Already in GHC, and describe in our paper on scoped type variables:
http://research.microsoft.com/~simonpj/papers/scoped-tyvar
Simon
| -----Original Message-----
| From: Ashley Yakeley [mailto:[EMAIL PROTECTED]]
| Sent: 11 April 2002 12:11
| To: Haskell List
| Subject: More Haskell Blue Sky: Partial Type Annotations
|
|
| I sometimes come across a situation when I only want to provide a
| _partial_ type annotation, perhaps because the full type has some
| variable unified with some variable in some other type annotation, or
| something. For instance:
|
| f :: forall a. [a] -> [a] -> [a]
| f x y = g x where
| g [] = y
| g (_:_) = x
|
| ...there doesn't seem to be a way of giving a type annotation
| for 'g'.
| Theoretically it's '[a] -> [a]', but where 'a' is the same as
| the 'a' in
| the type signature for 'f'.
|
| Under this proposal, unqualified type variables would be
| considered to be
| 'free' rather than implicitly forall-qualified. So for instance, any
| function could be given the partial type annotation 'a -> b'. For
| instance, all these annotations would be the same given this function:
|
| k :: a
| k :: a -> ba
| k :: a -> b -> a
| k :: forall b. a -> b -> a
| k :: forall a b. a -> b -> a
|
| k x y = x
|
| ...and these would both be the same, but different from the previous:
|
| k' :: Int -> ba
| k' :: forall b. Int -> b -> Int
|
| k' x y = x
|
| And so we could write this:
|
| f :: forall a. [a] -> [a] -> [a]
| f x y = g x where
| g :: [a] -> [a] -- partial annotation may be sufficient
| g [] = y
| g (_:_) = x
|
| I admit that's a bit of a change, and might break programs... In
| addition, it doesn't address the occasional need to tie type
| annotations
| together, something that has often made me add ugly dummy
| arguments used
| just for their type.
|
| --
| Ashley Yakeley, Seattle WA
|
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