other functions can be called, either with the -main-is flag or the :run command:

Prelude> foo = putStrLn "foo" >> System.Environment.getArgs >>= print
Prelude> bar = putStrLn "bar" >> System.Environment.getArgs >>= print
Prelude> :set -main-is foo
Prelude> :main foo "bar baz"
foo
["foo","bar baz"]
Prelude> :run bar ["foo", "bar baz"]
bar
["foo","bar baz"]

4.4.7. The it variable

Whenever an expression (or a non-binding statement, to be precise) is typed at the prompt, GHCi implicitly binds its value to the variable it. For example:

Prelude> 1+2
3
Prelude> it * 2
6

What actually happens is that GHCi typechecks the expression, and if it doesn’t have an IO type, then it transforms it as follows: an expression e turns into

let it = e;
print it

which is then run as an IO-action.

Hence, the original expression must have a type which is an instance of the Show class, or GHCi will complain:

Prelude> id

<interactive>:1:0:
    No instance for (Show (a -> a))
      arising from use of `print' at <interactive>:1:0-1
    Possible fix: add an instance declaration for (Show (a -> a))
    In the expression: print it
    In a 'do' expression: print it

The error message contains some clues as to the transformation happening internally.

If the expression was instead of type IO a for some a, then it will be bound to the result of the IO computation, which is of type a. eg.:

Prelude> Time.getClockTime
Wed Mar 14 12:23:13 GMT 2001
Prelude> print it
Wed Mar 14 12:23:13 GMT 2001

The corresponding translation for an IO-typed e is

it <- e

Note that it is shadowed by the new value each time you evaluate a new expression, and the old value of it is lost.

4.4.8. Type defaulting in GHCi

-XExtendedDefaultRules

Allow defaulting to take place for more than just numeric classes.

Consider this GHCi session:

ghci> reverse []

What should GHCi do? Strictly speaking, the program is ambiguous. show (reverse []) (which is what GHCi computes here) has type Show a => String and how that displays depends on the type a. For example:

ghci> reverse ([] :: String)
""
ghci> reverse ([] :: [Int])
[]

However, it is tiresome for the user to have to specify the type, so GHCi extends Haskell’s type-defaulting rules (Section 4.3.4 of the Haskell 2010 Report) as follows. The standard rules take each group of constraints (C1 a, C2 a, ..., Cn a) for each type variable a, and defaults the type variable if

1. The type variable a appears in no other constraints
2. All the classes Ci are standard.
3. At least one of the classes Ci is numeric.

At the GHCi prompt, or with GHC if the -XExtendedDefaultRules flag is given, the following additional differences apply:

• Rule 2 above is relaxed thus: All of the classes Ci are single-parameter type classes.
• Rule 3 above is relaxed this: At least one of the classes Ci is an interactive class (defined below).
• The unit type () and the list type [] are added to the start of the standard list of types which are tried when doing type defaulting.

The last point means that, for example, this program:

main :: IO ()
main = print def

instance Num ()

def :: (Num a, Enum a) => a
def = toEnum 0

prints () rather than 0 as the type is defaulted to () rather than Integer.

The motivation for the change is that it means IO a actions default to IO (), which in turn means that ghci won’t try to print a result when running them. This is particularly important for printf, which has an instance that returns IO a. However, it is only able to return undefined (the reason for the instance having this type is so that printf doesn’t require extensions to the class system), so if the type defaults to Integer then ghci gives an error when running a printf.

See also I/O actions at the prompt for how the monad of a computational expression defaults to IO if possible.

4.4.8.1. Interactive classes

The interactive classes (only relevant when -XExtendedDefaultRules is in effect) are: any numeric class, Show, Eq, Ord, Foldable or Traversable.

As long as a type variable is constrained by one of these classes, defaulting will occur, as outlined above.

4.4.8.2. Extended rules around default declarations

Since the rules for defaulting are relaxed under -XExtendedDefaultRules, the rules for default declarations are also relaxed. According to Section 4.3.4 of the Haskell 2010 Report, a default declaration looks like default (t1, ..., tn) where, for each ti, Num ti must hold. This is relaxed to say that for each ti, there must exist an interactive class C such that C ti holds. This means that type constructors can be allowed in these lists. For example, the following works if you wish your Foldable constraints to default to Maybe but your Num constraints to still default to Integer or Double:

default (Maybe, Integer, Double)

4.4.9. Using a custom interactive printing function

Since GHC 7.6.1, GHCi prints the result of expressions typed at the prompt using the function System.IO.print. Its type signature is Show a => a -> IO (), and it works by converting the value to String using show.

This is not ideal in certain cases, like when the output is long, or contains strings with non-ascii characters.

The -interactive-print flag allows to specify any function of type C a => a -> IO (), for some constraint C, as the function for printing evaluated expressions. The function can reside in any loaded module or any registered package, but only when it resides in a registered package will it survive a :cd, :add, :load, :reload or, :set.

-interactive-print ⟨expr⟩

Set the function used by GHCi to print evaluation results. Expression must be of type C a => a -> IO ().

As an example, suppose we have following special printing module:

module SpecPrinter where
import System.IO

sprint a = putStrLn $ show a ++ "!"

The sprint function adds an exclamation mark at the end of any printed value. Running GHCi with the command:

ghci -interactive-print=SpecPrinter.sprinter SpecPrinter

will start an interactive session where values with be printed using sprint:

*SpecPrinter> [1,2,3]
[1,2,3]!
*SpecPrinter> 42
42!

A custom pretty printing function can be used, for example, to format tree-like and nested structures in a more readable way.

The -interactive-print flag can also be used when running GHC in -e mode:

% ghc -e "[1,2,3]" -interactive-print=SpecPrinter.sprint SpecPrinter
[1,2,3]!

4.4.10. Stack Traces in GHCi

[ This is an experimental feature enabled by the new -fexternal-interpreter flag that was introduced in GHC 8.0.1. It is currently not supported on Windows.]

GHCi can use the profiling system to collect stack trace information when running interpreted code. To gain access to stack traces, start GHCi like this:

ghci -fexternal-interpreter -prof

This runs the interpreted code in a separate process (see Running the interpreter in a separate process) and runs it in profiling mode to collect call stack information. Note that because we’re running the interpreted code in profiling mode, all packages that you use must be compiled for profiling. The -prof flag to GHCi only works in conjunction with -fexternal-interpreter.

There are three ways to get access to the current call stack.

• error and undefined automatically attach the current stack to the error message. This often complements the HasCallStack stack (see HasCallStack), so both call stacks are shown.
• Debug.Trace.traceStack is a version of Debug.Trace.trace that also prints the current call stack.
• Functions in the module GHC.Stack can be used to get the current stack and render it.

You don’t need to use -fprof-auto for interpreted modules, annotations are automatically added at a granularity fine enough to distinguish individual call sites. However, you won’t see any call stack information for compiled code unless it was compiled with -fprof-auto or has explicit SCC annotations (see Inserting cost centres by hand).

4.5. The GHCi Debugger

GHCi contains a simple imperative-style debugger in which you can stop a running computation in order to examine the values of variables. The debugger is integrated into GHCi, and is turned on by default: no flags are required to enable the debugging facilities. There is one major restriction: breakpoints and single-stepping are only available in interpreted modules; compiled code is invisible to the debugger [5].

The debugger provides the following:

• The ability to set a breakpoint on a function definition or expression in the program. When the function is called, or the expression evaluated, GHCi suspends execution and returns to the prompt, where you can inspect the values of local variables before continuing with the execution.
• Execution can be single-stepped: the evaluator will suspend execution approximately after every reduction, allowing local variables to be inspected. This is equivalent to setting a breakpoint at every point in the program.
• Execution can take place in tracing mode, in which the evaluator remembers each evaluation step as it happens, but doesn’t suspend execution until an actual breakpoint is reached. When this happens, the history of evaluation steps can be inspected.
• Exceptions (e.g. pattern matching failure and error) can be treated as breakpoints, to help locate the source of an exception in the program.

