Depth-first schema rewriter.

Peri.walk/2 (delegated here) traverses a schema in pre-order, invoking the supplied callback on every subtree. Callback contract depends on context:

• Map / keyword schema entries — invoked as {:field, key, value}. Return one of:

  • {:cont, {:field, new_key, new_value}} — replace the entry; the walker recurses into new_value as a type expression. new_key may differ from the original (rename) and may be any term valid as a key for the surrounding container.
  • :drop — remove the entry from the parent map / keyword list.

• Every other subtree — invoked as the type expression itself (e.g. :string, {:list, …}, {:multi, …}, a nested map). Return:

  • {:cont, new_node} — replace and continue walking children of new_node.
  • :drop — not allowed in this context; raises.

Map keys are not visited as standalone nodes; constraint option lists (e.g. [gte: 18, error: "…"]), :enum members, :literal values, :ref names, :multi tags, callbacks, and transforms are not visited either — only sub-schemas and field entries are.

Example — make every required field optional:

Peri.walk(schema, fn
  {:required, t} -> {:cont, t}
  {:required, t, _opts} -> {:cont, t}
  other -> {:cont, other}
end)

Example — strip private fields from a map schema:

Peri.walk(schema, fn
  {:field, k, _v} when k in [:internal_id, :secret] -> :drop
  other -> {:cont, other}
end)

Example — rename email to :login:

Peri.walk(schema, fn
  {:field, :email, v} -> {:cont, {:field, :login, v}}
  other -> {:cont, other}
end)