do-stage-1.md

Motivation

• expression-oriented programming one of the great advances of FP
• expressions plug together like legos, making more malleable programming experience in-the-small

Examples

Write in an expression-oriented style, scoping variables as locally as possible:

let x = do {
  let tmp = f();
  tmp * tmp + 1
};

Use conditional statements as expressions, instead of awkward nested ternaries:

let x = do {
  if (foo()) { f() }
  else if (bar()) { g() }
  else { h() }
};

Especially nice for templating languages like JSX:

return (
  <nav>
    <Home />
    {
      do {
        if (loggedIn) {
          <LogoutButton />
        } else {
          <LoginButton />
        }
      }
    }
  </nav>
)

Tennant's Correspondence Principle

• key refactoring principles:
  • do { <expr>; } equivalent to <expr>
  • (do { <stmt> };) equivalent to { <stmt> }
• this semantic transparency is demonstrated by the semantics:
  1. Return the result of evaluating Body.

Further considerations

How to avoid either parsing conflict in statement context with do-while, or dangling-else type of ambiguity:

do do f(); while (x);

I have several alternatives I intend to explore here.

I'd also love to see things like do switch. fav would be without explicit return but i guess then you'd need break again?

const color = do switch(id) {
    case 0: return 'blue';
    case 1: return 'black';
    case 2: return 'yellow';
    default: return 'white';
}

if explicit return is used, the do would really just be syntax sugar for IIFEs

Personally I feel like it should be that do expressions introduce a new function-like scope thing but also still give eval result e.g.:

const x = do {
    3
}
// Still 3

function foo() { 
    const x = do {
        return 3
    }
    // x is 3 and this code is reached feels more useful than either returning 3 from foo or outright banning control flow in do expressions
}

Also I was playing around in the d8 shell with their experimental version of do expressions, and I realized default function arguments is a case that really needs considering when deciding the semantics for example currently what d8 does for a few cases:

function foo(x=do { var y = 3 }) {
    print(y)
}

foo() // prints 3

function foo(x=do { return 'Early return' }) {
    return 'Normal return'
}

print(foo()) // Early return

function* foo(x=do { return 'Early return' }) {
    yield 1
}
print(foo().value) // Early return, obviously this isn't what we'd want
print(foo().done) // true

function* foo(x= do { yield 'Early value' }) {
    yield 1
}
foo() // SegFault, obviously v8 doesn't have any notion of a parameters environment yet 
        // which isn't surprising as arbitrary statements were never able to be put into default parameters position

async function(x=do { await 3 }) {
    print(x)
}
foo() //SegFault again

So this proposal will need to define what a parameter scope environment looks like in addition to the semantics of what happens inside a do expression block.

Would it be a good idea to convert some statements such as if switch block into ExpressionStatement if they are on the right side of the AssignmentExpression? So that we can write functional code easily and omit do in most cases.

@isiahmeadows your approch to use do inside immediatly invoked function is making a little sense because most point to use do is not to use immediatly invoked function

I have run in to the same issue as Jamesernator and I'm also think that async do will fit very well. Like do is a replacement of () => and async do is a replacement of async () =>

@remhume return inside a do expression would return the outer function (or throw if the do expression is at the top level). You would need break statements instead of the return.

do is not to use immediatly invoked function

@Alexsey That's not entirely true, essentially I'd like semantics similar to arrow functions but that currently don't exist in the language essentially I'd want the following to happen:

const num = do {
    if (true) {
        3
    } else {
        4
    }
}
// I'd like to see num === 3 here

const num = do {
    return 3
}

// I'd also like num to be 3 here

for (const x of [1,2,3,4]) {
    const square = do {
        break
    }
}
// I'd like to be a syntax error about invalid break

---

The only things I'm undecided on is how I'd want to see yield / await in a plain do expression:

One: Simply transfer outer control of await / yield to the do expression:

function* foo() {
    console.log(do {
        yield null
    })
}
const g = foo()
g.next()
g.next(10) // Would print 10

let x = do { yield 1 } // SyntaxError

async function foo() {
    console.log(do {
       await fetch('foo.json').then(res => res.text())
    }) // Contents of foo.json
    console.log(async do {
        await fetch('foo.json').then(res => res.text())
    }) // Promise eventually resolving to contents of foo.json
        // unlike previous form doesn't pause the async function
}

Two: do is like an iife in almost every way except with special returning rules (although =>* doesn't have any proposal linked so I'd wait for this behaviour):

function foo() {
    // This would be identical to one's behaviour
    console.log(yield* do* {
       yield null
    })
}

const values = do* { yield 1; yield 2; 3 }
values.next() // { value: 1, done: false }
values.next() // { value: 2, done: false }
values.next() // { value: 3, done: true }

async function foo() {
    console.log(do {
        await fetch('foo.json').then(res => res.text())
     }) // SyntaxError await outside of async function
     console.log(await async do /* That's some keyword chaining that is */ {
         await fetch('foo.json').then(res => res.text())
      }) // Promise resolving to contents of foo.json
}

Three: Neither of the above, simply shorthand for immediately invoked normal arrow functions:

const values = do { yield 1; 3 } // SyntaxError
const body = do { let res = await fetch('foo.json'); await res.text() } // SyntaxError
const body = async do { let res = await fetch('foo.json'); await res.text() } Promise resolving to contents of foo.json

So maybe we can vote on "do return"?

