Using GHCiGHCiinterpreterGHCiinteractiveGHCiGHCiThe ‘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 famililar with HugsHugs, then you'll be right at home with GHCi. However, GHCi
also has support for interactively loading compiled code, as well as
supporting allexcept foreign export, at the momentthe language extensions that GHC provides.FFIGHCi supportForeign Function InterfaceGHCi supportIntroduction to GHCiLet's start with an example GHCi session. You can fire up
GHCi with the command ghci:
$ ghci
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 5.00, For Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package std ... linking ... done.
Prelude>
There may be a short pause while GHCi loads the prelude and
standard libraries, after which the prompt is shown. If we follow
the instructions and type :? for help, we
get:
Commands available from the prompt:
<stmt> evaluate/run <stmt>
:cd <dir> change directory to <dir>
:def <cmd> <expr> define a macro :<cmd>
:help, :? display this list of commands
:load <filename> load a module (and it dependents)
:module <mod> set the context for expression evaluation to <mod>
:reload reload the current module set
:set <option> ... set options
:type <expr> show the type of <expr>
:unset <option> ... unset options
:quit exit GHCi
:!<command> run the shell command <command>
Options for `:set' and `:unset':
+r revert top-level expressions after each evaluation
+s print timing/memory stats after each evaluation
+t print type after evaluation
-<flag> most GHC command line flags can also be set here
(eg. -v2, -fglasgow-exts, etc.)
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:promptGHCi
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 filesSuppose we have the following Haskell source code, which we
place in a file Main.hs in the current
directory:
main = print (fac 20)
fac 0 = 1
fac n = n * fac (n-1)
To load a Haskell source file into GHCi, use the
:load command::load
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. filenamesmodulesand filenamesfilenamesof modulesQuestion: 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 option on the GHCi command line, like
so:ghci -idir1:...:dirnor it can be set using the :set command
from within GHCi (see )Note that in
GHCi, and mode, the
option is used to specify the search path for
source files, whereas in standard
batch-compilation mode the 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.One final note: if you load a module called Main, it must
contain a main function, just like in
GHC.Making changes and recompilation:reloadIf 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 codecompiled codein GHCiWhen 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 package std, which contains the Prelude
and standard libraries.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 files with :load,
GHCi 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
Skipping D ( D.hs, D.o )
Compiling C ( C.hs, interpreted )
Compiling B ( B.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
*Main>
In the messages from the compiler, we see that it skipped D,
and used the object file D.o. The message
Skippingmodule
indicates that compilation for module
isn't necessary, 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 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 )
Skipping C ( C.hs, interpreted )
Skipping B ( B.hs, interpreted )
Skipping A ( A.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 C ( C.hs, interpreted )
Compiling B ( B.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
Skipping D ( D.hs, D.o )
Skipping C ( C.hs, C.o )
Compiling B ( B.hs, interpreted )
Compiling A ( A.hs, interpreted )
Ok, modules loaded: A, B, C, D.
HINT: since GHCi will only use a compiled object file if it
can 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 new modules will be
interpreted, but the rest of the project will remain
compiled.Interactive evaluation at the promptWhen you type an expression at the prompt, GHCi immediately
evaluates and prints the result. But that's not the whole story:
if you type something of type IO a for some
a, then GHCi executes it
as an IO-computation, and doesn't attempt to print the
result:.
Prelude> "hello"
"hello"
Prelude> putStrLn "hello"
hello
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
No instance for `Show (a -> a)'
arising from use of `print'
in a `do' expression pattern binding: print it
The error message contains some clues as to the
transformation happening internally.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. For technical reasons, GHCi
can only support the *-form for modules which
are interpreted, so compiled modules and package modules can
only contribute their exports to the current scope.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: :module can be shortened to
:m). The full syntax of the
:module command is:
:module +|-*mod1 ... *modnUsing 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.Qualified namesTo 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.Using do-notation at the promptdo-notationin GHCistatementsin GHCiGHCi 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.Here's an example:
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.Of course, you can also bind normal non-IO expressions
using the let-statement:
Prelude> let x = 42
Prelude> print x
42
Prelude>
An 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>
Any exceptions raised during the evaluation or execution
of the statement are caught and printed by the GHCi command line
interface (see for more
information on GHC's Exception support).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:+t
Prelude> :set +t
Prelude> let (x:xs) = [1..]
x :: Integer
xs :: [Integer]
The it variableitWhenever 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
This is a result of the translation mentioned earlier,
namely that an expression e is
translated to
let it = e;
print it
before execution, resulting in a binding for
it.If the expression was 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
Prelude> print it
Wed Mar 14 12:23:13 GMT 2001
The corresponding translation for an IO-typed
e is
it <- eNote that it is shadowed by the new
value each time you evaluate a new expression, and the old value
of it is lost.Invoking GHCiinvokingGHCiGHCi 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; for example, GHCi
doesn't generate interface files, so options related to interface
file generation won't have any effect.Packagespackageswith GHCiGHCi can make use of all the packages that come with GHC,
For example, to start up GHCi with the text
package loaded:
$ ghci -package text
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 5.00, For Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package std ... linking ... done.
