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
to a numeric literal
then the type system concludes that
, which in principle can represent 2 for every number type, must also be an integer,
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
the type checker finds out that the type variable
must be bound to the type
must be bound to
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.
The type inference mechanism is very similar to unification in PROLOG.