Use Base.Callable instead of Function in signatures

[cmcaine]

It's probably a better idea?

[hustf]

A slightly off-topic comment here:

I actually didn't know that the more general Callable existed at all, but I suppose it includes types, which are callable with an argument. If Callable is usefully more general, perhaps more function argument types should be elevated? What's a more general use case than Function?

Mux has a short and elegant codebase which I don't know well at all. One area of potential improvement (which would have been helpful in the past) could be tracing errors. I found this (from websockets) to be useful when condensing error messages:

function _show(io::IO, f::Function)
   m = methods(f)
   if length(m) > 1
       print(io, f, " has ", length(m), " methods: ")
       Base.show_method_table(io, m, 4, false)
   else
       method = first(m)
       argnames = join(method_argnames(method)[2:end], ", ")
       print(io, method.name, "(", argnames, ")")
   end
end

[cmcaine]

Base.Callable is just Union{Function, Type}, I think the general advice is not to dispatch on either :)

Thanks for the advice about errors, I'm still wondering how we might handle them better in Mux.