proposal-joint-iteration/spec.emu at main · tc39/proposal-joint-iteration · GitHub

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<!doctype html>
<meta charset="utf8">
<pre class="metadata">
title: Joint Iteration
status: proposal
stage: 3
contributors: Michael Ficarra
location: https://tc39.es/proposal-joint-iteration/
copyright: false
</pre>

<emu-clause id="sec-iterator.zip">
  <h1>Iterator.zip ( _iterables_ [ , _options_ ] )</h1>
  <p>This method performs the following steps when called:</p>
  <emu-alg>
    1. If _iterables_ is not an Object, throw a *TypeError* exception.
    1. Set _options_ to ? GetOptionsObject(_options_).
    1. Let _mode_ be ? Get(_options_, *"mode"*).
    1. If _mode_ is *undefined*, set _mode_ to *"shortest"*.
    1. If _mode_ is not one of *"shortest"*, *"longest"*, or *"strict"*, throw a *TypeError* exception.
    1. Let _paddingOption_ be *undefined*.
    1. If _mode_ is *"longest"*, then
      1. Set _paddingOption_ to ? Get(_options_, *"padding"*).
      1. If _paddingOption_ is not *undefined* and _paddingOption_ is not an Object, throw a *TypeError* exception.
    1. Let _iters_ be a new empty List.
    1. Let _padding_ be a new empty List.
    1. Let _inputIter_ be ? GetIterator(_iterables_, ~sync~).
    1. Let _next_ be ~not-started~.
    1. Repeat, while _next_ is not ~done~,
      1. Set _next_ to Completion(IteratorStepValue(_inputIter_)).
      1. IfAbruptCloseIterators(_next_, _iters_).
      1. If _next_ is not ~done~, then
        1. Let _iter_ be Completion(GetIteratorFlattenable(_next_, ~reject-primitives~)).
        1. IfAbruptCloseIterators(_iter_, the list-concatenation of « _inputIter_ » and _iters_).
        1. Append _iter_ to _iters_.
    1. Let _iterCount_ be the number of elements in _iters_.
    1. If _mode_ is *"longest"*, then
      1. If _paddingOption_ is *undefined*, then
        1. Perform the following steps _iterCount_ times:
          1. Append *undefined* to _padding_.
      1. Else,
        1. Let _paddingIter_ be Completion(GetIterator(_paddingOption_, ~sync~)).
        1. IfAbruptCloseIterators(_paddingIter_, _iters_).
        1. Let _usingIterator_ be *true*.
        1. Perform the following steps _iterCount_ times:
          1. If _usingIterator_ is *true*, then
            1. Set _next_ to Completion(IteratorStepValue(_paddingIter_)).
            1. IfAbruptCloseIterators(_next_, _iters_).
            1. If _next_ is ~done~, then
              1. Set _usingIterator_ to *false*.
            1. Else,
              1. Append _next_ to _padding_.
          1. If _usingIterator_ is *false*, append *undefined* to _padding_.
        1. If _usingIterator_ is *true*, then
          1. Let _completion_ be Completion(IteratorClose(_paddingIter_, NormalCompletion(~unused~))).
          1. IfAbruptCloseIterators(_completion_, _iters_).
    1. Let _finishResults_ be a new Abstract Closure with parameters (_results_) that captures nothing and performs the following steps when called:
      1. Return CreateArrayFromList(_results_).
    1. Return IteratorZip(_iters_, _mode_, _padding_, _finishResults_).
  </emu-alg>
</emu-clause>

<emu-clause id="sec-iterator.zipkeyed">
  <h1>Iterator.zipKeyed ( _iterables_ [ , _options_ ] )</h1>
  <p>This method performs the following steps when called:</p>
  <emu-alg>
    1. If _iterables_ is not an Object, throw a *TypeError* exception.
    1. Set _options_ to ? GetOptionsObject(_options_).
    1. Let _mode_ be ? Get(_options_, *"mode"*).
    1. If _mode_ is *undefined*, set _mode_ to *"shortest"*.
    1. If _mode_ is not one of *"shortest"*, *"longest"*, or *"strict"*, throw a *TypeError* exception.
    1. Let _paddingOption_ be *undefined*.
    1. If _mode_ is *"longest"*, then
      1. Set _paddingOption_ to ? Get(_options_, *"padding"*).
      1. If _paddingOption_ is not *undefined* and _paddingOption_ is not an Object, throw a *TypeError* exception.
    1. Let _iters_ be a new empty List.
    1. Let _padding_ be a new empty List.
    1. Let _allKeys_ be ? _iterables_.[[OwnPropertyKeys]]().
    1. Let _keys_ be a new empty List.
    1. For each element _key_ of _allKeys_, do
      1. Let _desc_ be Completion(_iterables_.[[GetOwnProperty]](_key_)).
      1. IfAbruptCloseIterators(_desc_, _iters_).
      1. If _desc_ is not *undefined* and _desc_.[[Enumerable]] is *true*, then
        1. Let _value_ be Completion(Get(_iterables_, _key_)).
        1. IfAbruptCloseIterators(_value_, _iters_).
        1. If _value_ is not *undefined*, then
          1. Append _key_ to _keys_.
          1. Let _iter_ be Completion(GetIteratorFlattenable(_value_, ~reject-primitives~)).
          1. IfAbruptCloseIterators(_iter_, _iters_).
          1. Append _iter_ to _iters_.
    1. Let _iterCount_ be the number of elements in _iters_.
    1. If _mode_ is *"longest"*, then
      1. If _paddingOption_ is *undefined*, then
        1. Perform the following steps _iterCount_ times:
          1. Append *undefined* to _padding_.
      1. Else,
        1. For each element _key_ of _keys_, do
          1. Let _value_ be Completion(Get(_paddingOption_, _key_)).
          1. IfAbruptCloseIterators(_value_, _iters_).
          1. Append _value_ to _padding_.
    1. Let _finishResults_ be a new Abstract Closure with parameters (_results_) that captures _keys_ and _iterCount_ and performs the following steps when called:
      1. Let _obj_ be OrdinaryObjectCreate(*null*).
      1. For each integer _i_ such that 0 ≤ _i_ &lt; _iterCount_, in ascending order, do
        1. Perform ! CreateDataPropertyOrThrow(_obj_, _keys_[_i_], _results_[_i_]).
      1. Return _obj_.
    1. Return IteratorZip(_iters_, _mode_, _padding_, _finishResults_).
  </emu-alg>
</emu-clause>

