Cases where it makes more sense to use concrete types rather than generics, even if you can be generic:
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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.
I'm working on arity errors in Garden, my toy programming language.
Rather than just saying "expected 3, got 4 arguments", I'm trying to report where the extra argument is, or what extra argument was expected.
Counter-intuitively, if you're writing a parser for a programming language, you need it to be a total function. As soon as you build IDE tooling, you need ASTs from invalid or incomplete input.
The parser should return (Ast, List<Error>) rather than Result<Ast, Error>.

