Feature request: Scheme-like anonymous recursive functions with initial arguments

[ellbur]

In functional languages like rescript, recursion is an important mode of control flow, on par with (or even more important than) loops. For that reason, some languages (like Scheme) have syntactic sugar for creating and immediately calling a recursive function. This reduces the need for complicated (and less flexible) loop syntax.

Motivating example:

type rec tree<'t> = Leaf('t) | Branch(promise<tree<'t>>, promise<tree<'t>>)

let delay = (ms, x) => Promise.make((resolve, _reject) => setTimeout(
  () => resolve(x),
  ms
)->ignore)

let myTree = Branch(
  delay(100, Branch(
    delay(100, Leaf(1)),
    delay(100, Leaf(2))
  )),
  delay(100, Leaf(3))
)

// Current syntax: declare and use a helper function
let rec flatten = async tree => switch tree {
  | Leaf(x) => [x]
  | Branch(x, y) => Array.concat(await flatten(await x), await flatten(await y))
}
let myFlatTree = await flatten(myTree)

// Proposed syntax: anonymous recursive function with initial parameters
let myFlatTree = async rec flatten (tree = myTree) => switch tree {
  | Leaf(x) => [x]
  | Branch(x, y) => Array.concat(await flatten(await x), await flatten(await y))
}

[josher8a]

I really like it! I think it should be an optional labeled argument. https://rescript-lang.org/docs/manual/latest/function#optional-with-default-value