> Even given a simple thing like foo:: int -> int -> int, there's really no way to tell what it's doing.

Ahh, this is an important point. The reason you can't tell what it's doing is because it is quite concrete. One really cool thing about this kind of type system is that the more generic your functions, the more you can tell about what it's doing. Consider this function:

    foo :: (a, b) -> a

There's only one possible thing this function can do! And there's only one possible implementation--the correct one. This is the case because the types involved are completely general. Type signatures can communicate a lot more than one might think.