Do Expressions

[hzoo]

Babel docs: http://babeljs.io/docs/plugins/transform-do-expressions
Stage 1 gist reproduced below: https://gist.github.com/dherman/1c97dfb25179fa34a41b5fff040f9879 by @dherman

In Babel

interface DoExpression <: Expression {
  type: "DoExpression";
  body: BlockStatement
}

---

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>
)

[mikesherov]

Seems fairly straightforward to me. @dherman any footguns to be aware of? @RReverser and @ariya any thoughts?

[ariya]

I haven't investigated do expressions yet.

BTW, why is the type Identifier?

[michaelficarra]

One of the major controversies with the proposal within the committee is whether abrupt completions such as return/break/continue statements should be allowed within do expressions.

[RReverser]

BTW, why is the type Identifier?

Seems like a copy-paste mistake to me.

[hzoo]

Oops, fixed (good catch @ariya)

[gibson042]

type: "BlockStatement"

Don't you mean type: "DoExpression"?

[mikesherov]

@gibson042, isn't the body of a do Expression just a normal block for the case of tree representation?

[gibson042]

Yes, subject to the concerns about completions @michaelficarra mentioned. But { "type: "BlockStatement", "body": […] } is an existing AST node, and it isn't an Expression.

[mikesherov]

The body is a blockStatement (subject to abrupt completions), the type is "DoExpression"... no? what am I missing?

[RReverser]

@mikesherov I think when @gibson042 wrote his previous comment

Don't you mean type: "DoExpression"?

main type of node was "BlockStatement", but @hzoo edited it afterwards, so it's now "DoExpression", thus misunderstanding :)