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.
Abandons the current evaluation (only available when stopped at a breakpoint).
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.
Travel back one step in the history. See Section 2.5.5, “Tracing and history”. See also:
Set a breakpoint on the specified function or line and column. See Section 126.96.36.199, “Setting breakpoints”.
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 (Section 2.4.5, “What's really in scope at the prompt?”).
There are two variants of the browse command:
* symbol is placed before
the module name, then all the
identifiers in scope in
(rather that just its exports) are shown.
*-form is only available for modules
which are interpreted; for compiled modules (including
modules from packages) only the non-
: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
they are listed individually.
!-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
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 Section 2.4.5, “What's really in scope at the prompt?”), whereas the second group of items are in scope
Prelude) and are therefore available either
unqualified, or with a
Changes the current working directory to
˜’ symbol at the
dir will be replaced
by the contents of the environment variable
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.
expr as a computation of
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
Continue the current evaluation, when stopped at a breakpoint.
Generates a “tags” file for Vi-style editors
Emacs-style editors (
no filename is specified, the default
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 is used to define new
commands, or macros, in GHCi. The command
expr defines a new GHCi command
implemented by the Haskell expression
expr, which must have type
String -> IO String. When
: is typed at the
prompt, GHCi will run the expression
(, take the
String, and feed it back into
GHCi as a new sequence of commands. Separate commands in
the result must be separated by
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
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
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.
: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 * |
Delete one or more breakpoints by number (use
breaks to see the number of each breakpoint). The
* form deletes all the breakpoints.
Opens an editor to edit the file
file, or the most recently loaded
file is omitted. The
editor to invoke is taken from the
environment variable, or a default editor on your system if
EDITOR is not set. You can change the
Prints the value of
the same way as
: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).
Move forward in the history. See Section 2.5.5, “Tracing and history”. See also:
Displays a list of the available commands.
Repeat the previous command.
Display the history of evaluation steps. With a number,
displays that many steps (default: 20). For use with
:trace; see Section 2.5.5, “Tracing and history”.
Displays information about the given name(s). For
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
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
and (b) all the other things mentioned in the instance
are in scope (either qualified or otherwise) as a result of
Infers and prints the kind of
type. The latter can be an arbitrary
type expression, including a partial application of a type constructor,
Either Int. If you specify the
!", GHC will in addition normalise the type
by expanding out type synonyms and evaluating type-function applications,
and display the normalised result.
Recursively loads the specified
modules, and all the modules they
depend on. Here, each
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
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.
:load command, the current
context is set to:
module, if it was loaded
the most recently successfully loaded module, if
any other modules were loaded as a result of the current
When a program is compiled and executed, it can use the
getArgs function to access the
However, we cannot simply pass the arguments to the
main function while we are testing in ghci,
main function doesn't take its
Instead, we can use the
This runs whatever
main is in scope, with
any arguments being treated the same as command-line arguments,
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
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 [+|-] [*],
Sets or modifies the current context for statements
typed at the prompt. The form
is equivalent to
See Section 2.4.5, “What's really in scope at the prompt?” for
Prints a value without forcing its evaluation.
to each thunk. See Section 2.5.1, “Breakpoints and inspecting variables” for more
information. See also the
which works like
Quits GHCi. You can also quit by typing 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.
Executes the lines of a file as a series of GHCi commands. This command
is compatible with multiline statements as set by
Sets various options. See Section 2.8, “The
:set command” for a list of
available options and Section 4.20.10, “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.
Sets the command used by
Sets the string to be used as the prompt in GHCi.
prompt, the sequence
%s is replaced by the names of the
modules currently in scope, and
starts with " then it is parsed as a Haskell String;
otherwise it is treated as a literal string.
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
: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 the bindings made at the prompt and their types.
List the active breakpoints.
List the active evaluations that are stopped at breakpoints.
Show the imports that are currently in force, as
Show the list of modules currently loaded.
Show the currently active package flags, as well as the list of packages currently loaded.
Show the currently active language flags.
Displays the specified setting (see
Prints a value without forcing its evaluation.
:sprint is similar to
Single-step from the last breakpoint. With an expression argument, begins evaluation of the expression with a single-step.
Evaluates the given expression (or from the last breakpoint if
no expression is given), and additionally logs the evaluation
steps for later inspection using
Section 2.5.5, “Tracing and history”.
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
Unsets certain options. See Section 2.8, “The
for a list of available options.
Executes the shell command