Am Sonntag 29 M?rz 2009 18:14:00 schrieb Zachary Turner:

> Given the following two function definitions:

>

> let func4 l = map (\y -> y+2) (filter (\z -> z `elem` [1..10]) (5:l))

> let func4_pf = map (+2) . filter (`elem` [1..10]) . (5 :)

>

> which are equivalent, why does the first one have a type of

>

> func4 :: (Num a, Enum a) => [a] -> [a]

>

> while the second one has a type of

>

> func4_pf :: [Integer] -> [Integer]

>

> Shouldn't the types be the same?

It's the dreaded monomorphism restriction.

Since the second is defined without arguments, it looks like a constant

and to avoid recomputation, it must have a monomorphic type, except a

type signature is given. That monomorphic type is defaulted to [Integer].

If you start ghci with the flag -fno-monomorphism-restriction, it will infer

the same general type for the second function, too.