There is currently no support for obtaining a “stack trace”, but the tracing and history features provide a useful second-best, which will often be enough to establish the context of an error. For instance, it is possible to break automatically when an exception is thrown, even if it is thrown from within compiled code (see Debugging exceptions).

4.5.1. Breakpoints and inspecting variables

Let’s use quicksort as a running example. Here’s the code:

qsort [] = []
qsort (a:as) = qsort left ++ [a] ++ qsort right
  where (left,right) = (filter (<=a) as, filter (>a) as)

main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])

First, load the module into GHCi:

Prelude> :l qsort.hs
[1 of 1] Compiling Main             ( qsort.hs, interpreted )
Ok, modules loaded: Main.
*Main>

Now, let’s set a breakpoint on the right-hand-side of the second equation of qsort:

*Main> :break 2
Breakpoint 0 activated at qsort.hs:2:15-46
*Main>

The command :break 2 sets a breakpoint on line 2 of the most recently-loaded module, in this case qsort.hs. Specifically, it picks the leftmost complete subexpression on that line on which to set the breakpoint, which in this case is the expression (qsort left ++ [a] ++ qsort right).

Now, we run the program:

*Main> main
Stopped at qsort.hs:2:15-46
_result :: [a]
a :: a
left :: [a]
right :: [a]
[qsort.hs:2:15-46] *Main>

Execution has stopped at the breakpoint. The prompt has changed to indicate that we are currently stopped at a breakpoint, and the location: [qsort.hs:2:15-46]. To further clarify the location, we can use the :list command:

[qsort.hs:2:15-46] *Main> :list
1  qsort [] = []
2  qsort (a:as) = qsort left ++ [a] ++ qsort right
3    where (left,right) = (filter (<=a) as, filter (>a) as)

The :list command lists the source code around the current breakpoint. If your output device supports it, then GHCi will highlight the active subexpression in bold.

GHCi has provided bindings for the free variables [6] of the expression on which the breakpoint was placed (a, left, right), and additionally a binding for the result of the expression (_result). These variables are just like other variables that you might define in GHCi; you can use them in expressions that you type at the prompt, you can ask for their types with :type, and so on. There is one important difference though: these variables may only have partial types. For example, if we try to display the value of left:

[qsort.hs:2:15-46] *Main> left

<interactive>:1:0:
    Ambiguous type variable `a' in the constraint:
      `Show a' arising from a use of `print' at <interactive>:1:0-3
    Cannot resolve unknown runtime types: a
    Use :print or :force to determine these types

This is because qsort is a polymorphic function, and because GHCi does not carry type information at runtime, it cannot determine the runtime types of free variables that involve type variables. Hence, when you ask to display left at the prompt, GHCi can’t figure out which instance of Show to use, so it emits the type error above.

Fortunately, the debugger includes a generic printing command, :print, which can inspect the actual runtime value of a variable and attempt to reconstruct its type. If we try it on left:

[qsort.hs:2:15-46] *Main> :set -fprint-evld-with-show
[qsort.hs:2:15-46] *Main> :print left
left = (_t1::[a])

This isn’t particularly enlightening. What happened is that left is bound to an unevaluated computation (a suspension, or thunk), and :print does not force any evaluation. The idea is that :print can be used to inspect values at a breakpoint without any unfortunate side effects. It won’t force any evaluation, which could cause the program to give a different answer than it would normally, and hence it won’t cause any exceptions to be raised, infinite loops, or further breakpoints to be triggered (see Nested breakpoints). Rather than forcing thunks, :print binds each thunk to a fresh variable beginning with an underscore, in this case _t1.

The flag -fprint-evld-with-show instructs :print to reuse available Show instances when possible. This happens only when the contents of the variable being inspected are completely evaluated.

If we aren’t concerned about preserving the evaluatedness of a variable, we can use :force instead of :print. The :force command behaves exactly like :print, except that it forces the evaluation of any thunks it encounters:

[qsort.hs:2:15-46] *Main> :force left
left = [4,0,3,1]

Now, since :force has inspected the runtime value of left, it has reconstructed its type. We can see the results of this type reconstruction:

[qsort.hs:2:15-46] *Main> :show bindings
_result :: [Integer]
a :: Integer
left :: [Integer]
right :: [Integer]
_t1 :: [Integer]

Not only do we now know the type of left, but all the other partial types have also been resolved. So we can ask for the value of a, for example:

[qsort.hs:2:15-46] *Main> a
8

You might find it useful to use Haskell’s seq function to evaluate individual thunks rather than evaluating the whole expression with :force. For example:

[qsort.hs:2:15-46] *Main> :print right
right = (_t1::[Integer])
[qsort.hs:2:15-46] *Main> seq _t1 ()
()
[qsort.hs:2:15-46] *Main> :print right
right = 23 : (_t2::[Integer])

We evaluated only the _t1 thunk, revealing the head of the list, and the tail is another thunk now bound to _t2. The seq function is a little inconvenient to use here, so you might want to use :def to make a nicer interface (left as an exercise for the reader!).

Finally, we can continue the current execution:

[qsort.hs:2:15-46] *Main> :continue
Stopped at qsort.hs:2:15-46
_result :: [a]
a :: a
left :: [a]
right :: [a]
[qsort.hs:2:15-46] *Main>

The execution continued at the point it previously stopped, and has now stopped at the breakpoint for a second time.

4.5.1.1. Setting breakpoints

Breakpoints can be set in various ways. Perhaps the easiest way to set a breakpoint is to name a top-level function:

:break identifier

Where ⟨identifier⟩ names any top-level function in an interpreted module currently loaded into GHCi (qualified names may be used). The breakpoint will be set on the body of the function, when it is fully applied but before any pattern matching has taken place.

Breakpoints can also be set by line (and optionally column) number:

:break line
:break line column
:break module line
:break module line column

When a breakpoint is set on a particular line, GHCi sets the breakpoint on the leftmost subexpression that begins and ends on that line. If two complete subexpressions start at the same column, the longest one is picked. If there is no complete subexpression on the line, then the leftmost expression starting on the line is picked, and failing that the rightmost expression that partially or completely covers the line.

When a breakpoint is set on a particular line and column, GHCi picks the smallest subexpression that encloses that location on which to set the breakpoint. Note: GHC considers the TAB character to have a width of 1, wherever it occurs; in other words it counts characters, rather than columns. This matches what some editors do, and doesn’t match others. The best advice is to avoid tab characters in your source code altogether (see -Wtabs in 警告和 sanity-checking).

If the module is omitted, then the most recently-loaded module is used.

Not all subexpressions are potential breakpoint locations. Single variables are typically not considered to be breakpoint locations (unless the variable is the right-hand-side of a function definition, lambda, or case alternative). The rule of thumb is that all redexes are breakpoint locations, together with the bodies of functions, lambdas, case alternatives and binding statements. There is normally no breakpoint on a let expression, but there will always be a breakpoint on its body, because we are usually interested in inspecting the values of the variables bound by the let.

4.5.1.2. Listing and deleting breakpoints

The list of breakpoints currently enabled can be displayed using :show breaks:

*Main> :show breaks
[0] Main qsort.hs:1:11-12
[1] Main qsort.hs:2:15-46

To delete a breakpoint, use the :delete command with the number given in the output from :show breaks:

*Main> :delete 0
*Main> :show breaks
[1] Main qsort.hs:2:15-46

To delete all breakpoints at once, use :delete *.

