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

[Ketil Malde]

Ting Lei <tinlyx at hotmail.com> 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.

-k
-- 
If I haven't seen further, it is by standing in the footprints of giants