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Hi, Servant user here: Haskell's type system is sound, which means that you can have type safety. TypeScript's type system is unsound, so every guarantee that you may think you have wrt. type safety go out of the window at the moment you start the typechecker.

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Hey, don't be this way. Haskell's type system is also unsound (see: undefined, unsafePerformIO). Typescript has done an amazing job bolting an advanced static type system on top of a dynamically typed language. The edge cases of that type system exist, and are a bit more prevalent than Haskell's, but there exist a number of libraries in Typescript that take inspiration from Haskell libraries. This question is about the differences between the design of these two ways of statically representing APIs and not about the fundamental differences in the type systems of their host languages.

There's a huge gulf here. undefined doesn't introduce unsoundness, and unsafePerformIO isn't part of the language standard, it's strongly discouraged, and rarely used. I've been writing Haskell professionally for 10 years, for example, and I've never written unsafePerformIO.

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