Trying pyrefly, Meta's type checker #

I guess I like trying type checkers.

Meta announced that they are open sourcing their type checker, pyrefly. It was easy enough to try:

uvx pyrefly check

On my tiny 640 lines of castfit (which passes pyright and mypy --strict) I got 74 errors. Going through some of these was instructive.

== is not supported between type[@_] and type[MyList]

This was in response to code like:

assert castfit.get_origin_type(MyList) == MyList

Good catch. That == should is.

Can't apply arguments to non-class, got LegacyImplicitTypeAlias[TypeForm, type[type[TypeVar(T, invariant)] | Any]] [bad-specialization]

Not sure what this means or how to handle it.

Argument Forall[T, (value: Any, kind: Unknown) -> bool] is not assignable to parameter with type (Any, Unknown) -> bool [bad-argument-type]

Imagine my confusion when I'm trying to figure out why this is not assignable. Must be that Unknown can't be assigned to Unknown. I might try this again in the future, but the error messages could definitely use some work.

# python, typed, pyrefly