Functions: generics / parametric polymorphism

[sharkdp]

Add support for generic functions, to allow things like

fn sqr(x) = x^2

Syntax

We can go the Rust route and do something like

fn sqr<T>(x: T) -> T^2 = x^2

Or we could add a special syntax for type variables, e.g. $T:

fn sqr(x: $T) -> $T^2 = x^2

Or go the Haskell way and use lowercase expressions as variables:

fn sqr(x: d) -> d^2 = x^2

that could work well with inferred types (#29):

f(x: d0, y: d1, z: d2) -> dR = …

[sharkdp]

Generic parameter deduction is definitely not straightforward. Imagine:

fn foo<D>(x: Meter * D^3) -> Scalar = …

foo(3 * meter * time^2)  // D^3 = time^2   ==>   D = time^(2/3)

or even worse:

fn bar<D1, D2>(x: D1 * D2, y: D1, z: D2) -> Scalar = …

This is a potential use case and could probably be supported somehow but maybe we should first disallow this. Or only allow it in the opposite order.

[sharkdp]

Error cases to consider:

• fn foo<Length>(x: Length) = …
• fn foo<D1, D2>(x: D1*D2) = … (see above)
• fn foo<D1>(x: Scalar) -> Scalar = …

[sharkdp]