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Type inference

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Latest revision as of 07:35, 29 November 2007

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, 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
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.

[edit] See also

The type inference mechanism is very similar to unification in PROLOG.