Support transforming Draft3 schemas

[jviotti]

To support upgrading arbitrary draft 3 schemas, we need to define rules to upgrade draft 3 to draft 4.

[jviotti]

I have almost zero experience with draft3, so I'll be pasting my findings here so anybody can correct me if I'm getting any of it wrong.

[jviotti]

One key piece of Alterschema is teaching the tool how to "walk" over all the subschemas of a given schema by definition all possible applicator keywords. From skimming https://datatracker.ietf.org/doc/html/draft-zyp-json-schema-03, all the keywords that take subschemas in some way are:

• properties
• patternProperties
• additionalProperties
• items
• additionalItems
• dependencies
• extends

See #86 for my initial definition take.

[jviotti]

As per the rules, these are my findings so far (I'll be updating this comment with new findings):

• type supports any, which was removed in Draft4. We should take care of it both on the string and array form of type (Upgrade draft3 type: any #89)
• Draft3 has disallow, which was removed in Draft4
• Draft3 has divisibleBy, which I presume must be reworked as multipleOf (Upgrade draft3 divisibleBy to multipleOf #90)
• Draft3 has extends, which was removed in Draft4
• The format keyword has some differences. Also, it seems that format was optional in draft3
  • date, time, utc-millisec, regex, color, style and phone were removed in draft4
  • ip-address was renamed to ipv4
  • host-name was renamed to hostname
• The required keyword use to be a boolean at the property level

[jeremyfiel]

I'm going to mix in a bit of draft 2 and 3 references to build a timeline of some of the keywords.

I'm no expert, so please correct me if i'm wrong.

• optional keyword accepts boolean in draft 2, replaced by required in draft 3, required updated to accept array of properties in draft 04
• maxDecimal in draft 2 replaced by divisibleBy in draft 3 and replaced again by multipleOf in draft-04
• maximumCanEqual in draft 2 accepts boolean, replaced by exclusiveMaximum in draft 3, boolean
• minimumCanEqual in draft 2 accepts boolean, replaced by exclusiveMinimum in draft 3, boolean
• uniqueItems added in draft 2 accepts boolean
• additionalItems added to type: array in draft 3, accepts boolean or schema
• alternate removed in draft 3
• patternProperties added in draft 3, expects regex
• id added in draft 3, expects uri
• $ref added in draft 3, expects uri
• $schema added in draft 3, expects uri
• requires replaced with dependencies in draft 3

 **Dependencies**
  Two types available: 
  Simple Dependency  If the dependency value is a string, then the
  instance object MUST have a property with the same name as the
  dependency value.  If the dependency value is an array of strings,
  then the instance object MUST have a property with the same name
  as each string in the dependency value's array.

  Schema Dependency  If the dependency value is a schema, then the
  instance object MUST be valid against the schema

[jeremyfiel]

@jviotti You mentioned other work on adjacent projects would support this work, has there been any movement on those projects?

[jviotti]

@jeremyfiel Yes, there is! I'm implementing some dialect-agnostic JSON Schema foundations here: https://www.jsontoolkit.org. The idea is for that project to eventually replace the engine we have here, so we can:

• Better traverse schemas in a dialect agnostic way, even schemas embedding schemas of different dialects
• Perform schema bundling
• Allow for more expressive rules that can handle some edge cases from Draft 3 that are not representable right now
• etc

Feel free to watch that repo. Definitely not forgetting about this issue. I promise it will be fixed once those things are in place.