Function Types

Flow-Wing lets you pass functions as arguments using function type syntax.

Syntax

[(param_types) -> return_type]

Basic Callback

fun greet(name: str) -> nthg {
  print("Hello, " + name)
}

fun runCallback(cb: [(str) -> nthg]) -> nthg {
  cb("World")
}

runCallback(greet)

Output: Hello, World

Multi-Parameter Callbacks

fun add(x: int, y: int) -> int {
  return x + y
}

fun apply(op: [(int, int) -> int], a: int, b: int) -> int {
  return op(a, b)
}

var result: int = apply(add, 3, 4)
print(result)

Output: 7

Keep a function in a variable

A function is a value like any other: keep it in a variable, change it, call it, pass it on. The variable needs its type written out.

fun double(x: int) -> int {
  return x * 2
}

fun triple(x: int) -> int {
  return x * 3
}

var op: [(int) -> int] = double
println(op(5))
op = triple
println(op(5))

Output: 10, then 15

Keep one in a field

A field of a class or an object type can hold a function. Call it as you call a method: box.op(x), self.op(x), or op(x) inside the class.

fun double(x: int) -> int {
  return x * 2
}

class Box {
  var op: [(int) -> int]
  init(op: [(int) -> int]) -> nthg {
      self.op = op
  }
  apply(x: int) -> int {
      return self.op(x)
  }
}

var box: Box = new Box(double)
println(box.apply(4))

type Handlers = {
  onValue: [(int) -> int]
}

var handlers: Handlers = { onValue: double }
println(handlers.onValue(21))

Output: 8, then 42

A function can also return a function (-> [(int) -> int]), and spawn f(x) runs the function f holds.

A function value that was never set

A variable or a field of a function type that no one set holds no function. Calling it stops the program with a runtime error that names it:

Runtime Error: Cannot call 'op': no function was assigned to it.

Limitations

• Flow-Wing does not currently support anonymous functions (lambdas/closures). Functions must be declared with fun before being passed as callbacks.
• A C function (one declared with decl) is not a function value: it takes and returns its values another way. Wrap it in a Flow-Wing function to pass it on.
• A method is not a function value (box.apply alone), and neither is a built-in such as println.
• Arrays of function values are not supported yet.