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Fill in missing sections of propDeps proposal and refactor
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‎build/build.js‎

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‎jsonschema-core.md‎

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Clients SHOULD be able to make requests with a "From" header so that server
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operators can contact the owner of a potentially misbehaving script.
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## A Vocabulary for Applying Subschemas
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## A Vocabulary for Applying Subschemas {#applicatorvocab}
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This section defines a vocabulary of applicator keywords that are RECOMMENDED
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for use as the basis of other vocabularies.
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the instance against this keyword, for either validation or annotation
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collection purposes, in such cases.
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##### `dependentSchemas`
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##### `dependentSchemas` {#dependent-schemas}
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This keyword specifies subschemas that are evaluated if the instance is an
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object and contains a certain property.
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Omitting this keyword has the same behavior as an empty object.
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##### `propertyDependencies`
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This keyword specifies subschemas that are evaluated if the instance is an
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object and contains a certain property with a certain string value.
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This keyword's value MUST be an object. Each value in the object MUST be an
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object whose values MUST be valid JSON Schemas.
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If the outer object key is a property in the instance and the inner object key
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is equal to the value of that property, the entire instance must validate
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against the schema. Its use is dependent on the presence and value of the
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property.
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Omitting this keyword has the same behavior as an empty object.
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### Keywords for Applying Subschemas to Child Instances
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Each of these keywords defines a rule for applying its subschema(s) to child

