On 18/12/2004, at 4:51 AM, Simon Peyton-Jones wrote:
This message is about lexically scoped type variables. I've gradually become convinced that if you write
f :: [a] -> [a] f x = <body>
then the type variable 'a' should be in scope in <body>.
I don't have a particularly strong opinion about this, but I've always thought that your suggestion is the way it should have been done. I actually find it confusing when someone "re-declares" the type variables in the function body; e.g. in your example of ...
f :: [a] -> [a] f x = my_id x where my_id :: a -> a my_id y = y
I would be very much inclined to use a type signature of "my_id :: b -> b", which makes it 100% obvious that it's using a fresh type variable. Considering that normal variables are lexically scoped, it seems intuitive to me that type variables should also be lexically scoped. -- % Andre Pang : trust.in.love.to.save