3.6. GHCi commands

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

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.

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.

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")

Displays a list of the available commands.

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.

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.

Sets or modifies the current context for statements typed at the prompt. See Section 3.4.1 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.

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

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

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

Show the bindings made at the prompt and their types.

Show the list of modules currently load.

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.

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

Unsets certain options. See Section 3.7 for a list of available options.

Executes the shell command command.