Ryan Ingram wrote:

There is such a tool, it's called ghci :)
It just takes a bit of massaging to do what you want:

ghci> :set -fglasgow-exts
ghci> :t (?f some_func [?a .. ?b])

Here's an example:
Prelude> :t ?f map [?a .. ?b]
?f map [?a .. ?b] :: forall t a b t1.
                    (Enum t1,
                   ?b::t1,
                   ?a::t1,
                   ?f::((a -> b) -> [a] -> [b]) -> [t1] -> t)
        => t

This tells you the types the variables have to have, and the type of
the expression.

Judicious use of (undefined :: type_signature) can also help.
 

Using undefined is already a standard technique for me. But what it doesn't let you do is

 foo (undefined :: Bar x) (undefined) :: Bar y   -- What type is the second argument?

I'm curios as to how the example you give actually works - I don't recognise that syntax at all...

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