Pleasant Notation for promise pipelining.

  • Mark S. Miller @erights, Agoric
  • Michael Fig @michaelfig, Agoric
  • Chip Morningstar @FUDCo, Evernote


Presented to TC39 (Javascript standards committee), achieving stage 1.

Presentation to TC39



This is a follow on proposal to proposal-eventual-send, providing syntactic sugar for the APIs of that proposal.

The 2011 ECMAScript strawman concurrency proposal also described a simple desugaring of an infix bang (!) operator to support promise pipelining. To avoid conflict with TypeScript, this proposal instead introduces the wavy dot (~.) syntax.

Wavy Dot

Like the (?.) of the optional chaining proposal, wavy dot (~.) is a proposed infix operator with the same precedence as dot (.). Both can be understood as adjective dot, i.e., an operation that is dot-like, but differs according to the adjective. Once the optional chaining proposal is accepted, we will add to this proposal an operator combining the two adjectives, such as (?~.) or (~?.).

When the wavy dot expression occurs in a syntactic context in which the value of the expression might be used, the syntax has the following equivalences

Syntax Internal Method p.[[GetSend]]('name')
p~.[prop] p.[[GetSend]](prop)
p~.(...args) p.[[ApplyFunctionSend]](args) p.[[ApplyMethodSend]]('name', args)
p~.[prop](...args) p.[[ApplyMethodSend]](prop, args)

When the expression occurs in a syntactic context where the value of the expression is obviously ignored, such as an ExpressionStatement, the equivalences are as above, but using the [[*SendOnly]] variant of these internal methods.

Syntax Internal Method
void p~.(...args); p.[[ApplyFunctionSendOnly]](args)
void; p.[[ApplyMethodSendOnly]]('name', args)
void p~.[prop](...args); p.[[ApplyMethodSendOnly]](prop, args)

Proposed Syntax

Abstract Syntax:

 Expression : ...
      Expression ~. [ Expression ]              // eventual get
      Expression ~. Arguments                   // eventual apply function
      Expression ~. [ Expression ] Arguments    // eventual apply method

Attempted Concrete Syntax, where "..." signifies the existing productions of that non-terminal. Our intention is that this syntax follow the pattern of the optional chaining proposal.

  WavyDot ::
      ~. [lookahead ∉ DecimalDigit]

  MemberExpression : ...
      MemberExpression WavyDot [ Expression ]
      MemberExpression WavyDot IdentifierName
  CallExpression : ...
      CallExpression WavyDot [ Expression ] Arguments
      CallExpression WavyDot IdentifierName Arguments
      MemberExpression WavyDot Arguments
      CallExpression WavyDot Arguments
      CallExpression WavyDot [ Expression ]
      CallExpression WavyDot IdentifierName


A Babel playground implements this proposal using static methods on the HandledPromise object that is in the current scope. The Work-In-Progress Babel pull request for this syntax is also available.