Untypeable Haskell 98

From HaskellWiki

Revision as of 02:45, 29 August 2006

Here we document code that looks like it should be valid Haskell98, but isn't typeable without extensions:

f :: a -> a
f x = g x
     where
--      g :: a -> a
        g y = bind x y
 
bind :: a -> a -> a
bind a b = a

The above Haskell code is Haskell 98, rank-1 types, but cannot be given a type signature. Try commenting out the type signature for g and everything will go wrong.

In GHC making the type of f :: forall a . a -> a, and adding -fglasgow-exts will make this code work. No such luck in Hugs.

With pattern type annotations, however, the code works in both Hugs and GHC:

f :: a -> a
f (x :: a) = g x
    where
          g :: a -> a
          g y = bind x y
 
bind :: a -> a -> a
bind a b = a