4.5.2. Single-stepping

Single-stepping is a great way to visualise the execution of your program, and it is also a useful tool for identifying the source of a bug. GHCi offers two variants of stepping. Use :step to enable all the breakpoints in the program, and execute until the next breakpoint is reached. Use :steplocal to limit the set of enabled breakpoints to those in the current top level function. Similarly, use :stepmodule to single step only on breakpoints contained in the current module. For example:

*Main> :step main
Stopped at qsort.hs:5:7-47
_result :: IO ()

The command :step expr begins the evaluation of ⟨expr⟩ in single-stepping mode. If ⟨expr⟩ is omitted, then it single-steps from the current breakpoint. :steplocal and :stepmodule commands work similarly.

The :list command is particularly useful when single-stepping, to see where you currently are:

[qsort.hs:5:7-47] *Main> :list
4
5  main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
6
[qsort.hs:5:7-47] *Main>

In fact, GHCi provides a way to run a command when a breakpoint is hit, so we can make it automatically do :list:

[qsort.hs:5:7-47] *Main> :set stop :list
[qsort.hs:5:7-47] *Main> :step
Stopped at qsort.hs:5:14-46
_result :: [Integer]
4
5  main = print (qsort [8, 4, 0, 3, 1, 23, 11, 18])
6
[qsort.hs:5:14-46] *Main>

4.5.3. Nested breakpoints

When GHCi is stopped at a breakpoint, and an expression entered at the prompt triggers a second breakpoint, the new breakpoint becomes the “current” one, and the old one is saved on a stack. An arbitrary number of breakpoint contexts can be built up in this way. For example:

[qsort.hs:2:15-46] *Main> :st qsort [1,3]
Stopped at qsort.hs:(1,0)-(3,55)
_result :: [a]
... [qsort.hs:(1,0)-(3,55)] *Main>

While stopped at the breakpoint on line 2 that we set earlier, we started a new evaluation with :step qsort [1,3]. This new evaluation stopped after one step (at the definition of qsort). The prompt has changed, now prefixed with ..., to indicate that there are saved breakpoints beyond the current one. To see the stack of contexts, use :show context:

... [qsort.hs:(1,0)-(3,55)] *Main> :show context
--> main
  Stopped at qsort.hs:2:15-46
--> qsort [1,3]
  Stopped at qsort.hs:(1,0)-(3,55)
... [qsort.hs:(1,0)-(3,55)] *Main>

To abandon the current evaluation, use :abandon:

... [qsort.hs:(1,0)-(3,55)] *Main> :abandon
[qsort.hs:2:15-46] *Main> :abandon
*Main>

4.5.4. The _result variable

When stopped at a breakpoint or single-step, GHCi binds the variable _result to the value of the currently active expression. The value of _result is presumably not available yet, because we stopped its evaluation, but it can be forced: if the type is known and showable, then just entering _result at the prompt will show it. However, there’s one caveat to doing this: evaluating _result will be likely to trigger further breakpoints, starting with the breakpoint we are currently stopped at (if we stopped at a real breakpoint, rather than due to :step). So it will probably be necessary to issue a :continue immediately when evaluating _result. Alternatively, you can use :force which ignores breakpoints.

4.5.5. Tracing and history

A question that we often want to ask when debugging a program is “how did I get here?”. Traditional imperative debuggers usually provide some kind of stack-tracing feature that lets you see the stack of active function calls (sometimes called the “lexical call stack”), describing a path through the code to the current location. Unfortunately this is hard to provide in Haskell, because execution proceeds on a demand-driven basis, rather than a depth-first basis as in strict languages. The “stack“ in GHC’s execution engine bears little resemblance to the lexical call stack. Ideally GHCi would maintain a separate lexical call stack in addition to the dynamic call stack, and in fact this is exactly what our profiling system does (Profiling), and what some other Haskell debuggers do. For the time being, however, GHCi doesn’t maintain a lexical call stack (there are some technical challenges to be overcome). Instead, we provide a way to backtrack from a breakpoint to previous evaluation steps: essentially this is like single-stepping backwards, and should in many cases provide enough information to answer the “how did I get here?” question.

To use tracing, evaluate an expression with the :trace command. For example, if we set a breakpoint on the base case of qsort:

*Main> :list qsort
1  qsort [] = []
2  qsort (a:as) = qsort left ++ [a] ++ qsort right
3    where (left,right) = (filter (<=a) as, filter (>a) as)
4
*Main> :b 1
Breakpoint 1 activated at qsort.hs:1:11-12
*Main>

and then run a small qsort with tracing:

*Main> :trace qsort [3,2,1]
Stopped at qsort.hs:1:11-12
_result :: [a]
[qsort.hs:1:11-12] *Main>

We can now inspect the history of evaluation steps:

[qsort.hs:1:11-12] *Main> :hist
-1  : qsort.hs:3:24-38
-2  : qsort.hs:3:23-55
-3  : qsort.hs:(1,0)-(3,55)
-4  : qsort.hs:2:15-24
-5  : qsort.hs:2:15-46
-6  : qsort.hs:3:24-38
-7  : qsort.hs:3:23-55
-8  : qsort.hs:(1,0)-(3,55)
-9  : qsort.hs:2:15-24
-10 : qsort.hs:2:15-46
-11 : qsort.hs:3:24-38
-12 : qsort.hs:3:23-55
-13 : qsort.hs:(1,0)-(3,55)
-14 : qsort.hs:2:15-24
-15 : qsort.hs:2:15-46
-16 : qsort.hs:(1,0)-(3,55)
<end of history>

To examine one of the steps in the history, use :back:

[qsort.hs:1:11-12] *Main> :back
Logged breakpoint at qsort.hs:3:24-38
_result :: [a]
as :: [a]
a :: a
[-1: qsort.hs:3:24-38] *Main>

Note that the local variables at each step in the history have been preserved, and can be examined as usual. Also note that the prompt has changed to indicate that we’re currently examining the first step in the history: -1. The command :forward can be used to traverse forward in the history.

The :trace command can be used with or without an expression. When used without an expression, tracing begins from the current breakpoint, just like :step.

The history is only available when using :trace; the reason for this is we found that logging each breakpoint in the history cuts performance by a factor of 2 or more.

-fghci-hist-size

Default:50

Modify the depth of the evaluation history tracked by GHCi.

4.5.6. Debugging exceptions

Another common question that comes up when debugging is “where did this exception come from?”. Exceptions such as those raised by error or head [] have no context information attached to them. Finding which particular call to head in your program resulted in the error can be a painstaking process, usually involving Debug.Trace.trace, or compiling with profiling and using Debug.Trace.traceStack or +RTS -xc (see -xc).

The GHCi debugger offers a way to hopefully shed some light on these errors quickly and without modifying or recompiling the source code. One way would be to set a breakpoint on the location in the source code that throws the exception, and then use :trace and :history to establish the context. However, head is in a library and we can’t set a breakpoint on it directly. For this reason, GHCi provides the flags -fbreak-on-exception which causes the evaluator to stop when an exception is thrown, and -fbreak-on-error, which works similarly but stops only on uncaught exceptions. When stopping at an exception, GHCi will act just as it does when a breakpoint is hit, with the deviation that it will not show you any source code location. Due to this, these commands are only really useful in conjunction with :trace, in order to log the steps leading up to the exception. For example:

*Main> :set -fbreak-on-exception
*Main> :trace qsort ("abc" ++ undefined)
“Stopped at <exception thrown>
_exception :: e
[<exception thrown>] *Main> :hist
-1  : qsort.hs:3:24-38
-2  : qsort.hs:3:23-55
-3  : qsort.hs:(1,0)-(3,55)
-4  : qsort.hs:2:15-24
-5  : qsort.hs:2:15-46
-6  : qsort.hs:(1,0)-(3,55)
<end of history>
[<exception thrown>] *Main> :back
Logged breakpoint at qsort.hs:3:24-38
_result :: [a]
as :: [a]
a :: a
[-1: qsort.hs:3:24-38] *Main> :force as
*** Exception: Prelude.undefined
[-1: qsort.hs:3:24-38] *Main> :print as
as = 'b' : 'c' : (_t1::[Char])

The exception itself is bound to a new variable, _exception.

