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Normative: Permit Symbol as WeakMap key and WeakSet entry
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Specification PR for https://github.com/tc39/proposal-symbols-as-weakmap-keys
Note that both registered and unregistered Symbols are permitted here.

Closes tc39#1194
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@littledan @acutmore
littledan authored and acutmore committed May 12, 2022
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Showing 1 changed file with 21 additions and 11 deletions.
32 changes: 21 additions & 11 deletions spec.html
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Expand Up @@ -6037,6 +6037,16 @@ <h1>
</emu-alg>
</emu-clause>

<emu-clause id="sec-hasidentity" aoid="HasIdentity">
<h1>HasIdentity ( _argument_ )</h1>
<p>The abstract operation HasIdentity takes argument _argument_ (an ECMAScript language value). It determines if _argument_ is considered to have a unique identity, enabling _argument_ to be a key of a WeakMap or entry in a WeakSet. This operation performs the following steps when called:</p>
<emu-alg>
1. If Type(_argument_) is Object, return *true*.
1. If Type(_argument_) is Symbol, return *true*.
1. Return *false*.
</emu-alg>
</emu-clause>

<emu-clause id="sec-samevalue" type="abstract operation">
<h1>
SameValue (
Expand Down Expand Up @@ -40346,11 +40356,11 @@ <h1>%SetIteratorPrototype% [ @@toStringTag ]</h1>

<emu-clause id="sec-weakmap-objects">
<h1>WeakMap Objects</h1>
<p>WeakMaps are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. In certain conditions, objects which are not live are removed as WeakMap keys, as described in <emu-xref href="#sec-weakref-execution"></emu-xref>.</p>
<p>WeakMaps are collections of key/value pairs where the keys are objects and/or symbols and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. In certain conditions, values which are not live are removed as WeakMap keys, as described in <emu-xref href="#sec-weakref-execution"></emu-xref>.</p>
<p>An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.</p>
<p>WeakMaps must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this specification is only intended to describe the required observable semantics of WeakMaps. It is not intended to be a viable implementation model.</p>
<emu-note>
<p>WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not &ldquo;leak&rdquo; memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:</p>
<p>WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object or symbol in a manner that does not &ldquo;leak&rdquo; memory resources if, in the absence of the WeakMap or WeakSet, the object or symbol otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object/symbol mapping of weak map instances to keys. Alternatively, each WeakMap may internally store its key to value mappings, but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSet:</p>
<p>Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In <i>Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97)</i>, A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, <a href="http://doi.acm.org/10.1145/263698.263733">http://doi.acm.org/10.1145/263698.263733</a>.</p>
<p>Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in Weak Tables. Journal of Universal Computer Science - J.UCS, vol. 14, no. 21, pp. 3481-3497, 2008, <a href="http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak">http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak</a></p>
</emu-note>
Expand Down Expand Up @@ -40420,7 +40430,7 @@ <h1>WeakMap.prototype.delete ( _key_ )</h1>
1. Let _M_ be the *this* value.
1. Perform ? RequireInternalSlot(_M_, [[WeakMapData]]).
1. Let _entries_ be the List that is _M_.[[WeakMapData]].
1. If Type(_key_) is not Object, return *false*.
1. If HasIdentity(_key_) is *false*, return *false*.
1. For each Record { [[Key]], [[Value]] } _p_ of _entries_, do
1. If _p_.[[Key]] is not ~empty~ and SameValue(_p_.[[Key]], _key_) is *true*, then
1. Set _p_.[[Key]] to ~empty~.
Expand All @@ -40440,7 +40450,7 @@ <h1>WeakMap.prototype.get ( _key_ )</h1>
1. Let _M_ be the *this* value.
1. Perform ? RequireInternalSlot(_M_, [[WeakMapData]]).
1. Let _entries_ be the List that is _M_.[[WeakMapData]].
1. If Type(_key_) is not Object, return *undefined*.
1. If HasIdentity(_key_) is *false*, return *undefined*.
1. For each Record { [[Key]], [[Value]] } _p_ of _entries_, do
1. If _p_.[[Key]] is not ~empty~ and SameValue(_p_.[[Key]], _key_) is *true*, return _p_.[[Value]].
1. Return *undefined*.
Expand All @@ -40454,7 +40464,7 @@ <h1>WeakMap.prototype.has ( _key_ )</h1>
1. Let _M_ be the *this* value.
1. Perform ? RequireInternalSlot(_M_, [[WeakMapData]]).
1. Let _entries_ be the List that is _M_.[[WeakMapData]].
1. If Type(_key_) is not Object, return *false*.
1. If HasIdentity(_key_) is *false*, return *false*.
1. For each Record { [[Key]], [[Value]] } _p_ of _entries_, do
1. If _p_.[[Key]] is not ~empty~ and SameValue(_p_.[[Key]], _key_) is *true*, return *true*.
1. Return *false*.
Expand All @@ -40468,7 +40478,7 @@ <h1>WeakMap.prototype.set ( _key_, _value_ )</h1>
1. Let _M_ be the *this* value.
1. Perform ? RequireInternalSlot(_M_, [[WeakMapData]]).
1. Let _entries_ be the List that is _M_.[[WeakMapData]].
1. If Type(_key_) is not Object, throw a *TypeError* exception.
1. If HasIdentity(_key_) is *false*, throw a *TypeError* exception.
1. For each Record { [[Key]], [[Value]] } _p_ of _entries_, do
1. If _p_.[[Key]] is not ~empty~ and SameValue(_p_.[[Key]], _key_) is *true*, then
1. Set _p_.[[Value]] to _value_.
Expand All @@ -40494,8 +40504,8 @@ <h1>Properties of WeakMap Instances</h1>

