# Using GHCi

GHCi The ‘i’ stands for “Interactive” is GHC's interactive environment, in which Haskell expressions can be interactively evaluated and programs can be interpreted. If you're familiar with Hugs
, then you'll be right at home with GHCi. However, GHCi also has support for interactively loading compiled code, as well as supporting all except `foreign export`, at the moment the language extensions that GHC provides.
. GHCi also includes an interactive debugger (see ).

## Introduction to GHCi

Let's start with an example GHCi session. You can fire up GHCi with the command `ghci`:

    $ ghci
    GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
    Loading package ghc-prim ... linking ... done.
    Loading package integer-gmp ... linking ... done.
    Loading package base ... linking ... done.
    Loading package ffi-1.0 ... linking ... done.
    Prelude>

There may be a short pause while GHCi loads the prelude and standard libraries, after which the prompt is shown. As the banner says, you can type `:?` to see the list of commands available, and a half line description of each of them.

We'll explain most of these commands as we go along. For Hugs users: many things work the same as in Hugs, so you should be able to get going straight away.

Haskell expressions can be typed at the prompt:

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

GHCi interprets the whole line as an expression to evaluate. The expression may not span several lines - as soon as you press enter, GHCi will attempt to evaluate it.

## Loading source files

Suppose we have the following Haskell source code, which we place in a file `Main.hs`:

    main = print (fac 20)

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

You can save `Main.hs` anywhere you like, but if you save it somewhere other than the current directory If you started up GHCi from the command line then GHCi's current directory is the same as the current directory of the shell from which it was started. If you started GHCi from the “Start” menu in Windows, then the current directory is probably something like `C:\Documents and Settings\user name`. then we will need to change to the right directory in GHCi:

    Prelude> :cd dir

where dir is the directory (or folder) in which you saved `Main.hs`.

To load a Haskell source file into GHCi, use the `:load` command:

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

GHCi has loaded the `Main` module, and the prompt has changed to “`*Main>`” to indicate that the current context for expressions typed at the prompt is the `Main` module we just loaded (we'll explain what the `*` means later in ). So we can now type expressions involving the functions from `Main.hs`:

    *Main> fac 17
    355687428096000

Loading a multi-module program is just as straightforward; just give the name of the “topmost” module to the `:load` command (hint: `:load` can be abbreviated to `:l`). The topmost module will normally be `Main`, but it doesn't have to be. GHCi will discover which modules are required, directly or indirectly, by the topmost module, and load them all in dependency order.

### Modules vs. filenames

Question: How does GHC find the filename which contains module M? Answer: it looks for the file `M.hs`, or `M.lhs`. This means that for most modules, the module name must match the filename. If it doesn't, GHCi won't be able to find it.

There is one exception to this general rule: when you load a program with `:load`, or specify it when you invoke `ghci`, you can give a filename rather than a module name. This filename is loaded if it exists, and it may contain any module you like. This is particularly convenient if you have several `Main` modules in the same directory and you can't call them all `Main.hs`.

The search path for finding source files is specified with the `-i` option on the GHCi command line, like so:

    ghci -idir1:...:dirn

or it can be set using the `:set` command from within GHCi (see ) Note that in GHCi, and `––make` mode, the `-i` option is used to specify the search path for source files, whereas in standard batch-compilation mode the `-i` option is used to specify the search path for interface files, see .

One consequence of the way that GHCi follows dependencies to find modules to load is that every module must have a source file. The only exception to the rule is modules that come from a package, including the `Prelude` and standard libraries such as `IO` and `Complex`. If you attempt to load a module for which GHCi can't find a source file, even if there are object and interface files for the module, you'll get an error message.

### Making changes and recompilation

If you make some changes to the source code and want GHCi to recompile the program, give the `:reload` command. The program will be recompiled as necessary, with GHCi doing its best to avoid actually recompiling modules if their external dependencies haven't changed. This is the same mechanism we use to avoid re-compiling modules in the batch compilation setting (see ).

## Loading compiled code

When you load a Haskell source module into GHCi, it is normally converted to byte-code and run using the interpreter. However, interpreted code can also run alongside compiled code in GHCi; indeed, normally when GHCi starts, it loads up a compiled copy of the `base` package, which contains the `Prelude`.

