2.4. Interactive evaluation at the prompt

When you type an expression at the prompt, GHCi immediately evaluates and prints the result:

Prelude> reverse "hello"
Prelude> 5+5

2.4.1. I/O actions at the prompt

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"
Prelude> putStrLn "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"
Prelude> do { putStrLn "hello"; return "yes" }

2.4.2. Using do-notation at the prompt

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

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

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!

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

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]

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]

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

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]

2.4.3. 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:


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 )

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.

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"
Prelude IO>

(Note: you can use import M as an alternative to :module +M, and :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 ”. :module and :load

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”. Qualified names

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. The :main and :run commands

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

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","bar baz"]
Prelude> :run bar ["foo", "bar baz"]
["foo","bar baz"]

2.4.4. The it variable

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
Prelude> it * 2

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' 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.

2.4.5. Type defaulting in GHCi

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 => a 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 98 Report (Revised)) 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

  1. The type variable a appears in no other constraints

  2. All the classes Ci are standard.

  3. 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.