<emu-clause id="sec-weakset-objects">
<h1>WeakSet Objects</h1>
<p>WeakSets are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. In certain conditions, objects which are not live are removed as WeakSet elements, as described in <emu-xref href="#sec-weakref-execution"></emu-xref>.</p>
<p>An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.</p>
<p>WeakSets are collections of objects and/or symbols. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific value, but no mechanism is provided for enumerating the values it holds. In certain conditions, values which are not live are removed as WeakSet elements, as described in <emu-xref href="#sec-weakref-execution"></emu-xref>.</p>
<p>An implementation may impose an arbitrarily determined latency between the time a value contained in a WeakSet becomes inaccessible and the time when the value is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular value that does not require the observer to present the observed value.</p>
<p>WeakSets must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this specification is only intended to describe the required observable semantics of WeakSets. It is not intended to be a viable implementation model.</p>
<emu-note>
<p>See the NOTE in <emu-xref href="#sec-weakmap-objects"></emu-xref>.</p>
Expand Down Expand Up @@ -40564,7 +40574,7 @@ <h1>WeakSet.prototype.add ( _value_ )</h1>
<emu-alg>
1. Let _S_ be the *this* value.
1. Perform ? RequireInternalSlot(_S_, [[WeakSetData]]).
1. If Type(_value_) is not Object, throw a *TypeError* exception.
1. If HasIdentity(_value_) is *false*, throw a *TypeError* exception.
1. Let _entries_ be the List that is _S_.[[WeakSetData]].
1. For each element _e_ of _entries_, do
1. If _e_ is not ~empty~ and SameValue(_e_, _value_) is *true*, then
Expand All @@ -40585,7 +40595,7 @@ <h1>WeakSet.prototype.delete ( _value_ )</h1>
<emu-alg>
1. Let _S_ be the *this* value.
1. Perform ? RequireInternalSlot(_S_, [[WeakSetData]]).
1. If Type(_value_) is not Object, return *false*.
1. If HasIdentity(_value_) is *false*, return *false*.
1. Let _entries_ be the List that is _S_.[[WeakSetData]].
1. For each element _e_ of _entries_, do
1. If _e_ is not ~empty~ and SameValue(_e_, _value_) is *true*, then
Expand All @@ -40605,7 +40615,7 @@ <h1>WeakSet.prototype.has ( _value_ )</h1>
1. Let _S_ be the *this* value.
1. Perform ? RequireInternalSlot(_S_, [[WeakSetData]]).
1. Let _entries_ be the List that is _S_.[[WeakSetData]].
1. If Type(_value_) is not Object, return *false*.
1. If HasIdentity(_value_) is *false*, return *false*.
1. For each element _e_ of _entries_, do
1. If _e_ is not ~empty~ and SameValue(_e_, _value_) is *true*, return *true*.
1. Return *false*.
Expand Down

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