Why should we want to run compiled code? Well, compiled code is roughly 10x faster than interpreted code, but takes about 2x longer to produce (perhaps longer if optimisation is on). So it pays to compile the parts of a program that aren't changing very often, and use the interpreter for the code being actively developed.

When loading up source modules with `:load`, GHCi normally looks for any corresponding compiled object files, and will use one in preference to interpreting the source if possible. For example, suppose we have a 4-module program consisting of modules A, B, C, and D. Modules B and C both import D only, and A imports both B & C:

          A
         / \
        B   C
         \ /
          D

We can compile D, then load the whole program, like this:

    Prelude> :! ghc -c 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.
    *Main>

In the messages from the compiler, we see that there is no line for `D`. This is because it isn't necessary to compile `D`, because the source and everything it depends on is unchanged since the last compilation.

At any time you can use the command `:show modules` to get a list of the modules currently loaded into GHCi:

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

If we now modify the source of D (or pretend to: using the Unix command `touch` on the source file is handy for this), the compiler will no longer be able to use the object file, because it might be out of date:

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

Note that module D was compiled, but in this instance because its source hadn't really changed, its interface remained the same, and the recompilation checker determined that A, B and C didn't need to be recompiled.

So let's try compiling one of the other modules:

    *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.

We didn't get the compiled version of C! What happened? Well, in GHCi a compiled module may only depend on other compiled modules, and in this case C depends on D, which doesn't have an object file, so GHCi also rejected C's object file. Ok, so let's also compile D:

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

Nothing happened! Here's another lesson: newly compiled modules aren't picked up by `:reload`, only `:load`:

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

The automatic loading of object files can sometimes lead to confusion, because non-exported top-level definitions of a module are only available for use in expressions at the prompt when the module is interpreted (see ). For this reason, you might sometimes want to force GHCi to load a module using the interpreter. This can be done by prefixing a `*` to the module name or filename when using `:load`, for example

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

When the `*` is used, GHCi ignores any pre-compiled object code and interprets the module. If you have already loaded a number of modules as object code and decide that you wanted to interpret one of them, instead of re-loading the whole set you can use `:add *M` to specify that you want `M` to be interpreted (note that this might cause other modules to be interpreted too, because compiled modules cannot depend on interpreted ones).

To always compile everything to object code and never use the interpreter, use the `-fobject-code` option (see ).

HINT: since GHCi will only use a compiled object file if it can be sure that the compiled version is up-to-date, a good technique when working on a large program is to occasionally run `ghc ––make` to compile the whole project (say before you go for lunch :-), then continue working in the interpreter. As you modify code, the changed modules will be interpreted, but the rest of the project will remain compiled.

## 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

### I/O actions at the prompt

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

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

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"

### 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.

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.

Hint: you can also use `let`-statements to define functions at the prompt:

    Prelude> let 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 braces and semicolons instead of layout:

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

Such multiline commands can be used with any GHCi command, and the lines between `:{` and `:}` are simply merged into a single line for interpretation. That implies that each such group must form a single valid command when merged, and that no layout rule is used. The main purpose of multiline commands is not to replace module loading but to make definitions in .ghci-files (see ) 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.

WARNING: 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.

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

    Prelude> :show bindings
    x :: Int
    Prelude>

HINT: 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]

### 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. 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. 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 `*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.

We're 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.

NOTE: 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`.

The scope is manipulated using the `:module` command. For example, if the current scope is `Prelude`, then we can bring into scope the exports from the module `IO` like so:

    Prelude> :module +IO
    Prelude IO> hPutStrLn stdout "hello\n"
    hello
    Prelude IO>

(Note: you can use conventional haskell `import` syntax as well, but this does not support `*` forms). `:module` can also be shortened to `:m`. The full syntax of the `:module` command is:

    :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`, GHCi will assume that you really wanted the `Prelude` and add it in for you (if you don't want the `Prelude`, then ask to remove it with `:m -Prelude`).

The scope is automatically set after a `:load` command, to 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).

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.

Hint: 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` ”.

#### `:module` and `:load`

It might seem that `:module` and `:load` do similar things: you can use both to bring a module into scope. However, there is a clear 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`.

