Add logical "if" schema

[TaylanUB]

It would be great if there were a logical if schema in the vein of and/or/not. It could look like

{ "if": [ conditional, consequent ] }

whose semantics would be exactly equivalent to:

{ "or": [ { "not": conditional },
          { "and": [ conditional, consequent ] } ] }

Thank you!

[davek2]

How would this be used? Schemas are declarative languages, not procedural - there are no variables and no flow of control. "and" and "or" do not exist as keywords in JSON Schema, although schema validator tools would use the "or" operator to evaluate the set union:

{"type": ["number", "string"]}

"Not" exists, but not as a logical operator, it means set complementation:

"not": { "type": "string" } 

An example illustrating how "if" would be used in a schema language would be helpful.

[TaylanUB]

David Kay notifications@github.com writes:

How would this be used? Schemas are declarative languages, not
procedural - there are no variables and no flow of control. "and" and
"or" do not exist in JSON Schema; "not" exists, but not as a logical
operator, it is more like unary negation:

{ "not": { "type": "string" } }

validates all values except strings. An example illustrating how this
would be used to accomplish a schema-related task would be helpful.

Oops, with 'and' and 'or' I meant 'allOf' and 'anyOf'.

Propositional logic is declarative, and has 'if'. The proposition

If cats are blue, then the moon is made of cheese.

evaluates to true, because the proposition "cats are blue" is false.
(See also "vacuous truth".) The proposition

If cats have fur, then dogs have four legs.

evaluates to true, because both sub-propositions are true.

Anyhow, here's a useful schema example:

{ "type": "object",
  "required": [ "productType" ],
  "properties":
    { "productType": { "enum": [ "book", "vhs" ] },
      "numberOfPages": { "type": "integer" },
      "duration": { "type": "number" } },
  "allOf":
    [ { "if": [ { "properties": { "productType": { "enum": [ "book" ] } } },
                { "required": [ "numberOfPages" ] } ] },
      { "if": [ { "properties": { "productType": { "enum": [ "vhs" ] } } },
                { "required": [ "duration" ] } ] } ] }

This validates

{ "productType": "book",
  "numberOfPages": 242 }

and

{ "productType": "vhs",
  "duration": 9330 }

but not

{ "productType": "book",
  "duration": 9430 }

or

{ "productType": "vhs",
  "numberOfPages": 230 }

which is pretty neat!

To repeat, the semantics of

{ "if": [ conditional, consequent ] }

should be equivalent to

{ "anyOf": [ { "not": conditional },
             { "allOf": [ conditional, consequent ] } ] }

although if my logic serves me right, this can actually be simplified to

{ "anyOf": [ { "not": conditional },
             consequent ] }

which is only slightly more verbose than the proposed "if", but the "if"
makes the intent a lot clearer.

Taylan

[epoberezkin]

See switch proposal, implemented in ajv (with v5 option).

[TaylanUB]

Ah, neat! I had searched through the issues a bit but didn't see that. Closing this.

[awwright]

@epoberezkin Aside, note that the next release won't be including any new functionality or change in semantics; and probably won't even be named v5 depending on how the IETF accepts the Internet-draft.