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Editorial: Model a binding as a Record
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@jmdyck
jmdyck committed May 13, 2021
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Expand Up @@ -8744,22 +8744,108 @@ <h1>Declarative Environment Records</h1>
[[Bindings]]
</td>
<td>
List of bindings
List of DeclarativeBindingRecord
</td>
<td>
Satisfies the invariant that no two bindings in [[Bindings]] are bindings for the same name.
Satisfies the invariant that no two records in [[Bindings]] have the same [[BoundName]].
</td>
</tr>
</tbody>
</table>
</emu-table>

<p>A <dfn>DeclarativeBindingRecord</dfn> has the following fields:</p>
<emu-table id="table-fields-of-declarative-binding-records" caption="DeclarativeBindingRecord Fields">
<table>
<tbody>
<tr>
<th>
Field Name
</th>
<th>
Value
</th>
<th>
Meaning
</th>
</tr>
<tr>
<td>
[[BoundName]]
</td>
<td>
a String
</td>
<td>
the name being bound.
</td>
</tr>
<tr>
<td>
[[IsInitialized]]
</td>
<td>
a Boolean
</td>
<td>
indicates whether the binding is initialized.
</td>
</tr>
<tr>
<td>
[[BoundValue]]
</td>
<td>
an ECMAScript language value
</td>
<td>
the value that the name is bound to.
</td>
</tr>
<tr>
<td>
[[IsMutable]]
</td>
<td>
a Boolean
</td>
<td>
if *true*, indicates that the binding is mutable.
</td>
</tr>
<tr>
<td>
[[IsDeletable]]
</td>
<td>
a Boolean
</td>
<td>
if *true*, indicates that the binding may be deleted by a subsequent DeleteBinding call.
</td>
</tr>
<tr>
<td>
[[IsStrict]]
</td>
<td>
a Boolean
</td>
<td>
if *true*, indicates that the binding is a strict binding.
</td>
</tr>
</tbody>
</table>
</emu-table>

