3.6. 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.

:add module ...

Add module(s) to the current target set, and perform a reload.

: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 the * symbol is placed before the module name, then all the identifiers defined 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.

: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.

:def name expr

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.

: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.

:help , :?

Displays a list of the available commands.

: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.

: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.

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

Sets or modifies the current context for statements typed at the prompt. See Section 3.4.3, “What's really in scope at the prompt?” for more details.


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


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.

:set [option...]

Sets various options. See Section 3.7, “The :set command” for a list of available options. The :set command by itself shows which options are currently set.

: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 %.

:show bindings

Show the bindings made at the prompt and their types.

:show modules

Show the list of modules currently load.

:ctags [filename] :etags [filename]

Generates a “tags” file for Vi-style editors (:ctags) or Emacs-style editors (etags). If no filename is specified, the defaulit 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.

See also Section 10.1, “Ctags and Etags for Haskell: hasktags.

: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.

: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.

:undef name

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

:unset option...

Unsets certain options. See Section 3.7, “The :set command” for a list of available options.

:! command...

Executes the shell command command.