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Fixes for dynamic scope and keywords #1041

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Dec 1, 2020
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Fixes for dynamic scope and keywords #1041

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71 changes: 25 additions & 46 deletions jsonschema-core.xml
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Original file line number Diff line number Diff line change
Expand Up @@ -654,25 +654,17 @@
appear in a schema resource's root schema.
</t>
<t>
Other keywords may take into account the dynamic scope that
exists during the evaluation of a schema, typically together
with an instance document. The outermost dynamic scope is the
root schema of the schema document in which processing begins.
The path from this root schema to any particular keyword (that
includes any "$ref" and "$dynamicRef" keywords that may have
been resolved) is considered the keyword's "validation path."
<cref>
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I removed the cref here because I'm mostly sure it's just left over from the recursive keywords and doesn't apply any more. At the least, it's not correct that, $dyanmicAnchor is only allowed in the root schema. If someone thinks there's still some value in this cref, let me know and we'll try to correct it instead of delete it.

Or should this be the schema object at which processing
begins, even if it is not a root? This has some implications
for the case where "$dynamicAnchor" is only allowed in the
root schema but processing begins in a subschema.
</cref>
Other keywords may take into account the dynamic scope that exists during the
evaluation of a schema. The outermost dynamic scope is the root schema of the
schema document in which processing begins. The path from this root schema to
any particular keyword (that includes any "$ref" and "$dynamicRef" keywords that
may have been resolved) is considered the keyword's "validation path."
</t>
<t>
Lexical and dynamic scopes align until a reference keyword
is encountered. While following the reference keyword moves processing
from one lexical scope into a different one, from the perspective
of dynamic scope, following reference is no different from descending
of dynamic scope, following a reference is no different from descending
into a subschema present as a value. A keyword on the far side of
that reference that resolves information through the dynamic scope
will consider the originating side of the reference to be their
Expand Down Expand Up @@ -741,7 +733,7 @@
<t>
Canonical schema URIs MUST NOT change while processing an instance, but
keywords that affect URI-reference resolution MAY have behavior that
is only fully determined at runtime.
is only fully determined dynamically.
</t>
<t>
While custom identifier keywords are possible, vocabulary designers should
Expand Down Expand Up @@ -798,12 +790,10 @@
of an instance against a schema.
</t>
<t>
For some by-reference applicators, such as
<xref target="ref">"$ref"</xref>, the referenced schema can be determined
by static analysis of the schema document's lexical scope. Others,
such as "$dynamicRef" (with "$dynamicAnchor"), may make use of dynamic
scoping, and therefore only be resolvable in the process of evaluating
the schema with an instance.
For some by-reference applicators, such as <xref target="ref">"$ref"</xref>, the
referenced schema can be determined by static analysis of the schema document's
lexical scope. Others, such as "$dynamicRef" (with "$dynamicAnchor"), are only
resolvable with knowledge of all the schemas in it's dynamic scope.
</t>
</section>
</section>
Expand Down Expand Up @@ -1397,7 +1387,6 @@
The "$anchor" and "$dynamicAnchor" keywords are used to specify such
fragments. They are identifier keywords that can only be used to create
plain name fragments, rather than absolute URIs as seen with "$id".
The behavior of the created fragment is identical for both keywords.
</t>
<t>
The base URI to which the resulting fragment is appended is the canonical
Expand Down Expand Up @@ -1471,51 +1460,41 @@
</cref>
</t>
<t>
The value of the "$ref" property MUST be a string which is a URI-Reference.
The value of the "$ref" keyword MUST be a string which is a URI-Reference.
Resolved against the current URI base, it produces the URI of the schema
to apply. This resolution is safe to perform on schema load, as the
process of evaluating an instance cannot change how the reference resolves.
to apply. This resolution is safe to perform on schema load as neither other
schemas nor the instance can change how the reference resolves.
</t>
</section>

<section title='Dynamic References with "$dynamicRef"' anchor="dynamic-ref">
<t>
The "$dynamicRef" keyword is an applicator that allows for deferring the
full resolution until runtime, at which point it is resolved each time it is
encountered while evaluating an instance.
The "$dynamicRef" keyword is an applicator that is used to reference a
dynamically identified schema.
</t>
<t>
Together with "$dynamicAnchor", "$dynamicRef" implements a cooperative
extension mechanism that is primarily useful with recursive schemas
(schemas that reference themselves). Both the extension point and the
runtime-determined extension target are defined with "$dynamicAnchor",
and only exhibit runtime dynamic behavior when referenced with
"$dynamicRef".
extension target are defined with "$dynamicAnchor", and only exhibit dynamic
behavior when referenced with "$dynamicRef".
</t>
<t>
The value of the "$dynamicRef" property MUST be a string which is
a URI-Reference. Resolved against the current URI base, it produces
the URI used as the starting point for runtime resolution. This initial
resolution is safe to perform on schema load.
The value of the "$dynamicRef" property MUST be a string which is a
URI-Reference. Resolved against the current URI base, it produces the URI used
as the starting point for resolution. This initial resolution is safe to perform
on schema load.
</t>
<t>
If the initially resolved starting point URI includes a fragment that
was created by the "$dynamicAnchor" keyword, the initial URI MUST be
replaced by the URI (including the fragment) for the outermost schema
resource in the <xref target="scopes">dynamic scope</xref> that defines
an identically named fragment with "$dynamicAnchor".
<cref>
Requiring both the initial and final URI fragment to be defined
by "$dynamicAnchor" ensures that the more common "$anchor"
never unexpectedly changes the dynamic resolution process
due to a naming conflict across resources. Users of
"$dynamicAnchor" are expected to be aware of the possibility
of such name collisions, while users of "$anchor" are not.
</cref>
Comment on lines -1507 to -1514
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I removed this cref because it appears to be wrong. There's no way that an $anchor can change dynamic scope resolution and even if it could, it's not clear how bookending could possibly solve that problem. If anyone knows what this means, please share and I'll put it back in.

</t>
<t>
Otherwise, its behavior is identical to "$ref", and no runtime
resolution is needed.
Otherwise, its behavior is identical to "$ref", and no dynamic resolution is
needed.
</t>
<t>
For a full example using these keyword, see appendix
Expand Down Expand Up @@ -3503,7 +3482,7 @@ https://example.com/schemas/common#/$defs/count/minimum
{
"$schema": "https://json-schema.org/draft/2020-11/schema",
"$id": "https://example.com/strict-tree",
"$dynamicAnchor": node,
"$dynamicAnchor": "node",

"$ref": "tree",
"unevaluatedProperties": false
Expand Down