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

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.

See also