<emu-clause id="sec-IteratorZip" type="abstract operation">
  <h1>
    IteratorZip (
      _iters_: a List of Iterator Records,
      _mode_: either *"shortest"*, *"longest"*, or *"strict"*,
      _padding_: a List of ECMAScript language values,
      _finishResults_: an Abstract Closure that takes a List of ECMAScript values and returns an ECMAScript value,
    ): a Generator
  </h1>
  <dl class="header">
  </dl>
  <emu-alg>
    1. Let _iterCount_ be the number of elements in _iters_.
    1. Let _openIters_ be a copy of _iters_.
    1. Let _closure_ be a new Abstract Closure with no parameters that captures _iters_, _iterCount_, _openIters_, _mode_, _padding_, and _finishResults_, and performs the following steps when called:
      1. If _iterCount_ = 0, return ReturnCompletion(*undefined*).
      1. Repeat,
        1. Let _results_ be a new empty List.
        1. Assert: _openIters_ is not empty.
        1. For each integer _i_ such that 0 ≤ _i_ &lt; _iterCount_, in ascending order, do
          1. Let _iter_ be _iters_[_i_].
          1. If _iter_ is *null*, then
            1. Assert: _mode_ is *"longest"*.
            1. Let _result_ be _padding_[_i_].
          1. Else,
            1. Let _result_ be Completion(IteratorStepValue(_iter_)).
            1. If _result_ is an abrupt completion, then
              1. Remove _iter_ from _openIters_.
              1. Return ? IteratorCloseAll(_openIters_, _result_).
            1. Set _result_ to ! _result_.
            1. If _result_ is ~done~, then
              1. Remove _iter_ from _openIters_.
              1. If _mode_ is *"shortest"*, then
                1. Return ? IteratorCloseAll(_openIters_, ReturnCompletion(*undefined*)).
              1. Else if _mode_ is *"strict"*, then
                1. If _i_ ≠ 0, then
                  1. Return ? IteratorCloseAll(_openIters_, ThrowCompletion(a newly created *TypeError* object)).
                1. For each integer _k_ such that 1 ≤ _k_ &lt; _iterCount_, in ascending order, do
                  1. Assert: _iters_[_k_] is not *null*.
                  1. Let _open_ be Completion(IteratorStep(_iters_[_k_])).
                  1. If _open_ is an abrupt completion, then
                    1. Remove _iters_[_k_] from _openIters_.
                    1. Return ? IteratorCloseAll(_openIters_, _open_).
                  1. Set _open_ to ! _open_.
                  1. If _open_ is ~done~, then
                    1. Remove _iters_[_k_] from _openIters_.
                  1. Else,
                    1. Return ? IteratorCloseAll(_openIters_, ThrowCompletion(a newly created *TypeError* object)).
                1. Return ReturnCompletion(*undefined*).
              1. Else,
                1. Assert: _mode_ is *"longest"*.
                1. If _openIters_ is empty, return ReturnCompletion(*undefined*).
                1. Set _iters_[_i_] to *null*.
                1. Set _result_ to _padding_[_i_].
          1. Append _result_ to _results_.
        1. Set _results_ to _finishResults_(_results_).
        1. Let _completion_ be Completion(Yield(_results_)).
        1. If _completion_ is an abrupt completion, then
          1. Return ? IteratorCloseAll(_openIters_, _completion_).
    1. Let _gen_ be CreateIteratorFromClosure(_closure_, *"Iterator Helper"*, %IteratorHelperPrototype%, « [[UnderlyingIterators]] »).
    1. Set _gen_.[[UnderlyingIterators]] to _openIters_.
    1. Return _gen_.
  </emu-alg>
</emu-clause>

<emu-clause id="sec-ifabruptcloseiterators" aoid="IfAbruptCloseIterators">
  <h1>IfAbruptCloseIterators ( _value_, _iteratorRecords_ )</h1>
  <p>IfAbruptCloseIterators is a shorthand for a sequence of algorithm steps that use a list of Iterator Records. An algorithm step of the form:</p>
  <emu-alg>
    1. IfAbruptCloseIterators(_value_, _iteratorRecords_).
  </emu-alg>
  <p>means the same thing as:</p>
  <emu-alg>
    1. Assert: _value_ is a Completion Record.
    1. If _value_ is an abrupt completion, return ? IteratorCloseAll(_iteratorRecords_, _value_).
    1. Else, set _value_ to _value_.[[Value]].
  </emu-alg>
</emu-clause>