Generic functions¶

A function written once for every type names a type parameter, one capital letter in angle brackets after its name. Each call fills it from its arguments:

fn first<T>(xs: List<T>) : Option<T> = xs | reduce((a, b) => a)
first(["Dune", "Emma"])

"Dune"

The same function over a list of numbers gives an Option<Int>:

fn first<T>(xs: List<T>) : Option<T> = xs | reduce((a, b) => a)
first([412, 280])

Option<Int>

Bounds¶

Inside the body T is a type of its own: the body may rely only on what the parameter promises. A bound, T : Orderable, promises an order, so the body may sort or find a minimum, and each call is checked against the bound:

fn shortest<T : Orderable>(xs: List<T>) : Option<T> = xs | min
shortest([412, 280, 310])

fn shortest<T : Orderable>(xs: List<T>) : Option<T> = xs | min
shortest([{1}, {2}])

check error[type-mismatch]: `shortest` needs T : Orderable, and T is Set<Int> here
 --> <expression>:2:1
  |
2 | shortest([{1}, {2}])
  | ^^^^^^^^^^^^^^^^^^^^

A bound is a type or a trait. Traits declares traits of a program's own and bounds by them.