3.4. Interactive evaluation at the prompt

When 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"
Prelude> putStrLn "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.

3.4.1. 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. 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"

(Note: :module can 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. 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.

3.4.2. 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.

Here's an example:

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.

Of course, you can also bind normal non-IO expressions using the let-statement:

Prelude> let x = 42
Prelude> print x

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!

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]

3.4.3. 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

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 <- 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.

3.4.4. 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. If the expression yields a set of type constraints that are all from standard classes (Num, Eq etc.), and at least one is either a numeric class or the Show, Eq, or Ord class, GHCi will try to use one of the default types, just as described in the Report.