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Moreover, designing the actual type system is at least as hard as implementing the type checker. Coming up with a static type system that is flexible/ergonomic, expressive/powerful, sound, easy to use, and also a good match for mutable state and imperative code is really, really difficult, as demonstrated by C, C++, and Java.

A flexible/ergonomic dynamic type system is trivial in comparison. Soundness is handled at runtime when the exact value of each expression is known, and ease of use is almost a given.