feat: control additional named elements in stabilize_lst()

[jonthegeek]

Summary

As a package developer, in order to model JSON Schema additionalProperties, I would like to control whether stabilize_lst() permits named elements that are not covered by a spec, and optionally validate them against a schema.

Details

By default stabilize_lst() rejects any extra named element:

stabilize_lst(list(a = 1L, z = 99L), a = specify_int())
#> Error: `list(...)` must not contain extra named elements. Extra element: "z"

JSON Schema additionalProperties is more flexible: it may be true (allow any extras — the JSON Schema default), false (forbid extras — stbl's current behavior), or a schema that every extra property must satisfy. stabilize_df() already has .extra_cols; stabilize_lst() needs the analogous control for named elements.

Proposed signature

stabilize_lst(x, ..., .extra = NULL)

Arguments

• .extra — Controls handling of named elements not named in ...: forbid (current default), allow unchecked, or a stabilizer / specify_*() spec that each extra element must satisfy.

Returns the (coerced) list.

Behavior

• Preserve the current "forbid extras" behavior as the default to avoid a breaking change (note: this differs from JSON Schema's permissive default; the converter layer can bridge that).
• When a spec is supplied, validate every extra named element against it, reporting failures with their names/locations.
• Also update specify_lst() to match this update.

References

• JSON Schema draft 2020-12, additionalProperties: https://www.ietf.org/archive/id/draft-bhutton-json-schema-01.html