- The set of modules that are currently in scope at the prompt. This set is modified by `:module`, and it is also set automatically after `:load`, `:add`, and `:reload`.

You cannot add a module to the scope if it is not loaded. This is why trying to use `:module` to load a new module results in the message “`module M is not loaded`”.

#### 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 flag `-fno-implicit-import-qualified`
.

#### 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> let 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> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
    Prelude> let 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"]

### 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.

### Type defaulting in GHCi

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 numeric, or is `Show`, `Eq`, or `Ord`.

- The unit type `()` is 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.

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

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 ).

### 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 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. 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 ). 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.

#### 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 `-fwarn-tabs` in ).

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.

#### 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 *`.

### 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. `:stepover` works 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>

### 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>

### 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.

### 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 (), 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. GHCi remembers the last 50 steps in the history (perhaps in the future we'll make this configurable).

### 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 `+RTS -xc` (see ).

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.

### 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 (), 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.

### 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 ) are only available at the scope of a breakpoint if there is an explicit type signature.

## 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 ). 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 ) also make sense in interactive mode. The ones that don't make sense are mostly obvious.

### Packages

Most packages (see ) 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`.

### 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 .) 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 `-Lpath` 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 ).

## 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.

`:back`

  Travel back one step in the history. See . See also: `:trace`, `:history`, `:forward`.

`:break [identifier | [module] line [column]]`

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

`:browse``!` `*`module ...

  Displays the identifiers defined 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.

  If the `*` symbol is placed before the module name, then all the identifiers in scope in module are shown; otherwise the list is limited to the exports of module. 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. If the `!` symbol is appended to the command, data constructors and class methods will be listed individually, otherwise, they will only be listed in the context of their data type or class declaration. The `!`-form also annotates the listing with comments giving possible imports for each group of entries.

      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, in the scope of `Prelude`, the first group of items from `Data.Maybe` have not been imported (but are available in fully qualified form in the GHCi session - see ), whereas the second group of items have been imported 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`.

  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`.

`:continue`

  Continue the current evaluation, when stopped at a breakpoint.

`:ctags` filename `:etags` 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 & 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. 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`

  Move forward in the history. See . 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 .

`: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.

`: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`.

`:load` `*`module ...

  Recursively loads the specified modules, 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.

  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.

`: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> let 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> let foo = putStrLn "foo" >> System.Environment.getArgs >>= print
      Prelude> let 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 ... *modn`
`import mod`

  Sets or modifies the current context for statements typed at the prompt. The form `import mod` is equivalent to `:module +mod`. See 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 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.

`:run`

  See `:main`.

`:set` option...

  Sets various options. See for a list of available options and 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 sequence `%s` is replaced by the names of the modules currently in scope, and `%%` is replaced by `%`. If prompt starts with " then it is parsed as a Haskell String; otherwise it is treated as a literal string.

`: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.

`: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 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 languages`

  Show the currently active language flags.

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

  Displays the specified setting (see `:set`).

`:sprint`

  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]`

  Single-step from the last breakpoint. With an expression argument, begins evaluation of the expression with a single-step.

`: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 .

`:type` expression

  Infers and prints the type of expression, including explicit forall quantifiers for polymorphic types. The monomorphism restriction is not applied to the expression during type inference.

`:undef` name

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

`:unset` option...

  Unsets certain options. See for a list of available options.

`:!` command...

  Executes the shell command command.

## The `:set` command

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

NOTE: 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.

### GHCi options

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

The available GHCi options are:

`+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.

`+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.

`+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`’.

### Setting GHC command-line options in GHCi

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

    Prelude> :set -fglasgow-exts

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

    Prelude> :set -fno-glasgow-exts

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.

## The `.ghci` file

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 (eg. `:set +s`), and defining useful macros. 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 everytime you start GHCi: eg. if your project uses GHC extensions and CPP, and has source files in three subdirectories A, B and C, you might put the following lines in `.ghci`:

    :set -fglasgow-exts -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

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.

`-read-dot-ghci`

  Read `./.ghci` and the other startup files (see above). This is normally the default, but the `-read-dot-ghci` option may be used to override a previous `-ignore-dot-ghci` option.

## 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.

## 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` again 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 .

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; ... }