Loading package lang ... linking ... done.
Loading package text ... linking ... done.
Prelude>
Note that GHCi also loaded the lang
package even though we didn't ask for it: that's because the
text package makes use of one or more of the
modules in lang, and therefore has a
dependency on it.The following command works to load new packages into a
running GHCi:
Prelude> :set -package nameBut note that doing this will cause all currently loaded
modules to be unloaded, and you'll be dumped back into the
Prelude.Extra librarieslibrarieswith GHCiExtra libraries may be specified on the command line using
the normal -llib
option. 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 overriden 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.GHCi commandsGHCi commands all begin with
‘:’ and consist of a single command
name followed by zero or more parameters. The command name may be
abbreviated, as long as the abbreviation is not ambiguous. All of
the builtin commands, with the exception of
:unset and :undef, may be
abbreviated to a single letter.:addmodule ...:addAdd module(s) to the
current target set, and perform a
reload.:browse*module
...:browseDisplays 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.:cddir:cdChanges the current working directory to
dir. A
‘˜’ symbol at the
beginning of dir will be replaced
by the contents of the environment variable
HOME.:defnameexpr:defThe command :defnameexpr defines a new GHCi command
:name,
implemented by the Haskell expression
expr, which must have type
String -> IO String. When
:nameargs is typed at the
prompt, GHCi will run the expression
(nameargs), 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")
:help:help:?:?Displays a list of the available commands.:infoname
...:infoDisplays 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.:loadmodule ...:loadRecursively 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, orthe most recently successfully loaded module, if
any other modules were loaded as a result of the current
:load, orPrelude otherwise.:module +|-*mod1 ... *modn:moduleSets or modifies the current context for statements
typed at the prompt. See for
more details.:quit:quitQuits GHCi. You can also quit by typing a control-D
at the prompt.:reload:reloadAttempts 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.:setoption...:setSets various options. See
for a list of available options. The
:set command by itself shows which
options are currently set.:setargsarg ...:setSets the list of arguments which are returned when the
program calls System.getArgsgetArgs.:setprogprog:setSets the string to be returned when the program calls
System.getProgNamegetProgName.:show bindings:show bindingsShow the bindings made at the prompt and their
types.:show modules:show modulesShow the list of modules currently load.:typeexpression:typeInfers 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.:undefname:undefUndefines the user-defined command
name (see :def
above).:unsetoption...:unsetUnsets certain options. See
for a list of available options.:!command...:!shell commandsin GHCiExecutes the shell command
command.The :set command:setThe :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 optionsoptionsGHCiGHCi options may be set using :set and
unset using :unset.The available GHCi options are:+r+rCAFsin GHCiConstant Applicative FormCAFsNormally, 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+sDisplay 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+tDisplay 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 GHCiNormal GHC command-line options may also be set using
:set. For example, to turn on
, 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:dynamicoptions
Prelude> :set -fno-glasgow-exts
lists the reverse for each
option where applicable.Certain static options (,
, , and
in particular) will also work, but some may
not take effect until the next reload.staticoptionsThe .ghci file.ghcifilestartupfiles, GHCiWhen it starts, GHCi always reads and executes commands from
$HOME/.ghci, followed by
./.ghci.The .ghci in your home directory 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 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.)Two command-line options control whether the
.ghci files are read:Don't read either ./.ghci or
$HOME/.ghci when starting up.Read .ghci and
$HOME/.ghci. This is normally the
default, but the option may
be used to override a previous
option.FAQ and Things To Watch Out ForGHCi complains about main not being
in scope when I load a module.mainwith GHCiYou probably omitted the module
declaration at the top of the module, which causes the
module name to default to Main. In
Haskell, the Main module must define a
function called main. Admittedly this
doesn't make a great deal of sense for an interpreter, but
the rule was kept for compatibility with GHC.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 GHCiThat'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.No, they don't. This is because the Haskell binding
to the GNU readline library doesn't support reading from the
terminal in a non-blocking way, which is required to work
properly with GHC's concurrency model.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).