<p>The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.</p>

<emu-clause id="sec-declarative-environment-records-hasbinding-n">
<h1>HasBinding ( _N_ )</h1>
<p>The HasBinding concrete method of a declarative Environment Record _envRec_ takes argument _N_ (a String). It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:</p>
<emu-alg>
1. If _envRec_.[[Bindings]] contains a binding for the name that is the value of _N_, return *true*.
1. If _envRec_.[[Bindings]] contains a DeclarativeBindingRecord whose [[BoundName]] field is equal to the value of _N_, return *true*.
1. Return *false*.
</emu-alg>
</emu-clause>
Expand All @@ -8769,7 +8855,7 @@ <h1>CreateMutableBinding ( _N_, _D_ )</h1>
<p>The CreateMutableBinding concrete method of a declarative Environment Record _envRec_ takes arguments _N_ (a String) and _D_ (a Boolean). It creates a new mutable binding for the name _N_ that is uninitialized. A binding must not already exist in this Environment Record for _N_. If _D_ has the value *true*, the new binding is marked as being subject to deletion. It performs the following steps when called:</p>
<emu-alg>
1. Assert: _envRec_.HasBinding(_N_) is *false*.
1. Let _binding_ be a mutable binding for _N_ and record that it is uninitialized. If _D_ is *true*, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
1. Let _binding_ be DeclarativeBindingRecord { [[BoundName]]: _N_, [[IsInitialized]]: *false*, [[BoundValue]]: *undefined*, [[IsMutable]]: *true*, [[IsDeletable]]: _D_, [[IsStrict]]: *false* }.
1. Append _binding_ to _envRec_.[[Bindings]].
1. Return NormalCompletion(~empty~).
</emu-alg>
Expand All @@ -8780,7 +8866,7 @@ <h1>CreateImmutableBinding ( _N_, _S_ )</h1>
<p>The CreateImmutableBinding concrete method of a declarative Environment Record _envRec_ takes arguments _N_ (a String) and _S_ (a Boolean). It creates a new immutable binding for the name _N_ that is uninitialized. A binding must not already exist in this Environment Record for _N_. If _S_ has the value *true*, the new binding is marked as a strict binding. It performs the following steps when called:</p>
<emu-alg>
1. Assert: _envRec_.HasBinding(_N_) is *false*.
1. Let _binding_ be an immutable binding for _N_ and record that it is uninitialized. If _S_ is *true*, record that the newly created binding is a strict binding.
1. Let _binding_ be DeclarativeBindingRecord { [[BoundName]]: _N_, [[IsInitialized]]: *false*, [[BoundValue]]: *undefined*, [[IsMutable]]: *false*, [[IsDeletable]]: *false*, [[IsStrict]]: _S_ }.
1. Append _binding_ to _envRec_.[[Bindings]].
1. Return NormalCompletion(~empty~).
</emu-alg>
Expand All @@ -8791,10 +8877,10 @@ <h1>InitializeBinding ( _N_, _V_ )</h1>
<p>The InitializeBinding concrete method of a declarative Environment Record _envRec_ takes arguments _N_ (a String) and _V_ (an ECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument _N_ to the value of argument _V_. An uninitialized binding for _N_ must already exist. It performs the following steps when called:</p>
<emu-alg>
1. Assert: _envRec_.HasBinding(_N_) is *true*.
1. Let _binding_ be the binding for _N_ in _envRec_.[[Bindings]].
1. Assert: _binding_ is uninitialized.
1. Set the bound value of _binding_ to _V_.
1. <emu-not-ref>Record</emu-not-ref> that _binding_ has been initialized.
1. Let _binding_ be the DeclarativeBindingRecord in _envRec_.[[Bindings]] whose [[BoundName]] field equals _N_.
1. Assert: _binding_.[[IsInitialized]] is *false*.
1. Set _binding_.[[BoundValue]] to _V_.
1. Set _binding_.[[IsInitialized]] to *true*.
1. Return NormalCompletion(~empty~).
</emu-alg>
</emu-clause>
Expand All @@ -8808,10 +8894,10 @@ <h1>SetMutableBinding ( _N_, _V_, _S_ )</h1>
1. Perform _envRec_.CreateMutableBinding(_N_, *true*).
1. Perform _envRec_.InitializeBinding(_N_, _V_).
1. Return NormalCompletion(~empty~).
1. Let _binding_ be the binding for _N_ in _envRec_.[[Bindings]].
1. If _binding_ is a strict binding, set _S_ to *true*.
1. If _binding_ has not yet been initialized, throw a *ReferenceError* exception.
1. Else if _binding_ is a mutable binding, change its bound value to _V_.
1. Let _binding_ be the DeclarativeBindingRecord in _envRec_.[[Bindings]] whose [[BoundName]] field equals _N_.
1. If _binding_.[[IsStrict]] is *true*, set _S_ to *true*.
1. If _binding_.[[IsInitialized]] is *false*, throw a *ReferenceError* exception.
1. Else if _binding_.[[IsMutable]] is *true*, change its bound value to _V_.
1. Else,
1. Assert: This is an attempt to change the value of an immutable binding.
1. If _S_ is *true*, throw a *TypeError* exception.
Expand All @@ -8828,9 +8914,9 @@ <h1>GetBindingValue ( _N_, _S_ )</h1>
<p>The GetBindingValue concrete method of a declarative Environment Record _envRec_ takes arguments _N_ (a String) and _S_ (a Boolean). It returns the value of its bound identifier whose name is the value of the argument _N_. If the binding exists but is uninitialized a *ReferenceError* is thrown, regardless of the value of _S_. It performs the following steps when called:</p>
<emu-alg>
1. Assert: _envRec_.HasBinding(_N_) is *true*.
1. Let _binding_ be the binding for _N_ in _envRec_.[[Bindings]].
1. If _binding_ is an uninitialized binding, throw a *ReferenceError* exception.
1. Return the value currently bound in _binding_.
1. Let _binding_ be the DeclarativeBindingRecord in _envRec_.[[Bindings]] whose [[BoundName]] field equals _N_.
1. If _binding_.[[IsInitialized]] is *false*, throw a *ReferenceError* exception.
1. Return _binding_.[[BoundValue]].
</emu-alg>
</emu-clause>

