When you type an expression at the prompt, GHCi immediately evaluates and prints the result:
Prelude> reverse "hello" "olleh" Prelude> 5+5 10
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"
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 Section 2.9, “The .ghci
file”) 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]
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
*
indicates
that it is the full top-level scope of
module
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
”.
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
”.
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
.
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"]
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.
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
The type variable a
appears in no
other constraints
All the classes Ci
are standard.
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.