Simon,
2015-02-05 17:44 GMT+01:00 Simon Peyton Jones
3. It interferes with generalisation.
For (3), consider
let f :: _a -> _a
f xs = reverse xs
in (f True, f ‘x’)
Here, f gets the type f :: forall b. [b] -> [b], and _a is unifed with [b].
So it simply doesn’t make sense for _a to appear in the body. What would it mean to say
let f :: _a -> _a
f xs = reverse xs
in (f (True :: _a), f ‘x’)
Isn't this a different case than Thomas' example? As I understand it, an equivalent of his example would have the wildcard in scope in the body of f, not in the body of the let. Something like this: let f :: _a -> _a f xs = reverse (xs :: _a) in (f [True], f "x") or let f :: _a -> _a f xs = let ys :: _a ys = tail xs in reverse ys in (f [True], f "x") I agree with what you say about _a being in scope in the body of the if, but I don't see a problem with _a being in scope in the body of f. Do you? Note also that I haven't yet checked which of both is actually implemented. Regards, Dominique