If I’m writing a hash map module that’s generic over what it accepts, and later someone else is going to want to use it with Foo keys, how am I able to write my hash map such that the hash function for Foos are in scope in my hash map module?

With function overloading, templates and late static binding.

You just use a `hash` function in your library code and user has to implement a version of it that accepts the Foo type.

To resolve the scope problem, Nim uses templates[1] and a `mixin`[2] statement to bind user-defined hash procedure.

0 - https://github.com/nim-lang/Nim/lib/pure/collections/tables....

1 - https://github.com/nim-lang/Nim/blob/652edb229ae3ca736b19474...

2 - https://nim-lang.org/docs/manual.html#generics-mixin-stateme...