metta.casting

Source: extensions/python/metta/casting.py.

Runtime typecasting against the engine's own type discipline. cast(space, value, type) answers value, narrowed to its Python-most spelling, when the engine admits it as that type: the exact ('get-type' then 'get-metatype') acceptance the translator compiles for a typed argument position, run in the space's scope so its ':' declarations and &self's both answer. Protocol types registered through metta.integrate.register_object_type participate, which makes this duck typing through the type system: an object satisfying the predicate casts to the protocol's name. A refused cast raises CastError naming the value's actual type candidates, the loud spelling of what a typed call does silently (a mismatched argument reduces to nothing). Targets the translator never checks (Atom, %Undefined%, _) pass unchecked here too, and a Python type spells its MeTTa reading: bool is Bool before int is Number, str is String, any other class its own name, the names get-type itself answers.

The entries below reproduce the source signatures and docstrings.

CastError

class CastError(MettaError, TypeError):

A cast the engine's type discipline refuses.

cast

def cast(space: Any, value: Any, type_: Any, /) -> Any:

Answer value, narrowed, when space's type discipline admits it as type_; raise CastError naming its actual types otherwise.

m.run("(: Ann Person)")
assert m.cast(S.Ann, "Person") is S.Ann
assert m.cast(3, int) == 3