Breaking on exceptions is particularly useful for finding out what your program was doing when it was in an infinite loop. Just hit Control-C, and examine the history to find out what was going on.

-fbreak-on-exception

-fbreak-on-error

Causes GHCi to halt evaluation and return to the interactive prompt in the event of an exception. While -fbreak-on-exception breaks on all exceptions, -fbreak-on-error breaks on only those which would otherwise be uncaught.

4.5.7. Example: inspecting functions

It is possible to use the debugger to examine function values. When we are at a breakpoint and a function is in scope, the debugger cannot show you the source code for it; however, it is possible to get some information by applying it to some arguments and observing the result.

The process is slightly complicated when the binding is polymorphic. We show the process by means of an example. To keep things simple, we will use the well known map function:

import Prelude hiding (map)

map :: (a->b) -> [a] -> [b]
map f [] = []
map f (x:xs) = f x : map f xs

We set a breakpoint on map, and call it.

*Main> :break 5
Breakpoint 0 activated at  map.hs:5:15-28
*Main> map Just [1..5]
Stopped at map.hs:(4,0)-(5,12)
_result :: [b]
x :: a
f :: a -> b
xs :: [a]

GHCi tells us that, among other bindings, f is in scope. However, its type is not fully known yet, and thus it is not possible to apply it to any arguments. Nevertheless, observe that the type of its first argument is the same as the type of x, and its result type is shared with _result.

As we demonstrated earlier (Breakpoints and inspecting variables), the debugger has some intelligence built-in to update the type of f whenever the types of x or _result are discovered. So what we do in this scenario is force x a bit, in order to recover both its type and the argument part of f.

*Main> seq x ()
*Main> :print x
x = 1

We can check now that as expected, the type of x has been reconstructed, and with it the type of f has been too:

*Main> :t x
x :: Integer
*Main> :t f
f :: Integer -> b

From here, we can apply f to any argument of type Integer and observe the results.

*Main> let b = f 10
*Main> :t b
b :: b
*Main> b
<interactive>:1:0:
    Ambiguous type variable `b' in the constraint:
      `Show b' arising from a use of `print' at <interactive>:1:0
*Main> :p b
b = (_t2::a)
*Main> seq b ()
()
*Main> :t b
b :: a
*Main> :p b
b = Just 10
*Main> :t b
b :: Maybe Integer
*Main> :t f
f :: Integer -> Maybe Integer
*Main> f 20
Just 20
*Main> map f [1..5]
[Just 1, Just 2, Just 3, Just 4, Just 5]

In the first application of f, we had to do some more type reconstruction in order to recover the result type of f. But after that, we are free to use f normally.

4.5.8. Limitations

• When stopped at a breakpoint, if you try to evaluate a variable that is already under evaluation, the second evaluation will hang. The reason is that GHC knows the variable is under evaluation, so the new evaluation just waits for the result before continuing, but of course this isn’t going to happen because the first evaluation is stopped at a breakpoint. Control-C can interrupt the hung evaluation and return to the prompt.

  The most common way this can happen is when you’re evaluating a CAF (e.g. main), stop at a breakpoint, and ask for the value of the CAF at the prompt again.

• Implicit parameters (see Implicit parameters) are only available at the scope of a breakpoint if there is an explicit type signature.

4.6. Invoking GHCi

GHCi is invoked with the command ghci or ghc --interactive. One or more modules or filenames can also be specified on the command line; this instructs GHCi to load the specified modules or filenames (and all the modules they depend on), just as if you had said :load modules at the GHCi prompt (see GHCi commands). For example, to start GHCi and load the program whose topmost module is in the file Main.hs, we could say:

$ ghci Main.hs

Most of the command-line options accepted by GHC (see Using GHC) also make sense in interactive mode. The ones that don’t make sense are mostly obvious.

-flocal-ghci-history

By default, GHCi keeps global history in ~/.ghc/ghci_history or %APPDATA%/<app>/ghci_history, but you can use current directory, e.g.:

$ ghci -flocal-ghci-history

It will create .ghci-history in current folder where GHCi is launched.

4.6.1. Packages

Most packages (see Using Packages) are available without needing to specify any extra flags at all: they will be automatically loaded the first time they are needed.

For hidden packages, however, you need to request the package be loaded by using the -package flag:

$ ghci -package readline
GHCi, version 6.8.1: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Loading package readline-1.0 ... linking ... done.
Prelude>

The following command works to load new packages into a running GHCi:

Prelude> :set -package name

But note that doing this will cause all currently loaded modules to be unloaded, and you’ll be dumped back into the Prelude.

4.6.2. Extra libraries

Extra libraries may be specified on the command line using the normal -llib option. (The term library here refers to libraries of foreign object code; for using libraries of Haskell source code, see 模块 vs. 文件名.) For example, to load the “m” library:

$ ghci -lm

On systems with .so-style shared libraries, the actual library loaded will the liblib.so. GHCi searches the following places for libraries, in this order:

• Paths specified using the -L command-line option,
• the standard library search path for your system, which on some systems may be overridden by setting the LD_LIBRARY_PATH environment variable.

On systems with .dll-style shared libraries, the actual library loaded will be lib.dll. Again, GHCi will signal an error if it can’t find the library.

GHCi can also load plain object files (.o or .obj depending on your platform) from the command-line. Just add the name the object file to the command line.

Ordering of -l options matters: a library should be mentioned before the libraries it depends on (see Options affecting linking).

4.7. GHCi commands

GHCi commands all begin with “:” and consist of a single command name followed by zero or more parameters. The command name may be abbreviated, with ambiguities being resolved in favour of the more commonly used commands.

:abandon

Abandons the current evaluation (only available when stopped at a breakpoint).

:add[*] ⟨module⟩

Add ⟨module⟩(s) to the current target set, and perform a reload. Normally pre-compiled code for the module will be loaded if available, or otherwise the module will be compiled to byte-code. Using the * prefix forces the module to be loaded as byte-code.

:all-types

List all types collected for expressions and (local) bindings currently loaded (while :set +c was active) with their respective source-code span, e.g.

GhciTypes> :all-types
GhciTypes.hs:(38,13)-(38,24): Maybe Id
GhciTypes.hs:(45,10)-(45,29): Outputable SpanInfo
GhciTypes.hs:(45,10)-(45,29): (Rational -> SpanInfo -> SDoc) -> Outputable SpanInfo

:back ⟨n⟩

Travel back ⟨n⟩ steps in the history. ⟨n⟩ is one if omitted. See Tracing and history for more about GHCi’s debugging facilities. See also: :trace, :history, :forward.

:break [⟨identifier⟩ | [⟨module⟩] ⟨line⟩ [⟨column⟩]]

Set a breakpoint on the specified function or line and column. See Setting breakpoints.

:browse[!] [[*] ⟨module⟩]

Displays the identifiers exported by the module ⟨module⟩, which must be either loaded into GHCi or be a member of a package. If ⟨module⟩ is omitted, the most recently-loaded module is used.

Like all other GHCi commands, the output is always displayed in the current GHCi scope (What’s really in scope at the prompt?).

There are two variants of the browse command:

• If the * symbol is placed before the module name, then all the identifiers in scope in ⟨module⟩ (rather that just its exports) are shown.

  The *-form is only available for modules which are interpreted; for compiled modules (including modules from packages) only the non-* form of :browse is available.

