Type inference

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Type inference is a feature of the type system which means that concrete types are deduced by the type system whereever it is obvious.

If you add an integer variable

x

to a numeric literal

2

, then the type system concludes that

2

, which in principle can represent 2 for every number type, must also be an integer, since

+

supports only addition of numbers of the same type.

(This restriction is a good thing, as we explain for the idea of a Generic number type.)

Another example: There are the following standard functions:

map :: (a -> b) -> [a] -> [b]
Char.ord :: (Char -> Int)

For the expression

map ord

the type checker finds out that the type variable

a

must be bound to the type

Char

and

b

must be bound to

Int

and thus it concludes

map ord :: [Char] -> [Int]

You can play with the type inference mechanism in Hugs and GHCi. See Determining the type of an expression.

@@ Line 19: / Line 19: @@
 == See also ==
-The type inference mechanism is very similar to [http://en.wikipedia.org/Unification unification] in [http://en.wikipedia.org/PROLOG PROLOG].
+The type inference mechanism is very similar to [http://en.wikipedia.org/wiki/Unification unification] in [http://en.wikipedia.org/wiki/PROLOG PROLOG].
 * Haskell-Cafe: [http://www.haskell.org/pipermail/haskell-cafe/2007-September/031255.html Learn Prolog ...]
 [[Category:Glossary]]