SchemaOf

The JSON schema of A, derived from the type itself so that it cannot drift from it.

case class Invoice(vendor: String, amount: Double, lines: List[String], note: Option[String]) derives SchemaOf

SchemaOf[Invoice].toJsonSchema()    // the schema `completeStructured` sends

The schema describes what uPickle's derived ReadWriter reads and writes (field names, Option as the value or null, an enum as its case name, a BigDecimal as a string), so a reply that matches the schema reads back into A. The module's tests check that round trip for every supported shape.

Supported: String, Int, Long, Double, Float, Boolean, BigDecimal, BigInt; Option; List, Seq, Vector and Set of a supported type; case classes whose fields are supported; enums and sealed hierarchies of singletons (as a string with the case names, or a case's @upickle.implicits.key). Not supported, and refused at compile time with a message naming the type: Map (core's ObjectSchema has a fixed set of properties and cannot describe map values), recursive types and sealed hierarchies with fields (both need a JSON Schema construct, $ref and oneOf, that core's schema model lacks), Option directly inside Option and tuples (uPickle writes both as JSON arrays), and named tuples. Other types need a given SchemaOf of their own: see SchemaOf.string and SchemaOf.stringEnum. A hand-written given is used as it is, so it can also describe a type the derivation would refuse.

A recursion that runs through hand-written givens which themselves call SchemaOf.derived (for mutually recursive P and Q, given SchemaOf[P] = SchemaOf.derived[P] and given SchemaOf[Q] = SchemaOf.derived[Q]) cannot be seen at compile time, because the check does not look inside a hand-written given. Building that schema (schema, definition, toJsonSchema) throws an IllegalStateException naming the recursive type instead, on first use; it never hangs.

A generic case class may be nested in itself at most 16 deep: W[W[...[Int]]] with 16 Ws is described, with 17 it is taken for a recursion. Spelled out in a field's type, the deeper nesting is refused at compile time; reached through the generic derives SchemaOf instance (SchemaOf[W[...[Int]]]) it is refused when the schema is built, with the same IllegalStateException.