JavaScript code generation for methods

[chengluyu]

Todos

• Make it work on basic examples.
• Translate nullary methods to get property. Because we didn't implement calling nullary methods so far.
• Inheritance.
• Method override.

Examples

Input

class Box[T]: { inner: T }
  method Map f = Box { inner = f this.inner }

def Box value = Box { inner = value }

def box1 = Box 1
def box2 = box1.Map (fun x -> add x 1)

Output

val Box: 'a -> box & {inner: 'a} = (value) => new Box({ inner: value })
val box1: box & {inner: 1} = Box { "inner": 1 }
val box2: box & {inner: int} = Box { "inner": 2 }

Generated

class Box {
  constructor(fields) {
    this.inner = fields.inner;
  }
  Map(f) {
    return new Box({ inner: f(this.inner) });
  }
}
const Box0 = (value) => new Box({ inner: value });
const box1 = Box0(1);
const box2 = box1.Map((x) => x + 1);

[LPTK]

Cool! Now try it with inheritance and overriding :^D

Tip: you can just ignore method signatures in the source (those declarations of the form Mtd: <some type>).

[LPTK]

Technical note: for this one, let's wait until #22 is merged and squashed (Cc @fo5for). Then we can merge directly into the main mlscript branch. But the PR is fine as it is right now.

[chengluyu]

Test Cases

Translate Nullary Methods into Getters

Input

class Box[T]: { inner: T }
  method Map f = Box { inner = f this.inner }
  method Get = this.inner

def Box value = Box { inner = value }

def boxOne = Box 1
def one = boxOne.Get
def boxTwo = boxOne.Map (fun x -> add x 1)
def two = boxTwo.Get

Output

val Box: 'a -> box & {inner: 'a} = (value) => new Box({ inner: value })
val boxOne: box & {inner: 1} = Box { "inner": 1 }
val one: 1 = 1
val boxTwo: box & {inner: int} = Box { "inner": 2 }
val two: int = 2

Inheritance

Input

class Box[T]: { inner: T }
  method Map f = Box { inner = f this.inner }
  method Get = this.inner

class DecoratedBox[T]: Box[T] & { motif: string }
  method GetBox: Box[T]
  method GetBox = this
  method GetMotif = this.motif
  method Decorate motif = DecoratedBox {
    inner = this.inner;
    motif
  }

def flowerBox = DecoratedBox { inner = 42; motif = "flower" }
def grassBox = flowerBox.Decorate "grass"

def FLOWER = flowerBox.GetMotif
def GRASS = grassBox.GetMotif

Output

val flowerBox: decoratedBox & {inner: 42, motif: "flower"} = DecoratedBox { "inner": 42, "motif": "flower" }
val grassBox: decoratedBox & {inner: 42, motif: "grass"} = DecoratedBox { "inner": 42, "motif": "grass" }
val FLOWER: string = "flower"
val GRASS: string = "grass"

Method Override

Input (from test cases)

class AbstractPair[A, B]: { x: A; y: B }
    method Test: (A -> B -> bool) -> bool
    method Map[C, D]: (A -> C) -> (B -> D) -> AbstractPair[C, D]

class Pair[A, B]: AbstractPair[A, B]
    method Test(f: A -> B -> bool) = f this.x this.y
    method Map fx fy = Pair { x = fx this.x; y = fy this.y }

class True[A, B]: Pair[A, B]
    method Test f = true
    method True = this.Test (fun x -> error)

Generated code

class AbstractPair {
  constructor(fields) {
    this.x = fields.x;
    this.y = fields.y;
  }
}
class Pair extends AbstractPair {
  constructor(fields) {
    super(fields);
  }
  Test(f) {
    return f(this.x)(this.y);
  }
  Map(fx) {
    return ((fy) => new Pair({
      x: fx(this.x),
      y: fy(this.y),
    }));
  }
}
class True extends Pair {
  constructor(fields) {
    super(fields);
  }
  Test(f) {
    return true;
  }
  get True() {
    return this.Test((x) => error);
  }
}

[chengluyu]

Hmm, I should implement error in generated code. Maybe in another PR?

[LPTK]

Great, looking good!

I should implement error in generated code. Maybe in another PR?

Yeah, it should probably be implemented as throwing an exception. Let's do it as part of this PR.

[LPTK]

Also, let's merge this first, and soon we'll add proper JS code-gen testing and revisit the tests you described above.

[LPTK]

@chengluyu This branch needs to be rebased on mlscript and remove all commits before 7ad32d4 (these commits were squashed in the previous PR). Let me know if you need help doing that. You can look into git rebase -i (interactive rebase).

[LPTK]

@chengluyu Actually since there are many conflicts, it would be easier to hard-reset this branch to the current HEAD of mlscript and then git-cherrypick your commits back one by one. Then force-push the new history. Or this can also be done as part of the git rebase -i command (drop all commits previously squashed).

[chengluyu]

[...] it would be easier to hard-reset this branch to the current HEAD of mlscript and then git-cherrypick your commits back one by one. Then force-push the new history. [...]

I proposed a lot of changes to JSBackend.scala in PR #29. How about do this after when that PR was merged?

[chengluyu]

BTW, I have an idea of merging multiple function defs into one. For example, we can translate

def f { x = 0 } = 1
def f { x = 1 } = 0

into

const f = (x) => {
  if (x === 0) { return 1; }
  else if (x === 1) { return 0; }
  else { /* unreachable */ }
}

[LPTK]

I have an idea of merging multiple function defs into one.

But this is not in line with MLscript semantics, which uses shadowing. In MLscript, f as defined above will always return value 0.

[LPTK]

To be more specific, what would you do in this case, for instance?

def f 0 = "oops"
def f x = x

f 0 + 1 // valid in MLscript