In principle you can create wrapper types in any PL, eg Path(Str) for filesystem functions. I only see it used w/ static types in practice.
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What are the most popular languages that have used an AST walker for their implementation?
I know Ruby used to do this, but there must be others.
(I'm interested in the lowest PL speed that users will tolerate if you have awesome features.)
Bootstrapping a language can be immensely satisfying.
I've added the ability to define stub types in the Garden stdlib and suddenly I don't need to special-case Int or String! They're just normal type declarations.
Playing with optional type signatures in Python, I realise that the return type is the most important to me.
I'd much rather have a function with only a return type instead of a function with only parameter types. It's often quick to add too.