using composition with multiple argument functions
Sun, 3 Feb 2002 03:04:18 -0800
[Moved to Cafe]
At 2002-02-02 17:42, Dean Herington wrote:
>With `h1 f g = f # g` and the type
>declaration for h1, the compiler sees that f1 and g1 must be used at the
>same type. With `h1 = f1 # g1`, that connection is missing.
Yes. Haskell cannot be sure that the last instance declaration is the one
it wants, because, even though it would work, there might be other
possible instance declarations that could apply.
Here's a simpler example:
class C f where
foo :: f -> Bool
instance C (Bool->Bool) where
foo f = f True
instance C (Int->Int) where
foo f = False
f :: Bool
f = foo id
Either instance declaration would work, but Haskell needs one
unambiguously for the general type of 'id'. You can fix it by restricting
the type of 'id':
f :: Bool
f = foo (id :: Int -> Int)
>So I guess my `Composable` class example can't work in general.
Maybe, maybe not.
Ashley Yakeley, Seattle WA