• Data constructors and class methods are usually displayed in the context of their data type or class declaration. However, if the ! symbol is appended to the command, thus :browse!, they are listed individually. The !-form also annotates the listing with comments giving possible imports for each group of entries. Here is an example:

  Prelude> :browse! Data.Maybe
  -- not currently imported
  Data.Maybe.catMaybes :: [Maybe a] -> [a]
  Data.Maybe.fromJust :: Maybe a -> a
  Data.Maybe.fromMaybe :: a -> Maybe a -> a
  Data.Maybe.isJust :: Maybe a -> Bool
  Data.Maybe.isNothing :: Maybe a -> Bool
  Data.Maybe.listToMaybe :: [a] -> Maybe a
  Data.Maybe.mapMaybe :: (a -> Maybe b) -> [a] -> [b]
  Data.Maybe.maybeToList :: Maybe a -> [a]
  -- imported via Prelude
  Just :: a -> Maybe a
  data Maybe a = Nothing | Just a
  Nothing :: Maybe a
  maybe :: b -> (a -> b) -> Maybe a -> b
  

  This output shows that, in the context of the current session (ie in the scope of Prelude), the first group of items from Data.Maybe are not in scope (althought they are available in fully qualified form in the GHCi session - see What’s really in scope at the prompt?), whereas the second group of items are in scope (via Prelude) and are therefore available either unqualified, or with a Prelude. qualifier.

:cd ⟨dir⟩

Changes the current working directory to ⟨dir⟩. A “~” symbol at the beginning of ⟨dir⟩ will be replaced by the contents of the environment variable HOME. See also the :show paths command for showing the current working directory.

Note: changing directories causes all currently loaded modules to be unloaded. This is because the search path is usually expressed using relative directories, and changing the search path in the middle of a session is not supported.

:cmd ⟨expr⟩

Executes ⟨expr⟩ as a computation of type IO String, and then executes the resulting string as a list of GHCi commands. Multiple commands are separated by newlines. The :cmd command is useful with :def and :set stop.

:complete ⟨type⟩ [⟨n⟩-][⟨m⟩] ⟨string-literal⟩

This command allows to request command completions from GHCi even when interacting over a pipe instead of a proper terminal and is designed for integrating GHCi’s completion with text editors and IDEs.

When called, :complete prints the ⟨n⟩^th to ⟨m⟩^th completion candidates for the partial input ⟨string-literal⟩ for the completion domain denoted by ⟨type⟩. Currently, only the repl domain is supported which denotes the kind of completion that would be provided interactively by GHCi at the input prompt.

If omitted, ⟨n⟩ and ⟨m⟩ default to the first or last available completion candidate respectively. If there are less candidates than requested via the range argument, ⟨n⟩ and ⟨m⟩ are implicitly capped to the number of available completition candidates.

The output of :complete begins with a header line containing three space-delimited fields:

• An integer denoting the number l of printed completions,
• an integer denoting the total number of completions available, and finally
• a string literal denoting a common prefix to be added to the returned completion candidates.

The header line is followed by ⟨l⟩ lines each containing one completion candidate encoded as (quoted) string literal. Here are some example invocations showing the various cases:

Prelude> :complete repl 0 ""
0 470 ""
Prelude> :complete repl 5 "import For"
5 21 "import "
"Foreign"
"Foreign.C"
"Foreign.C.Error"
"Foreign.C.String"
"Foreign.C.Types"
Prelude> :complete repl 5-10 "import For"
6 21 "import "
"Foreign.C.Types"
"Foreign.Concurrent"
"Foreign.ForeignPtr"
"Foreign.ForeignPtr.Safe"
"Foreign.ForeignPtr.Unsafe"
"Foreign.Marshal"
Prelude> :complete repl 20- "import For"
2 21 "import "
"Foreign.StablePtr"
"Foreign.Storable"
Prelude> :complete repl "map"
3 3 ""
"map"
"mapM"
"mapM_"
Prelude> :complete repl 5-10 "map"
0 3 ""

:continue

Continue the current evaluation, when stopped at a breakpoint.

:ctags [⟨filename⟩]

Generates a “tags” file for Vi-style editors (:ctags) or Emacs-style editors (:etags). If no filename is specified, the default tags or TAGS is used, respectively. Tags for all the functions, constructors and types in the currently loaded modules are created. All modules must be interpreted for these commands to work.

:def[!] ⟨name⟩ ⟨expr⟩

:def is used to define new commands, or macros, in GHCi. The command :def ⟨name⟩ ⟨expr⟩ defines a new GHCi command :name, implemented by the Haskell expression ⟨expr⟩, which must have type String -> IO String. When :name args is typed at the prompt, GHCi will run the expression (name args), take the resulting String, and feed it back into GHCi as a new sequence of commands. Separate commands in the result must be separated by “\n”.

That’s all a little confusing, so here’s a few examples. To start with, here’s a new GHCi command which doesn’t take any arguments or produce any results, it just outputs the current date and time:

Prelude> let date _ = Time.getClockTime >>= print >> return ""
Prelude> :def date date
Prelude> :date
Fri Mar 23 15:16:40 GMT 2001

Here’s an example of a command that takes an argument. It’s a re-implementation of :cd:

Prelude> let mycd d = Directory.setCurrentDirectory d >> return ""
Prelude> :def mycd mycd
Prelude> :mycd ..

Or I could define a simple way to invoke “ghc --make Main” in the current directory:

Prelude> :def make (\_ -> return ":! ghc --make Main")

We can define a command that reads GHCi input from a file. This might be useful for creating a set of bindings that we want to repeatedly load into the GHCi session:

Prelude> :def . readFile
Prelude> :. cmds.ghci

Notice that we named the command :., by analogy with the “.” Unix shell command that does the same thing.

Typing :def on its own lists the currently-defined macros. Attempting to redefine an existing command name results in an error unless the :def! form is used, in which case the old command with that name is silently overwritten.

:delete * | ⟨num⟩ ...

Delete one or more breakpoints by number (use :show breaks to see the number of each breakpoint). The * form deletes all the breakpoints.

:edit ⟨file⟩

Opens an editor to edit the file ⟨file⟩, or the most recently loaded module if ⟨file⟩ is omitted. If there were errors during the last loading, the cursor will be positioned at the line of the first error. The editor to invoke is taken from the EDITOR environment variable, or a default editor on your system if EDITOR is not set. You can change the editor using :set editor.

:etags

See :ctags.

:force ⟨identifier⟩ ...

Prints the value of ⟨identifier⟩ in the same way as :print. Unlike :print, :force evaluates each thunk that it encounters while traversing the value. This may cause exceptions or infinite loops, or further breakpoints (which are ignored, but displayed).

:forward ⟨n⟩

Move forward ⟨n⟩ steps in the history. ⟨n⟩ is one if omitted. See Tracing and history for more about GHCi’s debugging facilities. See also: :trace, :history, :back.

:help

:?

Displays a list of the available commands.

:

Repeat the previous command.

:history [num]

Display the history of evaluation steps. With a number, displays that many steps (default: 20). For use with :trace; see Tracing and history. To set the number of history entries stored by GHCi, use the -fghci-hist-size flag.

:info[!] ⟨name⟩

Displays information about the given name(s). For example, if ⟨name⟩ is a class, then the class methods and their types will be printed; if ⟨name⟩ is a type constructor, then its definition will be printed; if ⟨name⟩ is a function, then its type will be printed. If ⟨name⟩ has been loaded from a source file, then GHCi will also display the location of its definition in the source.

For types and classes, GHCi also summarises instances that mention them. To avoid showing irrelevant information, an instance is shown only if (a) its head mentions ⟨name⟩, and (b) all the other things mentioned in the instance are in scope (either qualified or otherwise) as a result of a :load or :module commands.

The command :info! works in a similar fashion but it removes restriction (b), showing all instances that are in scope and mention ⟨name⟩ in their head.

