Hello
On Sat, 29 Dec 2007 10:59:19 +0300, Jim Apple
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
Why?
Well, the wise people shall tell you precisely why. I think, it's because you cannot pattern match the existentials in a way that lets escape something out of (forall) context. Of course, in this particular case the pattern [] doesn't bind anything, so definitely nothing can escape. But it seems like ghc doesn't track this special case. Of course, isnull :: (forall a . [a] -> Bool) isnull ([] :: forall b . [b]) = True will work. Consider this -- inferred type is forall a. ([a] -> Bool) isn [] = True -- usual function isnull :: (forall a . [a]) -> Bool isnull x = isn x it works because you apply a function isn, which has (implicitly) Bool inside the forall context to a forall value. -- Daniil Elovkov