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Defeating Return Type Polymorphism (philipphagenlocher.de)

submitted 11 months ago by [email protected] to c/[email protected]

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This was already featured in the Weekly News a couple of weeks back, but I think maybe it deserves it's own thread. I've tried to explain this approach to some people before, but I think this article does a much better job than I have.

I do think the "Defeating" in the title might be a little bit negative, it's have preferred something neutral like "When your result type depends on your argument values", but it's still something useful to know from retaining your type safety.

This existentials and GADTs can be converted into a CPS style without type equality constraints (usually, with enough work) so that you can start from this description but use it in languages with less sophisticated type systems -- as long as they have parametricity -- like Haskell 2010.

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[–] [email protected] 1 points 11 months ago (1 children)

Ah, that's interesting. Although I can imagine not many people would want to write code in that style. And I also wonder how many languages support higher rank polymorphism in the first place.

[–] [email protected] 1 points 11 months ago

Yeah, I generally prefer pattern matching and constructor calls, but often languages don't have GADTs or existentials. Even in GHC, existentials are still a bit "wonky", though still generally nicer to use than CPS/Codensity.