Aliases and holes

Aliases

Most small functions are a pipeline fragment with a name. := defines an alias, and the checker writes its type: the input is the most general one the fragment accepts, and each parameter takes the type of the step argument it fills.

top(n) := sort | take(n)
[412, 280, 310, 145] | top(2)

[145, 280]

hql check prints the signature it wrote:

top(n) := sort | take(n)
1

top : (Collection<T>, Int) -> List<T> where T : Orderable
Int

Holes

A call with _ for an argument is a function of the arguments left open — a partial application:

fn add(a: Int, b: Int) : Int = a + b
[1, 2, 3] | map(add(10, _))

[11, 12, 13]

A hole is a whole argument. _ + 1 is not a hole, and a lambda, n => n + 1, writes that function.