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Allow <let/> in <constraint/> without requiring a sibling constraint #552

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Allow <let/> in <constraint/> without requiring a sibling constraint #552

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b471a22
Allow only let in constraint for metaschema module for #548
aj-stein-nist Feb 22, 2024
27c7a2b
Allow only let in constraint for XML schema for #548
aj-stein-nist Feb 22, 2024
bb3f957
Adjust grammar for constraint eval for let spec for #548
aj-stein-nist Mar 15, 2024
306e30b
Adjust spec language for #548
aj-stein-nist Mar 15, 2024
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12 changes: 3 additions & 9 deletions schema/metaschema/metaschema-module-metaschema.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -837,12 +837,10 @@
<define-assembly name="assembly-constraints">
<use-name>constraint</use-name>
<model>
<assembly ref="constraint-let-expression" max-occurs="unbounded">
<group-as name="lets" in-json="ARRAY"/>
</assembly>
<choice-group min-occurs="1" max-occurs="unbounded">
<group-as name="rules" in-json="ARRAY"/>
<discriminator>object-type</discriminator>
<assembly ref="constraint-let-expression"/>
<assembly ref="targeted-allowed-values-constraint"/>
<assembly ref="targeted-expect-constraint"/>
<assembly ref="targeted-index-has-key-constraint"/>
Expand All @@ -857,12 +855,10 @@
<define-assembly name="field-constraints">
<use-name>constraint</use-name>
<model>
<assembly ref="constraint-let-expression" max-occurs="unbounded">
<group-as name="lets" in-json="ARRAY"/>
</assembly>
<choice-group min-occurs="1" max-occurs="unbounded">
<group-as name="rules" in-json="ARRAY"/>
<discriminator>object-type</discriminator>
<assembly ref="constraint-let-expression"/>
<assembly ref="targeted-allowed-values-constraint"/>
<assembly ref="targeted-expect-constraint"/>
<assembly ref="targeted-index-has-key-constraint"/>
Expand All @@ -874,12 +870,10 @@
<define-assembly name="flag-constraints">
<use-name>constraint</use-name>
<model>
<assembly ref="constraint-let-expression" max-occurs="unbounded">
<group-as name="lets" in-json="ARRAY"/>
</assembly>
<choice-group min-occurs="1" max-occurs="unbounded">
<group-as name="rules" in-json="ARRAY"/>
<discriminator>object-type</discriminator>
<assembly ref="constraint-let-expression"/>
<assembly ref="flag-allowed-values"/>
<assembly ref="flag-expect"/>
<assembly ref="flag-index-has-key"/>
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29 changes: 13 additions & 16 deletions schema/xml/metaschema.xsd
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Original file line number Diff line number Diff line change
Expand Up @@ -1167,20 +1167,14 @@
<xs:attribute name="expression" type="MetaschemaPathType" use="required"/>
</xs:complexType>

<xs:group name="LetExpressionGroup">
<xs:sequence>
<xs:element name="let" type="ConstraintLetType" minOccurs="0" maxOccurs="unbounded">
<xs:annotation>
<xs:documentation>Declares a variable whose name is bound to the result of the Metapath expression. These variables can be used in target and other Metapath expressions used in constrains that are declared in the same context or in constraints declared in a child assembly, field, or flag.</xs:documentation>
</xs:annotation>
</xs:element>
</xs:sequence>
</xs:group>

<xs:complexType name="DefineFlagConstraintsType">
<xs:sequence>
<xs:group ref="LetExpressionGroup"/>
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:choice maxOccurs="unbounded">
<xs:element name="let" type="ConstraintLetType">
<xs:annotation>
<xs:documentation>Declares a variable whose name is bound to the result of the Metapath expression. These variables can be used in target and other Metapath expressions used in constrains that are declared in the same context or in constraints declared in a child assembly, field, or flag.</xs:documentation>
</xs:annotation>
</xs:element>
<xs:element minOccurs="0" name="allowed-values" type="AllowedValuesType">
<xs:annotation>
<xs:documentation>Constrains the allowed values for the flag.</xs:documentation>
Expand Down Expand Up @@ -1208,6 +1202,11 @@

<xs:group name="TargetedCommonConstraintsGroup">
<xs:choice>
<xs:element name="let" type="ConstraintLetType">
<xs:annotation>
<xs:documentation>Declares a variable whose name is bound to the result of the Metapath expression. These variables can be used in target and other Metapath expressions used in constrains that are declared in the same context or in constraints declared in a child assembly, field, or flag.</xs:documentation>
</xs:annotation>
</xs:element>
<xs:element name="allowed-values" type="TargetedAllowedValuesConstraintType">
<xs:annotation>
<xs:documentation>Constrains the allowed values for the flag or field referenced by the scope attribute.</xs:documentation>
Expand Down Expand Up @@ -1253,8 +1252,7 @@

<xs:complexType name="DefineFieldConstraintsType">
<xs:sequence>
<xs:group ref="LetExpressionGroup"/>
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:choice minOccurs="1" maxOccurs="unbounded">
<xs:group ref="TargetedCommonConstraintsGroup"/>
</xs:choice>
<xs:element name="remarks" type="RemarksType" minOccurs="0"/>
Expand All @@ -1263,8 +1261,7 @@

<xs:complexType name="DefineAssemblyConstraintsType">
<xs:sequence>
<xs:group ref="LetExpressionGroup"/>
<xs:choice minOccurs="0" maxOccurs="unbounded">
<xs:choice minOccurs="1" maxOccurs="unbounded">
<xs:group ref="TargetedCommonConstraintsGroup"/>
<xs:group ref="AssemblyConstraintsGroup"/>
</xs:choice>
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4 changes: 3 additions & 1 deletion website/content/specification/syntax/constraints.md
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Expand Up @@ -200,7 +200,9 @@ A `let` statement has a REQUIRED `@var` flag, which defines the variable name.

A `let` statement has a REQUIRED `@expression` flag, which defines an [Metapath expression](/specification/syntax/metapath), whose result is used to define the variable's value in the evaluation context.

During constraint evaluation, each `let` statement MUST be evaluated in encounter order. If a previous variable is bound with the same name in the evaluation context, the new value MUST bound in a sub-context to avoid side effects. This sub-context MUST be made available to any constraints following the `let` statement declaration, and to any constraints defined on child nodes of the current context.
During constraint evaluation, each `let` statement MUST be evaluated in encountered order. If a previous variable is bound with the same name in the evaluation context, the new value MUST bound in a sub-context to avoid side effects. This sub-context MUST be made available to any constraints following the `let` statement declaration, and to any constraints defined on child nodes of the current context.
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I am not sure if this is blocking and/or material to this PR or not.

This is technically out of scope for #548, but given today's conversation, do want to address this concern within this PR and adjust given discussion and around processing order here and now as it has been decided with scope rebinding being disallowed or defer that? Note, the diff on this paragraph is the result of me correcting encounter to encountered to be more grammatical. I think this was the intent.

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Ouch, I had a comment here, somehow I dropped it.

Can we codify a rule that no two let element siblings have the same name?

Can we write a constraint for that?

@david-waltermire also grateful for your input here.


A `let` statement MAY be defined in a constraint assembly by itself, before, or after other constraint types. One or more `let` statements SHOULD be declared before any other constraint types as a best practice, but it is not required.

During evaluation, when a variable is bound for a `let` statement, the variables value MUST be set to the result of evaluating the `@expression` using the current node as the Metapath evaluation focus.

Expand Down
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