Re: [Haskell-cafe] A voyage of undiscovery

Robert Greayer wrote:

f0 _ = (foo True, foo 'x') where foo = id

is well-typed.

Really? That actually works? How interesting... This suggests to me that where-clauses also do strange things to the type system.

whereas

f1 foo = (foo True, foo 'x')

requires 'foo' to be polymorphic in its first argument. This does require a higher rank type, which can't be inferred:

You could type f1 as f1 :: (forall a . a -> a) -> (Bool, Char)

and apply it to 'id'.

Or you could type it as something like: f1 :: (forall a . a -> ()) -> ((),())

and apply it to 'const ()'

...all of which is beyond Haskell-98, which is what I am limiting myself to at present. (Actually, even that is a lie. I don't have type-classes yet...)