It's very nice to see a small type checker in Python, for Python! This became much easier in the last 10 years, since the MyPy team basically "upstreamed" the typed_ast library they were using into the stdlib. I found that there are not enough good teaching materials on type checkers -- e.g. the second edition of the Dragon Book lacks a type checker, which is a glaring hole IMO - https://news.ycombinator.com/item?id=…
once you get used to it, visitors are a very pleasant way to write ast walking code in python. they are essentially generating your case statement for you, so instead of `case ast.Expr: handle_expr(node)` you just write a `self.visit_expr` method and have the visitor match the node type to the method name and call it.
If you search for pop(), you can see that
self.expected_ret.append(ret)
self.generic_visit(n)
self.expected_ret.pop()
self.push(narrows_true); [self.visit(s) for s in n.body]; self.pop()
self.push(narrows_false); [self.visit(s) for s in n.orelse]; self.pop()
It's not so bad here, but with a big enough language, and more complicated algorithms, the mutable member variables basically become "mutable globals".
And if you re-call visit() at arbitrary depths, IMO the algorithm gets obscured.
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That said, I agreed here that visitors are useful when you need to say traverse all string literals in an AST, at arbitrary depths: https://lobste.rs/s/jdgjjt/visitor_pattern_considered_pointl...
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A sign that this issue isn't settled is that two of the more complex type checkers make opposite decisions
- MyPy uses visitors extensively - https://github.com/python/mypy/tree/master/mypy
- TypeScript mostly uses switch/case functions - https://github.com/microsoft/TypeScript/blob/main/src/compil...
I'd be interested in analysis of why that is, but I suspect it's mainly style