We don't parameterize a function by it's input and return types because functions are extremely polymorphic in julia. However, there's lots of tricks one can do to dispatch on whether or not a given function have methods that satisfy some sort of interface, see here: https://github.com/oxinabox/Tricks.jl#we-can-use-static_hasm...

Using this technique, you can do compile time dispatch on things like input and return types.