[Haskell-cafe] A voyage of undiscovery

[Andrew Coppin]

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...)