Roberto Zunino wrote:
When you type some expression such as
\x -> x
you have to choose among an infinite number of valid types for it:
Int -> Int Char -> Char forall a . a -> a forall a b . (a,b) -> (a,b) ...
Yet the type inference is smart enough to choose the "best" one: forall a. a -> a because this is the "most general" type.
Alas, for code like yours:
foo = \f -> (f 'J', f True)
there are infinite valid types too:
(forall a. a -> Int) -> (Int, Int) (forall a. a -> Char)-> (Char, Char) (forall a. a -> (a,a)) -> ((Char,Char),(Bool,Bool)) ...
and it is much less clear if a "best", most general type exists at all.
Requiring a type annotation for these not-so-common code snippets seems to be a fair compromise, at least to me.
Thank you for coming up with a clear and comprehensible answer.