Another JSON Schema Gripe

Recently I wrote of my disgruntlement with JSON Schema. Since then I’ve learned that its authors plan more work, and that there are several other efforts to build a schema facility for JSON. This note is just a complaint about a particular use-case, with the hope that it might inform these efforts.

Here’s the problem; the language I’m building includes a big object whose fields are also objects, and each of these child objects has a Type field, whose value is, in effect, an enum; a constrained set of string values. The rest of the fields in each child object depend on the Type value.

Like so:

{
  "o1": {
    "Type": "Animal",
    ... other fields specific to Animal ...
  },
  "o2": {
    "Type": "Vegetable",
    ... other fields specific to Vegetable ...
  },
  "o3": {
    "Type": "Mineral",
    ... other fields specific to Mineral
  }
}

In JSON Schema it’s easy enough to use a oneOf construct to describe what you want, but here’s the problem: when you syntax-check these, the error messages go right off the rails. For example, suppose my schema says that if Type is Mineral, then there must be a boolean-valued Organic field. And then suppose I try to validate a doc with an object that has "Type": "Mineral" and "Organic": 42.

The validators I’ve been working with give me a massively unhelpful flurry of messages saying that:

This can’t be Animal because the Type is wrong.

This can’t be Vegetable because the Type is wrong.

This can’t be Mineral because the Organic is wrong.

This can’t be a valid instance because the oneOf doesn’t match.

Of these, only one error message is helpful; but I don’t see any way to write a schema processor to report what’s really wrong in a human-friendly way.

Anyhow, I can think of specific recommendations for schema-language design that fall out of this observation. But I’m not building any of them so I’ll stick with just reporting the use-case.

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