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Add grammar flags to disallow mixing ?? with || and &&. (#39)
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Add grammar flags to disallow mixing ?? with || and &&.
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@DanielRosenwasser
DanielRosenwasser authored Jul 13, 2019
2 parents 2fc2f14 + 04359c2 commit 99ff664
Showing 1 changed file with 165 additions and 22 deletions.
187 changes: 165 additions & 22 deletions spec.html
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<!doctype html>
<meta charset="utf8">
<pre class=metadata>
title: Nullish coalescing operator
title: Nullish Coalescing Operator
stage: 2
contributors: Daniel Ehrenberg
contributors: Daniel Ehrenberg, Daniel Rosenwasser
</pre>

<emu-intro id=sec-intro>
Expand All @@ -12,33 +12,176 @@ <h1>Introduction</h1>
<p>The main design decisions made in this specification are:
<ol>
<li>The right argument of `??` is evaluated only if needed ("short circuiting").</li>
<li>`??` has the same precedence as `||`.</li>
<li>`??` has lower precedence than `||`.</li>
<li>`??` cannot immediately contain, or be contained within, an `&amp;&amp;` or `||` operation.</li>
<li>The right argument is selected if the left argument is `null` or `undefined`.</li>
</ol>
</p>
</emu-intro>

<emu-clause id=sec-nullish-coalescing-syntax>
<h1>Syntax</h1>
<emu-clause id="sec-binary-logical-operators">
<h1>Binary Logical Operators</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
<ins>
NullishExpression[In, Yield, Await, Nullish] :
LogicalORExpression[?In, ?Yield, ?Await, ?Nullish]
NullishExpression[?In, ?Yield, ?Await, +Nullish] `??` LogicalORExpression[?In, ?Yield, ?Await, +Nullish]
</ins>

<emu-grammar>
LogicalORExpression[In, Yield, Await] :
LogicalANDExpression[?In, ?Yield, ?Await]
LogicalORExpression[?In, ?Yield, ?Await] `||` LogicalANDExpression[?In, ?Yield, ?Await]
<ins>LogicalORExpression[?In, ?Yield, ?Await] `??` LogicalANDExpression[?In, ?Yield, ?Await]</ins>
LogicalORExpression[In, Yield, Await, <ins>Nullish</ins>] :
LogicalANDExpression[?In, ?Yield, ?Await, <ins>?Nullish</ins>]
<del>LogicalORExpression[?In, ?Yield, ?Await] `||` LogicalANDExpression[?In, ?Yield, ?Await]</del>
<ins>[~Nullish] LogicalORExpression[?In, ?Yield, ?Await, ~Nullish] `||` LogicalANDExpression[?In, ?Yield, ?Await, ~Nullish]</ins>

LogicalANDExpression[In, Yield, Await, <ins>Nullish</ins>] :
BitwiseORExpression[?In, ?Yield, ?Await]
<del>LogicalANDExpression[?In, ?Yield, ?Await] `&amp;&amp;` BitwiseORExpression[?In, ?Yield, ?Await]</del>
<ins>[~Nullish] LogicalANDExpression[?In, ?Yield, ?Await] `&amp;&amp;` BitwiseORExpression[?In, ?Yield, ?Await]</ins>
</emu-grammar>

<emu-clause id="sec-binary-logical-operators-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>
LogicalANDExpression : LogicalANDExpression `&amp;&amp;` BitwiseORExpression

LogicalORExpression : LogicalORExpression `||` LogicalANDExpression

<ins>NullishExpression : NullishExpression `??` LogicalORExpression</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>

<emu-clause oldids="sec-binary-logical-operators-static-semantics-isvalidsimpleassignmenttarget" id="sec-binary-logical-operators-static-semantics-assignmenttargettype">
<h1>Static Semantics: AssignmentTargetType</h1>
<emu-see-also-para op="AssignmentTargetType"></emu-see-also-para>
<emu-grammar>
LogicalANDExpression : LogicalANDExpression `&amp;&amp;` BitwiseORExpression

LogicalORExpression : LogicalORExpression `||` LogicalANDExpression

<ins>NullishExpression : NullishExpression `??` LogicalORExpression</ins>
</emu-grammar>
<emu-alg>
1. Return ~invalid~.
</emu-alg>
</emu-clause>

<emu-clause id="sec-binary-logical-operators-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>

<emu-grammar><ins>NullishExpression : NullishExpression `??` LogicalORExpression</ins></emu-grammar>
<emu-alg>
<ins>
1. Let _lref_ be the result of evaluating |LogicalORExpression|.
1. Let _lval_ be ? GetValue(_lref_).
1. If _lval_ is *undefined* or *null*,
1. Let _rref_ be the result of evaluating |LogicalANDExpression|.
1. Return ? GetValue(_rref_).
1. Otherwise, return _lval_.
</ins>
</emu-alg>
</emu-clause>
</emu-clause>

