`NamedTuple`s reify into `tuple`s

[rlouf]

While we can unify namedtuples with the same structure, reify returns a tuple when passed a namedtuple:

from typing import NamedTuple
from unification import unify, reify

class Test(NamedTuple):
    a: int
    b: int

test1 = Test(1, 4)
test2 = Test(var(), var())

res = unify(test1, test2)
print(res)

reified = reify(test2, res)
print(reified)
# (1, 4)

Trying to access elements by name obviously fails:

print(reified.a)

We would expect reify to return a namedtuple with the same structure instead.

[brandonwillard]

There's only explicit dispatch constructor/_reify support for tuple, and since NamedTuple's type is a subclass of tuple, it's being interpreted as the latter.

We could attempt to add broader support for subclasses by assuming that their constructors take the same arguments as their base class constructors, but that's bound to fail in some cases, and failure as a default is probably worse than a successful base class result.

[jarble]

@brandonwillard Is this reify method documented anywhere? The README doesn't seem to mention it.

[brandonwillard]

unification.core._reify is the dispatch function underlying unification.core.reify. It needs to be overloaded in order to add support for new types.