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What do you mean by "not first class"?

https://docs.julialang.org/en/v1/manual/functions/#man-anony...

It means I cannot fully specify the type of function my interface expects as an argument.

A function is of type ‘Function’. There is no way to define it with the type of its argument or the type of what it returns.

Actually, Julia functions are all subtypes of Function. Every julia function has a unique type.

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.

Functions are polymorphic in many languages that let you annotate the their types. The concern is not really for dispatch here, it's for readability/correctness.

> The concern is not really for dispatch here, it's for readability/correctness.

Then dispatch to an error.

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