‎proposals/propertyDependencies.md‎

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# JSON Schema Proposal: `propertyDependencies` Keyword
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# JSON Schema: The `propertyDependencies` Keyword
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## Abstract
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This document proposes a change to the JSON Schema Core specification and
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Applicator vocabulary by adding the `propertyDependencies` keyword.
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The `propertyDependencies` keyword is a more friendly way to select between two
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or more schemas to validate an instance against than is currently supported by
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JSON Schema.
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## Status
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**Current Status**: PROPOSAL
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TODO: We should have a short standard blurb outlining the stages involved in a
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feature making its way to stable status.
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TODO: Link to a document that describes the proposal => stable process in
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detail.
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## Note to Readers
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The issues list for this proposal can be found at
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The issues list for this document can be found at
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<https://github.com/json-schema-org/json-schema-spec/issues?q=is%3Aissue+propertydependencies>.
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For additional information, see <https://json-schema.org/>.
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## Conventions and Terminology
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All conventions and terms used and defined by the JSON Schema Core specification
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also apply to this document.
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All conventions and terms used and defined by the [JSON Schema Core
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specification](../jsonschema-core.html) also apply to this document.
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## Overview
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### Problem Statement
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A common need in JSON Schema is to select between one schema or another to
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validate an instance based on the value of some property in the JSON instance.
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There are a several patterns people use to accomplish this, but they all have
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significant [problems](#problems).
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<!-- What problem exists that needs solving? -->
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OpenAPI solves this problem with the `discriminator` keyword. However, their
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approach is more oriented toward code generation concerns, is poorly specified
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when it comes to validation, and is couple to OpenAPI concepts that don't exist
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is JSON Schema. Therefore, it's necessary to define something new rather than
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adopt `discriminator`.
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### Solution
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<!-- What is the solution? -->
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The `dependentSchemas` keyword is very close to what is needed except it checks
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for the presence of a property rather than it's value. The chosen solution is to
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build on that concept to solve this problem.
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### Alternatives
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```jsonschema
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{
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"propertyDependencies": {
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"foo": {
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"aaa": { "$ref": "#/$defs/foo-aaa" }
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}
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}
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}
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```
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<!-- What other options have been considered? (summary, not detailed) -->
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The validation result is equivalent to the following schema.
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### Limitations
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<!-- Are there any limitations inherent to the proposal? -->
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```jsonschema
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{
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"if": {
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"properties": {
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"foo": { "const": "aaa" }
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},
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"required": ["foo"]
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},
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"then": { "$ref": "#/$defs/foo-aaa" }
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}
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```
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### Examples
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<!-- How will this feature be used? -->
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### Limitations
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## Proposal
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The problem of choosing an alternative based on a property value could apply for
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a value of any JSON type, but `propertyDependencies` only solves this problem
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when the value is a string. One of the main goals of this keyword is to define
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something that's intuitive enough and easy enough to use that people will
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actually use it rather than fallback to `oneOf` because it's simple. Achieving
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those goals means that some trade-offs need to be made. {{alternatives}} lists
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some alternatives that we considered.
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### Target for Change
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## A Vocabulary for Applying Subschemas
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This proposal will add the {{propertydependencies}} section contained herein as
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a subsection of JSON Schema Core, section 10.2.2 "Keywords for Applying
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Subschemas Conditionally."
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This document adds the `propertyDependencies` keyword to the
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`https://json-schema.org/vocab/applicator` [applicator
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vocabulary](../jsonschema-core.html#applicatorvocab).
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### New Keyword: `propertyDependencies` {#propertydependencies}
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### `propertyDependencies`
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This keyword specifies subschemas that are evaluated if the instance is an
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object and contains a certain property with a certain string value.
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Omitting this keyword has the same behavior as an empty object.
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## [Appendix] Problems With Existing Patterns {#problems}
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### `oneOf`/`anyOf`
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The pattern of using `oneOf` to describe a choice between two schemas has become
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ubiquitous.
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```jsonschema
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{
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"oneOf": [
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{ "$ref": "#/$defs/aaa" },
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{ "$ref": "#/$defs/bbb" }
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]
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}
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```
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However, this pattern has several shortcomings. The main problem is that it
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tends to produce confusing error messages. Some implementations employ
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heuristics to guess the user's intent and provide better messaging, but that's
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not wide-spread or consistent behavior, nor is it expected or required from
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implementations.
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This pattern is also inefficient. Generally, there is a single value in the
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object that determines which alternative to chose, but the `oneOf` pattern has
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no way to specify what that value is and therefore needs to evaluate the entire
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schema. This is made worse in that every alternative needs to be fully validated
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to ensure that only one of the alternative passes and all the others fail. This
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last problem can be avoided by using `anyOf` instead, but that pattern is much
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less used.
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### `if`/`then`
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We can describe this kind of constraint more efficiently and with with better
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error messaging by using `if`/`then`. This allows the user to explicitly specify
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the constraint to be used to select which alternative the schema should be used
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to validate the schema. However, this pattern has problems of it's own. It's
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verbose, error prone, and not particularly intuitive, which leads most people to
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avoid it.
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```jsonschema
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{
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"allOf": [
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{
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"if": {
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"properties": {
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"foo": { "const": "aaa" }
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},
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"required": ["foo"]
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},
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"then": { "$ref": "#/$defs/foo-aaa" }
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},
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{
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"if": {
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"properties": {
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"foo": { "const": "bbb" }
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},
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"required": ["foo"]
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},
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"then": { "$ref": "#/$defs/foo-bbb" }
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}
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]
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}
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```
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## [Appendix] Alternatives Considered {#alternatives}
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Here are some alternatives that were considered that support all value types.
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All examples have the same validation behavior as the examples above.
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This version uses an array of objects. Each object is a collection of the
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variables needed to express a property dependency. This doesn't fit the style of
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JSON Schema. There aren't any keywords remotely like this. It's also still too
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verbose. It's a little more intuitive than `if`/`then` and definitely less error
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prone.
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```jsonschema
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{
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"propertyDependencies": [
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{
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"propertyName": "foo",
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"propertySchema": { "const": "aaa" },
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"apply": { "$ref": "#/$defs/foo-aaa" }
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},
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{
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"propertyName": "foo",
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"propertySchema": { "const": "bbb" },
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"apply": { "$ref": "#/$defs/foo-bbb" }
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}
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]
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}
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```
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A slight variation on that example is to make it a map of keyword to dependency
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object. It's still too verbose.
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```jsonschema
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{
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"propertyDependencies": {
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"foo": [
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{
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"propertySchema": { "const": "aaa" },
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"apply": { "$ref": "#/$defs/foo-aaa" }
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},
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{
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"propertySchema": { "const": "bbb" },
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"apply": { "$ref": "#/$defs/foo-bbb" }
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}
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]
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}
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}
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```
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This one is a little more consistent with the JSON Schema style (poor keyword
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naming aside), but otherwise has all the same problems as the other examples.
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```jsonschema
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{
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"allOf": [
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{
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"propertyDependencyName": "foo",
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"propertyDependencySchema": { "const": "aaa" },
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"propertyDependencyApply": { "$ref": "#/$defs/foo-aaa" }
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},
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{
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"propertyDependencyName": "foo",
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"propertyDependencySchema": { "const": "bbb" },
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"propertyDependencyApply": { "$ref": "#/$defs/foo-bbb" }
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}
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]
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}
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```
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This one is a variation of `if` that combines `if`, `properties`, and `required`
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to reduce boilerplate. It's also essentially a variation of the previous example
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with better names. This avoids to error proneness problem, but it's still too
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verbose.
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```jsonschema
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{
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"allOf": [
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{
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"ifProperties": {
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"foo": { "const": "aaa" }
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},
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"then": { "$ref": "#/$defs/foo-aaa" }
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},
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{
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"ifProperties": {
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"foo": { "const": "bbb" }
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},
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"then": { "$ref": "#/$defs/foo-aaa" }
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}
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]
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}
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```
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All of the previous alternatives use a schema as the discriminator. This
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alternative is a little less powerful in that it can only match on exact values,
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but it successfully addresses the problems we're concerned about with the
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current approaches. The only issue with this alternative is that it's not as
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intuitive as the chosen solution.
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```jsonschema
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{
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"propertyDepenencies": {
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"foo": [
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["aaa", { "$ref": "#/$defs/foo-aaa" }],
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["bbb", { "$ref": "#/$defs/foo-bbb" }]
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]
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}
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}
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```
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## [Appendix] Change Log
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* [October 2023] Created
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## Champions
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| Champion | Company | Email | URI |
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|----------------------------|---------|-------------------------|----------------------------------|
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| Jason Desrosiers | Postman | <tbd> | <https://github.com/jdesrosiers> |
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| Champion | Company | Email | URI |
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|----------------------------|---------|----------------------|----------------------------------|
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| Jason Desrosiers | Postman | <jdesrosi@gmail.com> | <https://github.com/jdesrosiers> |

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