You are going to miss some of the benefits of FP by doing functional style programming in JS, rather than relying on purity.

I myself have not noticed any reduced development speed in Haskell or other ML dialects. On the contrary, any non-trivial work is a lot easier because of the reduced complexity of reasoning about pure code with well-defined types.

Haskell specifically can be a little slower due to setup and library complexity as well as compile times.

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