:issafe [⟨module⟩]

Displays Safe Haskell information about the given module (or the current module if omitted). This includes the trust type of the module and its containing package.

:kind[!] ⟨type⟩

Infers and prints the kind of ⟨type⟩. The latter can be an arbitrary type expression, including a partial application of a type constructor, such as Either Int. In fact, :kind even allows you to write a partial application of a type synonym (usually disallowed), so that this works:

ghci> type T a b = (a,b,a)
ghci> :k T Int Bool
T Int Bool :: *
ghci> :k T
T :: * -> * -> *
ghci> :k T Int
T Int :: * -> *

If you specify the optional “!”, GHC will in addition normalise the type by expanding out type synonyms and evaluating type-function applications, and display the normalised result.

:list ⟨identifier⟩

Lists the source code around the definition of ⟨identifier⟩ or the current breakpoint if not given. This requires that the identifier be defined in an interpreted module. If your output device supports it, then GHCi will highlight the active subexpression in bold.

:list [⟨module⟩] ⟨line⟩

Lists the source code around the given line number of ⟨module⟩. This requires that the module be interpreted. If your output device supports it, then GHCi will highlight the active subexpression in bold.

:load[!] [*]⟨module⟩

Recursively loads the specified ⟨module⟩s, and all the modules they depend on. Here, each ⟨module⟩ must be a module name or filename, but may not be the name of a module in a package.

All previously loaded modules, except package modules, are forgotten. The new set of modules is known as the target set. Note that :load can be used without any arguments to unload all the currently loaded modules and bindings.

Normally pre-compiled code for a module will be loaded if available, or otherwise the module will be compiled to byte-code. Using the * prefix forces a module to be loaded as byte-code.

Adding the optional “!” turns type errors into warnings while loading. This allows to use the portions of the module that are correct, even if there are type errors in some definitions. Effectively, the “-fdefer-type-errors” flag is set before loading and unset after loading if the flag has not already been set before. See Deferring type errors to runtime for further motivation and details.

After a :load command, the current context is set to:

• ⟨module⟩, if it was loaded successfully, or
• the most recently successfully loaded module, if any other modules were loaded as a result of the current :load, or
• Prelude otherwise.

:loc-at ⟨module⟩ ⟨line⟩ ⟨col⟩ ⟨end-line⟩ ⟨end-col⟩ [⟨name⟩]

Tries to find the definition site of the name at the given source-code span, e.g.:

X> :loc-at X.hs 6 14 6 16 mu
X.hs:(8,7)-(8,9)

This command is useful when integrating GHCi with text editors and IDEs for providing a goto-definition facility.

The :loc-at command requires :set +c to be set.

:main ⟨arg1⟩ ... ⟨argn⟩

When a program is compiled and executed, it can use the getArgs function to access the command-line arguments. However, we cannot simply pass the arguments to the main function while we are testing in ghci, as the main function doesn’t take its arguments directly.

Instead, we can use the :main command. This runs whatever main is in scope, with any arguments being treated the same as command-line arguments, e.g.:

Prelude> main = System.Environment.getArgs >>= print
Prelude> :main foo bar
["foo","bar"]

We can also quote arguments which contains characters like spaces, and they are treated like Haskell strings, or we can just use Haskell list syntax:

Prelude> :main foo "bar baz"
["foo","bar baz"]
Prelude> :main ["foo", "bar baz"]
["foo","bar baz"]

Finally, other functions can be called, either with the -main-is flag or the :run command:

Prelude> foo = putStrLn "foo" >> System.Environment.getArgs >>= print
Prelude> bar = putStrLn "bar" >> System.Environment.getArgs >>= print
Prelude> :set -main-is foo
Prelude> :main foo "bar baz"
foo
["foo","bar baz"]
Prelude> :run bar ["foo", "bar baz"]
bar
["foo","bar baz"]

:module +|- [*]⟨mod1⟩ ...

import ⟨mod⟩

Sets or modifies the current context for statements typed at the prompt. The form import mod is equivalent to :module +mod. See What’s really in scope at the prompt? for more details.

:print ⟨names⟩

Prints a value without forcing its evaluation. :print may be used on values whose types are unknown or partially known, which might be the case for local variables with polymorphic types at a breakpoint. While inspecting the runtime value, :print attempts to reconstruct the type of the value, and will elaborate the type in GHCi’s environment if possible. If any unevaluated components (thunks) are encountered, then :print binds a fresh variable with a name beginning with _t to each thunk. See Breakpoints and inspecting variables for more information. See also the :sprint command, which works like :print but does not bind new variables.

:quit

Quits GHCi. You can also quit by typing Control-D at the prompt.

:reload[!]

Attempts to reload the current target set (see :load) if any of the modules in the set, or any dependent module, has changed. Note that this may entail loading new modules, or dropping modules which are no longer indirectly required by the target.

Adding the optional “!” turns type errors into warnings while loading. This allows to use the portions of the module that are correct, even if there are type errors in some definitions. Effectively, the “-fdefer-type-errors” flag is set before loading and unset after loading if the flag has not already been set before. See Deferring type errors to runtime for further motivation and details.

:run

See :main.

:script [⟨n⟩] ⟨filename⟩

Executes the lines of a file as a series of GHCi commands. This command is compatible with multiline statements as set by :set +m

:set [⟨option⟩ ...]

Sets various options. See The :set and :seti commands for a list of available options and Interactive-mode options for a list of GHCi-specific flags. The :set command by itself shows which options are currently set. It also lists the current dynamic flag settings, with GHCi-specific flags listed separately.

:set args ⟨arg⟩

Sets the list of arguments which are returned when the program calls System.getArgs.

:set editor ⟨cmd⟩

Sets the command used by :edit to ⟨cmd⟩.

:set prog ⟨prog⟩

Sets the string to be returned when the program calls System.getProgName.

:set prompt ⟨prompt⟩

Sets the string to be used as the prompt in GHCi. Inside ⟨prompt⟩, the next sequences are replaced:

• %s by the names of the modules currently in scope.
• %l by the line number (as referenced in compiler messages) of the current prompt.
• %d by the date in “Weekday Month Date” format (e.g., “Tue May 26”) .
• %t by the current time in 24-hour HH:MM:SS format.
• %T by the current time in 12-hour HH:MM:SS format.
• %@ by the current time in 12-hour am/pm format.
• %A by the current time in 24-hour HH:MM format.
• %u by the username of the current user.
• %w by the current working directory.
• %o by the operating system.
• %a by the machine architecture.
• %N by the compiler name.
• %V by the compiler version.
• %call(cmd [args]) by the result of calling cmd args.
• %% by %.

If ⟨prompt⟩ starts with " then it is parsed as a Haskell String; otherwise it is treated as a literal string.

:set prompt-cont ⟨prompt⟩

Sets the string to be used as the continuation prompt (used when using the :{ command) in GHCi.

:set prompt-function <prompt-function>

Sets the function to be used for the prompt displaying in GHCi. The function should be of the type [String] -> Int -> IO String. This function is called each time the prompt is being made. The first argument stands for the names of the modules currently in scope(the name of the “topmost” module will begin with a *; see What’s really in scope at the prompt? for more information). The second arguments is the line number (as referenced in compiler messages) of the current prompt.

:set prompt-cont-function <prompt-function>

Sets the function to be used for the continuation prompt (used when using the :{ command) displaying in GHCi.

:set stop ⟨num⟩ ⟨cmd⟩

Set a command to be executed when a breakpoint is hit, or a new item in the history is selected. The most common use of :set stop is to display the source code at the current location, e.g. :set stop :list.

