haskell - Monomorphism restriction in pattern bindings -

haskell - Monomorphism restriction in pattern bindings -

{-# language nomonomorphismrestriction #-} module seek f :: io (a -> io string) f = homecoming $ const getline main :: io () main = g <- f :: io (a -> io string) g "string" >>= print g 5 >>= print

even nomonomorphismrestriction flag , explicit type signature, module fails compile couldn't match expected type ‘[char]’ actual type ‘int’, despite g beingness polymorphic.

this not monomorphism restriction means. monomorphism restriction says if definition has no type signature , has left-hand side no parameters, specialized monomorphic type (or rather monomorphic plenty rid of class constraints). have given type signatures doesn't apply.

the problem here have given wrong type f.

f :: io (a -> io string)

actually means

f :: forall a. io (a -> io string)

that is, first pick type a, can bind monomorphic function of type a -> io string a. there no problem program, example:

main = g <- f g "string" >>= print g' <- f g' 5 >>= print

but usage illustration requires type:

f :: io (forall a. -> io string)

that is, want bind first , pick type later, i.e. utilize function @ multiple types. called "impredicative type" , unfortunately ghc has not supported them quite while, far know.

the way solve problem create newtype wrapper explicitly quantifies inner polymorphic type:

newtype r = r { getr :: forall a. -> io string } f :: io r f = homecoming $ r (const getline) main :: io () main = g <- f getr g "string" >>= print getr g 5 >>= print

haskell