A false dichotomy that hides the real request

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The common question of whether to use Hindley-Milner (HM) or Bidirectional (Bidir) typing for a new programming language is a false dichotomy. The real question is whether your language needs generics. Generics require unification, which is central to HM, but bidirectional typing is actually a superset of HM — it can incorporate unification just as well. Adding a `check` function alongside an HM `infer` function is all it takes to make a system bidirectional. The practical guidance: if you're building a general-purpose language, support generics and use unification within a bidirectional framework. If you're building a DSL or learning project where simplicity matters, annotation-driven bidirectional typing without unification is a valid, simpler choice.

How to Choose Between Hindley-Milner and Bidirectional Typing