If a number is given before the command, then the commands are run when the specified breakpoint (only) is hit. This can be quite useful: for example, :set stop 1 :continue effectively disables breakpoint 1, by running :continue whenever it is hit (although GHCi will still emit a message to say the breakpoint was hit). What’s more, with cunning use of :def and :cmd you can use :set stop to implement conditional breakpoints:

*Main> :def cond \expr -> return (":cmd if (" ++ expr ++ ") then return \"\" else return \":continue\"")
*Main> :set stop 0 :cond (x < 3)

Ignoring breakpoints for a specified number of iterations is also possible using similar techniques.

:seti [⟨option⟩ ...]

Like :set, but options set with :seti affect only expressions and commands typed at the prompt, and not modules loaded with :load (in contrast, options set with :set apply everywhere). See Setting options for interactive evaluation only.

Without any arguments, displays the current set of options that are applied to expressions and commands typed at the prompt.

:show bindings

Show the bindings made at the prompt and their types.

:show breaks

List the active breakpoints.

:show context

List the active evaluations that are stopped at breakpoints.

:show imports

Show the imports that are currently in force, as created by import and :module commands.

:show modules

Show the list of modules currently loaded.

:show packages

Show the currently active package flags, as well as the list of packages currently loaded.

:show paths

Show the current working directory (as set via :cd command), as well as the list of directories searched for source files (as set by the -i option).

:show language

Show the currently active language flags for source files.

:showi language

Show the currently active language flags for expressions typed at the prompt (see also :seti).

:show [args|prog|prompt|editor|stop]

Displays the specified setting (see :set).

:sprint ⟨expr⟩

Prints a value without forcing its evaluation. :sprint is similar to :print, with the difference that unevaluated subterms are not bound to new variables, they are simply denoted by _.

:step [⟨expr⟩]

Enable all breakpoints and begin evaluating an expression in single-stepping mode. In this mode evaluation will be stopped after every reduction, allowing local variables to be inspected. If ⟨expr⟩ is not given, evaluation will resume at the last breakpoint. See Single-stepping.

:steplocal

Enable only breakpoints in the current top-level binding and resume evaluation at the last breakpoint.

:stepmodule

Enable only breakpoints in the current module and resume evaluation at the last breakpoint.

:trace ⟨expr⟩

Evaluates the given expression (or from the last breakpoint if no expression is given), and additionally logs the evaluation steps for later inspection using :history. See Tracing and history.

:type ⟨expression⟩

Infers and prints the type of ⟨expression⟩, including explicit forall quantifiers for polymorphic types. The type reported is the type that would be inferred for a variable assigned to the expression, but without the monomorphism restriction applied.

*X> :type length
length :: Foldable t => t a -> Int

:type +v ⟨expression⟩

Infers and prints the type of ⟨expression⟩, but without fiddling with type variables or class constraints. This is useful when you are using -XTypeApplications and care about the distinction between specified type variables (available for type application) and inferred type variables (not available). This mode sometimes prints constraints (such as Show Int) that could readily be solved, but solving these constraints may affect the type variables, so GHC refrains.

*X> :set -fprint-explicit-foralls
*X> :type +v length
length :: forall (t :: * -> *). Foldable t => forall a. t a -> Int

:type +d ⟨expression⟩

Infers and prints the type of ⟨expression⟩, defaulting type variables if possible. In this mode, if the inferred type is constrained by any interactive class (Num, Show, Eq, Ord, Foldable, or Traversable), the constrained type variable(s) are defaulted according to the rules described under -XExtendedDefaultRules. This mode is quite useful when the inferred type is quite general (such as for foldr) and it may be helpful to see a more concrete instantiation.

*X> :type +d length
length :: [a] -> Int

:type-at ⟨module⟩ ⟨line⟩ ⟨col⟩ ⟨end-line⟩ ⟨end-col⟩ [⟨name⟩]

Reports the inferred type at the given span/position in the module, e.g.:

*X> :type-at X.hs 6 6 6 7 f
Int -> Int

This command is useful when integrating GHCi with text editors and IDEs for providing a show-type-under-point facility.

The last string parameter is useful for when the span is out of date, i.e. the file changed and the code has moved. In which case :type-at falls back to a general :type like lookup.

The :type-at command requires :set +c to be set.

:undef ⟨name⟩

Undefines the user-defined command ⟨name⟩ (see :def above).

:unset ⟨option⟩

Unsets certain options. See The :set and :seti commands for a list of available options.

:uses ⟨module⟩ ⟨line⟩ ⟨col⟩ ⟨end-line⟩ ⟨end-col⟩ [⟨name⟩]

Reports all module-local uses of the thing at the given position in the module, e.g.:

:uses GhciFind.hs 53 66 53 70 name
GhciFind.hs:(46,25)-(46,29)
GhciFind.hs:(47,37)-(47,41)
GhciFind.hs:(53,66)-(53,70)
GhciFind.hs:(57,62)-(57,66)

This command is useful for highlighting and navigating all uses of an identifier in editors and IDEs.

The :uses command requires :set +c to be set.

:! ⟨command⟩

Executes the shell command ⟨command⟩.

4.8. The :set and :seti commands

The :set command sets two types of options: GHCi options, which begin with “+”, and “command-line” options, which begin with “-”.

注解

At the moment, the :set command doesn’t support any kind of quoting in its arguments: quotes will not be removed and cannot be used to group words together. For example, :set -DFOO='BAR BAZ' will not do what you expect.

4.8.1. GHCi options

GHCi options may be set using :set and unset using :unset.

The available GHCi options are:

:set +c

Collect type and location information after loading modules. The commands :all-types, :loc-at, :type-at, and :uses require +c to be active.

:set +m

Enable parsing of multiline commands. A multiline command is prompted for when the current input line contains open layout contexts (see Multiline input).

:set +r

Normally, any evaluation of top-level expressions (otherwise known as CAFs or Constant Applicative Forms) in loaded modules is retained between evaluations. Turning on +r causes all evaluation of top-level expressions to be discarded after each evaluation (they are still retained during a single evaluation).

This option may help if the evaluated top-level expressions are consuming large amounts of space, or if you need repeatable performance measurements.

:set +s

Display some stats after evaluating each expression, including the elapsed time and number of bytes allocated. NOTE: the allocation figure is only accurate to the size of the storage manager’s allocation area, because it is calculated at every GC. Hence, you might see values of zero if no GC has occurred.

:set +t

Display the type of each variable bound after a statement is entered at the prompt. If the statement is a single expression, then the only variable binding will be for the variable it.

4.8.2. Setting GHC command-line options in GHCi

Normal GHC command-line options may also be set using :set. For example, to turn on -Wmissing-signatures, you would say:

Prelude> :set -Wmissing-signatures

Any GHC command-line option that is designated as dynamic (see the table in Flag reference), may be set using :set. To unset an option, you can set the reverse option:

Prelude> :set -Wno-incomplete-patterns -XNoMultiParamTypeClasses

Flag reference lists the reverse for each option where applicable.

Certain static options (-package, -I, -i, and -l in particular) will also work, but some may not take effect until the next reload.

4.8.3. Setting options for interactive evaluation only

GHCi actually maintains two sets of options:

• The loading options apply when loading modules
• The interactive options apply when evaluating expressions and commands typed at the GHCi prompt.

The :set command modifies both, but there is also a :seti command (for “set interactive”) that affects only the interactive options set.

It is often useful to change the interactive options, without having that option apply to loaded modules too. For example

:seti -XMonoLocalBinds

It would be undesirable if -XMonoLocalBinds were to apply to loaded modules too: that might cause a compilation error, but more commonly it will cause extra recompilation, because GHC will think that it needs to recompile the module because the flags have changed.

If you are setting language options in your .ghci file, it is good practice to use :seti rather than :set, unless you really do want them to apply to all modules you load in GHCi.

