Rank-2 polymorphism and pattern matching
jbapple+ghc-users at gmail.com
Sat Dec 29 02:59:19 EST 2007
The following won't compile for me
isnull :: (forall a . [a]) -> Bool
isnull ( :: forall b . [b]) = True
Couldn't match expected type `forall b. [b]'
against inferred type `[a]'
In the pattern: 
Wrapping it in a constructor doesn't help, though I can define a "null":
data NullList = NullList (forall a . [a])
null = NullList 
isnull2 :: NullList -> Bool
isnull2 (NullList ) = True
More information about the Glasgow-haskell-users