Walks one document source against its mapping: the structure half of casting, as Dowser.Type is the value half.

What a value is depends on its mapping entry, never on its shape:

• an entry with "properties", or of type object/nested, is an object — a map is walked key by key against those properties, a list element by element, and on the way out a struct is turned into a map first;
• an entry of an opaque type (opts[:opaque], the backend's flat_object/flattened), or one mapped "enabled": false, holds whatever the document put in it: it is returned exactly as it is;
• any other entry with a "type" is a leaf — the whole value goes to the type module (opts[:types]), even when it is a map (a range, a geo_point object), and an array of them element by element;
• no entry at all (a field the mapping doesn't know) is left as it is, only its keys going through opts[:key_fn] on the way in — or, with unknown_fields: :raise, raises a Dowser.UnknownFieldError.

A leaf whose type left a map uncast (no type for it, or a shape it didn't recognize) has its keys go through opts[:key_fn] like the rest of the document. An opaque subtree's keys never do: they are document content, not mapping names, and under keys: :atoms would turn any writer into a way to exhaust the atom table.

Options

• :key_fn — applied to every key load/3 walks (default: identity).
• :types — the Dowser.Type module leaves are cast through (default Dowser.Type).
• :opaque — the mapping types whose subtree is left alone (default []).
• :unknown_fields — what to do with a field an object's "properties" don't declare: :keep it as it is (default), or :raise a Dowser.UnknownFieldError. With no mapping at all, nothing is unknown.