Expand All @@ -8839,8 +8925,8 @@ <h1>DeleteBinding ( _N_ )</h1>
<p>The DeleteBinding concrete method of a declarative Environment Record _envRec_ takes argument _N_ (a String). It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:</p>
<emu-alg>
1. Assert: _envRec_.HasBinding(_N_) is *true*.
1. Let _binding_ be the binding for _N_ in _envRec_.[[Bindings]].
1. If _binding_ cannot be deleted, return *false*.
1. Let _binding_ be the DeclarativeBindingRecord in _envRec_.[[Bindings]] whose [[BoundName]] field equals _N_.
1. If _binding_.[[IsDeletable]] is *false*, return *false*.
1. Remove _binding_ from _envRec_.[[Bindings]].
1. Return *true*.
</emu-alg>
Expand Down Expand Up @@ -9598,6 +9684,46 @@ <h1>Module Environment Records</h1>
</tbody>
</table>
</emu-table>
<p>The [[Bindings]] of a Module Environment Record can include ImportDeclarativeBindingRecords. These have all the fields of DeclarativeBindingRecords, plus the additional state fields listed in the following table.</p>
<emu-table id="table-additional-fields-of-importdeclarativebindingrecords" caption="Additional Fields of ImportDeclarativeBindingRecords">
<table>
<tbody>
<tr>
<th>
Field Name
</th>
<th>
Value
</th>
<th>
Meaning
</th>
</tr>
<tr>
<td>
[[TargetModuleRec]]
</td>
<td>
a Module Record
</td>
<td>
?
</td>
</tr>
<tr>
<td>
[[TargetName]]
</td>
<td>
a String
</td>
<td>
the name of a binding that exists in [[TargetModuleRec]]'s module Environment Record.
</td>
</tr>
</tbody>
</table>
</emu-table>
<p>The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:</p>

<emu-clause id="sec-module-environment-records-getbindingvalue-n-s">
Expand All @@ -9606,14 +9732,15 @@ <h1>GetBindingValue ( _N_, _S_ )</h1>
<emu-alg>
1. Assert: _S_ is *true*.
1. Assert: _envRec_.HasBinding(_N_) is *true*.
1. Let _binding_ be the binding for _N_ in _envRec_.[[Bindings]].
1. If _binding_ is an indirect binding, then
1. Let _M_ and _N2_ be the indirection values provided when this binding for _N_ was created.
1. Let _binding_ be the DeclarativeBindingRecord in _envRec_.[[Bindings]] whose [[BoundName]] field equals _N_.
1. If _binding_ is an ImportDeclarativeBindingRecord, then
1. Let _M_ be _binding_.[[TargetModuleRec]].
1. Let _N2_ be _binding_.[[TargetName]].
1. Let _targetEnv_ be _M_.[[Environment]].
1. If _targetEnv_ is *undefined*, throw a *ReferenceError* exception.
1. Return ? _targetEnv_.GetBindingValue(_N2_, *true*).
1. If _binding_ is an uninitialized binding, throw a *ReferenceError* exception.
1. Return the value currently bound in _binding_.
1. If _binding_.[[IsInitialized]] is *false*, throw a *ReferenceError* exception.
1. Return _binding_.[[BoundValue]].
</emu-alg>
<emu-note>
<p>_S_ will always be *true* because a |Module| is always strict mode code.</p>
Expand Down Expand Up @@ -9654,7 +9781,7 @@ <h1>CreateImportBinding ( _N_, _M_, _N2_ )</h1>
1. Assert: _envRec_.HasBinding(_N_) is *false*.
1. Assert: _M_ is a Module Record.
1. Assert: When _M_.[[Environment]] is instantiated it will have a direct binding for _N2_.
1. Let _binding_ be an immutable indirect binding for _N_ that references _M_ and _N2_ as its target binding and record that the binding is initialized.
1. Let _binding_ be ImportDeclarativeBindingRecord { [[BoundName]]: _N_, [[IsInitialized]]: *true*, [[BoundValue]]: *undefined*, [[IsMutable]]: *false*, [[IsDeletable]]: *false*, [[IsStrict]]: *false*, [[TargetModuleRec]]: _M_, [[TargetName]]: _N2_ }.
1. Append _binding_ to _envRec_.[[Bindings]].
1. Return NormalCompletion(~empty~).
</emu-alg>
Expand Down Expand Up @@ -42908,8 +43035,8 @@ <h1>Changes to BlockDeclarationInstantiation</h1>
</emu-alg>
<p>During BlockDeclarationInstantiation the following steps are performed in place of step <emu-xref href="#step-blockdeclarationinstantiation-initializebinding"></emu-xref>:</p>
<emu-alg replaces-step="step-blockdeclarationinstantiation-initializebinding">
1. Let _binding_ be the binding for _fn_ in _env_.[[Bindings]].
1. If _binding_ is an uninitialized binding, then
1. Let _binding_ be the DeclarativeBindingRecord in _env_.[[Bindings]] whose [[BoundName]] field equals _fn_.
1. If _binding_.[[IsInitialized]] is *false*, then
1. Perform _env_.InitializeBinding(_fn_, _fo_).
1. Else,
1. Assert: _d_ is a |FunctionDeclaration|.
Expand Down

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