> The way a function is declares looks just like a variable assignment for instance, but in a compiled language it pushes the wrong paradigm on me, since that's pretty much immutable.

I think one of the problems with the syntax is that colon (ordinary used for type ascription in ML-family languages) is being used for... something else. For example, in

  foo: (x: int) -> int = (expression using x)

the introduction of the 'x' binding is in the type position of 'foo', whereas in OCaml it would be

  let foo (x: int): int = ...

Here the binding of 'x' is associated to 'foo' rather than first appearing in type position. I think this is more readable, and the component parts are more easily parsed by humans. One could argue that visually, using an arrow as in

  let foo (x: int) -> int = ...

is more readable, but the OCaml syntax captures the fact that the RHS of the binding is defining an 'int' rather than a function value.

> I must admit that I'm mostly writing Python and JS these days, but the way the colon is being used feels grossly inconsistent and confusing. Why not say std.vector for instance now instead of std::vector?)

One potential sensible distinction would be to use one for statically determined paths and the other for dynamically determined paths (including the separate dynamic phase of compile-time constexpr evaluation).