View source for Type inference

'''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 <hask>x</hask> to a numeric literal <hask>2</hask>,
then the type system concludes that <hask>2</hask>, which in principle can represent 2 for every number type, must also be an integer,
since <hask>+</hask> 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:
<haskell>
map :: (a -> b) -> [a] -> [b]
Char.ord :: (Char -> Int)
</haskell>
For the expression <hask>map ord</hask> the type checker finds out that the type variable <hask>a</hask> must be bound to the type <hask>Char</hask> and <hask>b</hask> must be bound to <hask>Int</hask> and thus it concludes
<haskell>
map ord :: [Char] -> [Int]
</haskell>

You can play with the type inference mechanism in [[Hugs]] and [[GHC]]i. See [[Determining the type of an expression]].

== See also ==

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]]