<emu-clause id="sec-nullish-coalescing-evaluation">
<h1>Runtime Semantics: Evaluation</h1>

<emu-grammar>LogicalORExpression : LogicalORExpression `??` LogicalANDExpression</emu-grammar>
<emu-alg>
1. Let _lref_ be the result of evaluating |LogicalORExpression|.
1. Let _lval_ be ? GetValue(_lref_).
1. If _lval_ is *undefined* or *null*,
1. Let _rref_ be the result of evaluating |LogicalANDExpression|.
1. Return ? GetValue(_rref_).
1. Otherwise, return _lval_.
</emu-alg>
<emu-clause id="sec-conditional-operator">
<h1>Conditional Operator ( `? :` )</h1>
<h2>Syntax</h2>
<emu-grammar type="definition">
ConditionalExpression[In, Yield, Await] :
<del>LogicalORExpression[?In, ?Yield, ?Await]</del>
<del>LogicalORExpression[?In, ?Yield, ?Await] `?` AssignmentExpression[+In, ?Yield, ?Await] `:` AssignmentExpression[?In, ?Yield, ?Await]</del>
<ins>NullishExpression[?In, ?Yield, ?Await, ~Nullish]</ins>
<ins>NullishExpression[?In, ?Yield, ?Await, ~Nullish] `?` AssignmentExpression[+In, ?Yield, ?Await] `:` AssignmentExpression[+In, ?Yield, ?Await]</ins>
</emu-grammar>

<emu-clause id="sec-conditional-operator-static-semantics-isfunctiondefinition">
<h1>Static Semantics: IsFunctionDefinition</h1>
<emu-see-also-para op="IsFunctionDefinition"></emu-see-also-para>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : NullishExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. Return *false*.
</emu-alg>
</emu-clause>

<emu-clause oldids="sec-conditional-operator-static-semantics-isvalidsimpleassignmenttarget" id="sec-conditional-operator-static-semantics-assignmenttargettype">
<h1>Static Semantics: AssignmentTargetType</h1>
<emu-see-also-para op="AssignmentTargetType"></emu-see-also-para>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : NullishExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. Return ~invalid~.
</emu-alg>
</emu-clause>

<emu-clause id="sec-conditional-operator-runtime-semantics-evaluation">
<h1>Runtime Semantics: Evaluation</h1>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : NullishExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. <del>Let _lref_ be the result of evaluating |LogicalORExpression|.</del>
1. <ins>Let _lref_ be the result of evaluating |NullishExpression|.</ins>
1. Let _lval_ be ToBoolean(? GetValue(_lref_)).
1. If _lval_ is *true*, then
1. Let _trueRef_ be the result of evaluating the first |AssignmentExpression|.
1. Return ? GetValue(_trueRef_).
1. Else,
1. Let _falseRef_ be the result of evaluating the second |AssignmentExpression|.
1. Return ? GetValue(_falseRef_).
</emu-alg>
</emu-clause>
</emu-clause>

<emu-clause id="sec-tail-position-calls">
<h1>Tail Position Calls</h1>

<emu-clause id="sec-static-semantics-hascallintailposition">
<h1>Static Semantics: HasCallInTailPosition</h1>
<p>With parameter _call_.</p>

<emu-clause id="sec-expression-rules">
<h1>Expression Rules</h1>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : NullishExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-alg>
1. Let _has_ be HasCallInTailPosition of the first |AssignmentExpression| with argument _call_.
1. If _has_ is *true*, return *true*.
1. Return HasCallInTailPosition of the second |AssignmentExpression| with argument _call_.
</emu-alg>
<emu-grammar>
<del>ConditionalExpression : LogicalORExpression `?` AssignmentExpression `:` AssignmentExpression</del>
<ins>ConditionalExpression : NullishExpression `?` AssignmentExpression `:` AssignmentExpression</ins>
</emu-grammar>
<emu-grammar>LogicalANDExpression : LogicalANDExpression `&amp;&amp;` BitwiseORExpression</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |BitwiseORExpression| with argument _call_.
</emu-alg>
<emu-grammar>LogicalORExpression : LogicalORExpression `||` LogicalANDExpression</emu-grammar>
<emu-alg>
1. Return HasCallInTailPosition of |LogicalANDExpression| with argument _call_.
</emu-alg>
<emu-grammar>
<ins>
NullishExpression : NullishExpression `??` LogicalORExpression
</ins>
</emu-grammar>
<emu-alg>
<ins>
1. Return HasCallInTailPosition of |LogicalORExpression| with argument _call_.
</ins>
</emu-alg>
</emu-clause>
</emu-clause>
</emu-clause>

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