The two sets of options can be inspected using the :set and :seti commands respectively, with no arguments. For example, in a clean GHCi session we might see something like this:

Prelude> :seti
base language is: Haskell2010
with the following modifiers:
  -XNoMonomorphismRestriction
  -XNoDatatypeContexts
  -XNondecreasingIndentation
  -XExtendedDefaultRules
GHCi-specific dynamic flag settings:
other dynamic, non-language, flag settings:
  -fimplicit-import-qualified
warning settings:

The two sets of options are initialised as follows. First, both sets of options are initialised as described in The .ghci and .haskeline files. Then the interactive options are modified as follows:

• The option -XExtendedDefaultRules is enabled, in order to apply special defaulting rules to expressions typed at the prompt (see Type defaulting in GHCi).
• The Monomorphism Restriction is disabled (see Switching off the dreaded Monomorphism Restriction).

4.9. The .ghci and .haskeline files

4.9.1. The .ghci files

When it starts, unless the -ignore-dot-ghci flag is given, GHCi reads and executes commands from the following files, in this order, if they exist:

1. ./.ghci
2. appdata/ghc/ghci.conf, where ⟨appdata⟩ depends on your system, but is usually something like C:/Documents and Settings/user/Application Data
3. On Unix: $HOME/.ghc/ghci.conf
4. $HOME/.ghci

The ghci.conf file is most useful for turning on favourite options (e.g. :set +s), and defining useful macros.

注解

When setting language options in this file it is usually desirable to use :seti rather than :set (see Setting options for interactive evaluation only).

Placing a .ghci file in a directory with a Haskell project is a useful way to set certain project-wide options so you don’t have to type them every time you start GHCi: eg. if your project uses multi-parameter type classes, scoped type variables, and CPP, and has source files in three subdirectories A, B and C, you might put the following lines in .ghci:

:set -XMultiParamTypeClasses -XScopedTypeVariables -cpp
:set -iA:B:C

(Note that strictly speaking the -i flag is a static one, but in fact it works to set it using :set like this. The changes won’t take effect until the next :load, though.)

Once you have a library of GHCi macros, you may want to source them from separate files, or you may want to source your .ghci file into your running GHCi session while debugging it

:def source readFile

With this macro defined in your .ghci file, you can use :source file to read GHCi commands from file. You can find (and contribute!-) other suggestions for .ghci files on this Haskell wiki page: GHC/GHCi

Additionally, any files specified with -ghci-script flags will be read after the standard files, allowing the use of custom .ghci files.

Two command-line options control whether the startup files files are read:

-ignore-dot-ghci

Don’t read either ./.ghci or the other startup files when starting up.

-ghci-script

Read a specific file after the usual startup files. Maybe be specified repeatedly for multiple inputs.

When defining GHCi macros, there is some important behavior you should be aware of when names may conflict with built-in commands, especially regarding tab completion.

For example, consider if you had a macro named :time and in the shell, typed :t 3 — what should happen? The current algorithm we use for completing commands is:

1. First, look up an exact match on the name from the defined macros.
2. Look for the exact match on the name in the built-in command list.
3. Do a prefix lookup on the list of built-in commands - if a built-in command matches, but a macro is defined with the same name as the built-in defined, pick the macro.
4. Do a prefix lookup on the list of built-in commands.
5. Do a prefix lookup on the list of defined macros.

Here are some examples:

1. You have a macro :time and enter :t 3

   You get :type 3

2. You have a macro :type and enter :t 3

   You get :type 3 with your defined macro, not the builtin.

3. You have a macro :time and a macro :type, and enter :t 3

   You get :type 3 with your defined macro.

4.9.2. The .haskeline file

GHCi uses Haskeline under the hood. You can configure it to, among other things, prune duplicates from GHCi history. See: Haskeline user preferences.

4.10. Compiling to object code inside GHCi

By default, GHCi compiles Haskell source code into byte-code that is interpreted by the runtime system. GHCi can also compile Haskell code to object code: to turn on this feature, use the -fobject-code flag either on the command line or with :set (the option -fbyte-code restores byte-code compilation again). Compiling to object code takes longer, but typically the code will execute 10-20 times faster than byte-code.

Compiling to object code inside GHCi is particularly useful if you are developing a compiled application, because the :reload command typically runs much faster than restarting GHC with --make from the command-line, because all the interface files are already cached in memory.

There are disadvantages to compiling to object-code: you can’t set breakpoints in object-code modules, for example. Only the exports of an object-code module will be visible in GHCi, rather than all top-level bindings as in interpreted modules.

4.11. Running the interpreter in a separate process

Normally GHCi runs the interpreted code in the same process as GHC itself, on top of the same RTS and sharing the same heap. However, if the flag -fexternal-interpreter is given, then GHC will spawn a separate process for running interpreted code, and communicate with it using messages over a pipe.

-fexternal-interpreter

Since:8.0.1

Run interpreted code (for GHCi, Template Haskell, Quasi-quoting, or Annotations) in a separate process. The interpreter will run in profiling mode if -prof is in effect, and in dynamically-linked mode if -dynamic is in effect.

There are a couple of caveats that will hopefully be removed in the future: this option is currently not implemented on Windows (it is a no-op), and the external interpreter does not support the GHCi debugger, so breakpoints and single-stepping don’t work with -fexternal-interpreter.

See also the -pgmi (Replacing the program for one or more phases) and -opti (Forcing options to a particular phase) flags.

Why might we want to do this? The main reason is that the RTS running the interpreted code can be a different flavour (profiling or dynamically-linked) from GHC itself. So for example:

• We can use the profiler to collect stack traces when using GHCi (see Stack Traces in GHCi).
• When compiling Template Haskell code with -prof we don’t need to compile the modules without -prof first (see Using Template Haskell with Profiling) because we can run the profiled object code in the interpreter.

This feature is experimental in GHC 8.0.x, but it may become the default in future releases.

4.12. FAQ and Things To Watch Out For

The interpreter can’t load modules with foreign export declarations!

Unfortunately not. We haven’t implemented it yet. Please compile any offending modules by hand before loading them into GHCi.

-O doesn’t work with GHCi!

For technical reasons, the bytecode compiler doesn’t interact well with one of the optimisation passes, so we have disabled optimisation when using the interpreter. This isn’t a great loss: you’ll get a much bigger win by compiling the bits of your code that need to go fast, rather than interpreting them with optimisation turned on.

Unboxed tuples don’t work with GHCi

That’s right. You can always compile a module that uses unboxed tuples and load it into GHCi, however. (Incidentally the previous point, namely that -O is incompatible with GHCi, is because the bytecode compiler can’t deal with unboxed tuples).

Concurrent threads don’t carry on running when GHCi is waiting for input.

This should work, as long as your GHCi was built with the -threaded switch, which is the default. Consult whoever supplied your GHCi installation.

After using getContents, I can’t use stdin, until I do :load or :reload

This is the defined behaviour of getContents: it puts the stdin Handle in a state known as semi-closed, wherein any further I/O operations on it are forbidden. Because I/O state is retained between computations, the semi-closed state persists until the next :load or :reload command.

You can make stdin reset itself after every evaluation by giving GHCi the command :set +r. This works because stdin is just a top-level expression that can be reverted to its unevaluated state in the same way as any other top-level expression (CAF).

I can’t use Control-C to interrupt computations in GHCi on Windows.

See Running GHCi on Windows.

The default buffering mode is different in GHCi to GHC.

In GHC, the stdout handle is line-buffered by default. However, in GHCi we turn off the buffering on stdout, because this is normally what you want in an interpreter: output appears as it is generated.

If you want line-buffered behaviour, as in GHC, you can start your program thus:

main = do { hSetBuffering stdout LineBuffering; ... }

[5]	Note that packages only contain compiled code, so debugging a package requires finding its source and loading that directly.

[6]	We originally provided bindings for all variables in scope, rather than just the free variables of the expression, but found that this affected performance considerably, hence the current restriction to just the free variables.