How to pass an optional argument to another function that accepts an optional argument?

[tom-sherman]

fun f(x=0)
  x

fun g(x: ?int)
  f(x)

fun main()
  println(g())

Error:

test.kk(5, 5): error: types do not match
  context      :   f(x)
  term         :     x
  inferred type: optional<optional<int>>
  expected type: optional<int>

How can I accept an optional param in g and pass it along to f?

[pmahoney]

not sure if there's a more elegant way, but this is one possibility:

fun f(x=0)
  x

fun g(x: ?int)
  match x
    Optional(v) -> f(v)
    None -> f()

fun main()
  println(g())

[xialvjun]

Watching for will the compiler do the sugar.

[tom-sherman]

Yep would be nice to not have to do the manual pattern match, that's what I was hoping to avoid.

[pmahoney]

A helper function can do it, but unfortunately it does not generalize; you need a new helper for every function signature.

fun opt(f : (optional<a>) -> e b, x : optional<optional<a>>) : e b
  match x
    Optional(Optional(v)) -> f(v)
    Optional(None) -> f()
    None -> f()

//////////

fun f(x = 0)
  x

fun g(x : ?int)
  f.opt(x)

fun main()
  println(g())
  println(g(12))

// output:
// 0
// 12