It's dependency injection. and yes, you can model dependecies like a monad but most people, even in less pure fp langs, don't.

i don't really say this to just be a pedant, but if you're an fp enjoyer, you will be disappointed if you get the picture that zig is fp-like, outside of a few squint-and-it-looks-like things

> ...

> What facilities does the language provide me to create correct-by-construction systems and how easily can I program the type-system.

Isn't programming the type-system orthogonal to the program's domain in the same way that manual memory management is?

Why write:

EqPoint.eql(a, c)

When you can write:

Point.eql(a, c)

  fn Maybe(comptime T: type) type {
    return union(enum) {
        value: T,
        nothing,

        const Self = @This();

        pub fn just(the_val: T) Self   { return .{ .value = the_val }; }
        pub fn nothing() Self          { return .nothing; }

      }
    }

    data Maybe a = Just a | Nothing

I haven't heard anyone writing code in Elixir complain about performance issues.