>
>
> # let y = z z ;;
> val y : (('_a -> '_b) -> '_a -> '_b) -> '_a -> '_b = <fun>
>
> Can anyone get rid of the pesky underscores in the type of y above, so
> that it becomes truly polymorphic?
>

Eta expansion does it...

# let y x = z z x;;
val y : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun>

Regards,
François