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.

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