Please transfer proposal to tc39

See tc39/proposals#44

If we are in an async function, would await work in a do expression?

(async function(){
    const foo = do{
        await anAsyncFunc()
   }
   // Error or the foo is the awaited anAsyncFunc()?
})();

Ok, guys, should let x = do {do { true }} resolve to true and if not – why?
Current Babel does not parse it and other authors aren't sure as well.

Is there any argument for why it shouldn't? Seems intuitive to me that it would...

@ivan-kleshnin This happens because do-expressions must strictly be in expression position, it's the same reason why you can't write an immediately invoked function expression without wrapping it brackets:

function() { console.log("Hello") }() // Does not work
(function() { console.log("Hello")()) // Does work

let x = do { do { true } } // Does not work
let x = do { (do { true } }) } // Does work

Of course there's no spec text yet so it could still happen, Babel's parser (and v8's experimental implementation) are thus just guesses as to what the exact grammar might be, if there's no async variant I don't really see the purpose of having it in statement position when you could just use a block:

do {
    const greeting = "Hello!"
    console.log(greeting)
}

// vs just

{
    const greeting = "Hello!"
    console.log(greeting)
}

Although if there's an async variant I think it'd be rather nice for top-level execution e.g.:

module.exports = function myCoolFunction() {

}

if (require.main === module) {
    do async {
        // Execute my cool cli tool
    }
}

@Jamesernator thank you for the explanation – you gave a lot of good points.

I don't really see the purpose of having it in statement position when you could just use a block:

The original question was caused by the use of do in JSX.

@dherman or anyone: I have a question about whether it is valid to use the do expression for an if statement WITHOUT AN else CONDITION. When I use a do expression without else, I get an eslint error for no-unused-expressions. I am using this code in my JSX for a React component, something like this (illustrative purposes only):

export default function myComponent(props) {
  return (
    <div>
      {do {
        if (true) {
          <p>If statement is true</p>
        }
      }}
      <p>I am always rendered</p>
    </div>
  );
}

I prefer to use the do construct over the unnatural, confusing usage of &&, as suggested by Facebook for React rendering. Is this a valid use case?

Note that babel/eslint-plugin-babel#13 may be related to this issue, but I don't know for sure yet until I understand more about the design of the do expression.

Thank you.

I also asked the question here: https://stackoverflow.com/questions/45667350/javascript-do-expression-if-statement-without-else

@ecbrodie There's no reason that wouldn't work, the completion value of if is already well defined, basically you see the result of anything a do-block could do just by eval-ing the body e.g.:

const x = eval(`
    if (true) {
        'banana'
    }
}
x // 'banana'

const y = eval(`
    if (false) {
        'banana'
    }
}

y // undefined

Regarding this JSX example:

return (
  <nav>
    <Home />
    {
      do {
        if (loggedIn) {
          <LogoutButton />
        } else {
          <LoginButton />
        }
      }
    }
  </nav>
)

I got triggered so badly on this:

Especially nice for templating languages like JSX...

First, the ternary expression is million times better in this situation:

return (
  <nav>
    <Home />
    {loggedIn ? <LogoutButton /> : <LoginButton />}
  </nav>
)

Second, no need to use JSX, just use the HyperScript function h, which makes it another million times better:

return h('nav',
  h(Home),
  loggedIn ? h(LogoutButton) : h(LoginButton),
);

Or JSON-ML (hint: JSON is built into JavaScript by default; no need to use the defunct XML syntax in JavaScript):

return ['nav',
  [Home],
  loggedIn ? [LogoutButton] : [LoginButton],
];

Reported for this example.

just kidding ;)

Is there any activity on this? Or similar proposals elsewhere I could look at or try out?

I need this really badly -- especially when working with JSX.

@streamich A ternary is pretty nice when there's only 2 branches (true or false), but if there's 3-4-5 it quickly becomes completely unreadable and you're forced to make a helper function with return to avoid using a temporary variable.

One could argue that splitting things into functions is "right" anyway, but I want refactoring into functions/consts a conscious decisions, not something I have to do because of language restrictions. Inline is often easier to read than having to jump back and forth between intermediary helper functions.