"Greg Buchholz" wrote:

I'm not quite sure why this is illegal...

...

foo :: Integer -> (forall a. Show a => a) foo 2 = ["foo"] foo x = x

The type signature promises that foo returns a function that has type a *for all a* (that are in Show). But neither a string list nor an integer can have all types. Rather, they are very specific. That is, what you have implemented actually could have type foo :: Integer -> (exists a. Show a => a) (if GHC supported such types). Note that only bottom can have a type like forall a.a.

...while this is just fine...

...

bar :: Integer -> (forall a. Show a => a->b) -> b bar 2 k = k ["bar"] bar x k = k x

Here, you declare bar to *take* a function that delivers some b for any a. That is, the burden of defining such a function now is on the caller, not bar itself. However, in this case, defining such an argument function actually is easy, for another reason: the signature says k returns a result of type b, but b is fixed. So it is always possible to come up with a trivial instance of such a function (which will just ignore its argument). For instance, bar 2 (\_ -> "boo") But try to call this for a different example: ;-) baz :: Integer -> (forall a b. Show a => a->b) -> c baz 2 k = k ["baz"] baz x k = k x Hope this helps, - Andreas