Hello
On Sat, 29 Dec 2007 10:59:19 +0300, Jim Apple
<[EMAIL PROTECTED]> wrote:
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
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