AvroPrism

.field(_.x) — drill via selector lambda.

.fieldNamed[B]("schema_name") — drill by the EXPLICIT schema field name, bypassing resolution entirely. The escape hatch for a hand-written codec the resolver cannot read (a field list that both renames beyond recognition and reorders, a column bearing another field's name, two columns normalising alike); the common (derived / order-preserving / name-transformed) codecs need .field(_.x) instead, which resolves the name for you.

The name is CHECKED against the schema it will be looked up in, at construction (issue #95): a name the record does not carry throws rather than Missing silently at runtime. A MAP parent is carved out — .fieldNamed is also how a map KEY is addressed, and an absent key is data, not a mistake. To feature-detect a RECORD field, ask the schema: codec.schema.getField(name).

.at(i) — drill into the i-th array element / map entry.

.union[Branch] — drill into a union alternative by branch type.

.each — split into an AvroTraversal over the iterated array.

.fields(_.a, _.b, ...) — focus a NamedTuple over selected fields.

The AvroCodec for the synthesised NamedTuple is summoned at the call site; with no hand-written given in scope it auto-derives through kindlings. The 22-selector ceiling is gone: up to 22 selectors the focus is spelled TupleN and hearth builds it with that tuple's constructor; at 23 and above the focus type IS a *: cons chain, which hearth ≥ 0.4.2 builds through Tuple.fromArray instead. (Both spellings are needed — hearth's constructor branch below 23 cannot build a cons chain, and there is no TupleN above 22.)

That is not the same as "unbounded". .fields selects from a case class, so '''254 selectors is a hard, permanent ceiling''' — the JVM caps a parameter list at 254 slots and a 255-field case class does not compile at all. Well below that, the binding limit is the compiler thread's stack, because the derivation recurses per field: measured, -Xss1m (the JVM default) tops out around 32 selectors, -Xss4m around 150, and -Xss8m — what this repo's .jvmopts sets — reaches 254. A downstream build gets none of that automatically, so raise -Xss there before concluding a cover is too wide. Compile time is the third cost: the derivation grows superlinearly in arity, so a wide cover may also want a higher -Xmacro-settings:avroDerivation.timeout=30s — the unit suffix is required, a bare integer is silently ignored. (Issue #96.)