Type Inference

map ::	(a -> b, [a]) -> [b]
f ::	a -> b
h ::	a
t ::	[a]

"map must return [b]."

        map(f, []) =  [] 
        map(f, h:t) = f(h) : [map(f, t)

"That fits with the return value in the other clause."

"Everything else checks out okay."

• If you ask the compiler, it will say that the type is:

         map :: (a -> b, [a]) -> [b]

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Copyright © 1999,2008 Mark Dominus