On Tue, 2004-06-22 at 20:52, Adrian Hey wrote:
On Tuesday 22 Jun 2004 6:20 pm, MR K P SCHUPKE wrote:
ahh but in this example:
f :: [Int] -> [Bool] f (i:is) = even i : f is f e@[] = e
e is an empty list of Ints not an empty list of Bools!
If the difference is significant (I don't believe it is) then consistency demands that this expression should give a type error..
let e=[] in (length e : e, null e : e)
It doesn't, so clearly e can be both an empty list of Ints and an empty list of Bools :-)
I think the point is that [] (or e in your example) has type forall a.[a] where as in the original example e was bound to an argument with the type [Int], so e could not be used where something of type [Bool] was required. On the other hand [] :: forall a.[a] could be used in either context. Duncan