Support for terms with `Literal` types

[kiranandcode]

While implementing part of pact I ran into a situation where it would be helpful to be able to construct terms of type Literal[...] fixed set of values.

from effectful.ops.syntax import defop, NotHandled

@defop
def Price() -> Literal[5,10,20,50,80,120,150,200]:
    raise NotHandled

Price()

Currently this fails with an error about __mro__:

  File "/Users/kirancodes/Documents/code/effectful/effectful/ops/syntax.py", line 515, in apply_cast
    return __dispatch(dispatch_type)(dispatch_type, op, *args, **kwargs)
           ~~~~~~~~~~^^^^^^^^^^^^^^^
...
  File "/Users/kirancodes/.local/share/uv/python/cpython-3.13.9-macos-aarch64-none/lib/python3.13/functools.py", line 765, in _compose_mro
    bases = set(cls.__mro__)
                ^^^^^^^^^^^
AttributeError: __mro__. Did you mean: '__or__'?

[kiranandcode]

Oh, wait looking at the code, I guess this is a subtype of UnionTypes which are explicitly mentioned as not supported:

    if isinstance(tp, types.UnionType):
        raise TypeError(f"Union types are not supported: {tp}")

This is a pretty useful annotation for describing dls, so it'd be nice to able to annotate types with such annotations.

If unions explicitly aren't allowed, could type aliases be supported instead?

type PriceValue = Literal[5,10,20,50,80,120,150,200]

@defop
def Price() -> PriceValue:
    raise NotHandled

Price()

[eb8680]

I think we can just normalize Literal to a Union of the type of its elements in _simple_type. In that case, the result of typeof(Price()) would just be int.

UnionType is disallowed because it's not clear how to implement a generic defdata pattern for it, and because no Python runtime object has type UnionType - a fundamental property of Python's gradual type system is that every value has exactly one concrete type at runtime.

[kiranandcode]

Oh, that makes sense! Okay, will update the PR I submitted accordingly.