Re: [Haskell-cafe] a code that cannot compile with or without NoMonomorphismRestriction

Ketil Malde wrote:

Ting Lei writes:

...

(f1, f2) = let commond_definitions = undefined in let f1 = id.show f2 x = (< x) in (f1, f2)

I think the type signatures should be:

f1 :: Show a => a -> String

and

f2 :: Ord b => b -> b -> Bool

When I define these separately, this works:

f1 :: Show a => a -> String f1 = id . show

f2 :: Ord b => b -> b -> Bool f2 = flip (<)

But when I define them as a pair

f1 :: Show a => a -> String f2 :: Ord b => b -> b -> Bool (f1,f2) = (id . show, flip (<))

I get an error message:

Line 9: 1 error(s), 0 warning(s)

Couldn't match expected type `forall a. Show a => a -> String' with actual type `a -> String' When checking that `f1' has the specified type `forall a1. Show a1 => a1 -> String'

Defining the pair at once works:

p :: (Show a, Ord b) => (a -> String, b -> b -> Bool) p = (id . show, flip (<))

I guess that didn't help a lot, somebody with deeper GHC-fu than me will have to step in.

The problem is that f1 and f2 are polymorphic functions. To put polymorphic functions in a pair, you need *impredicative polymorphism*. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com