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Why Swift's type checker is so slow

How a 10 year old design choice for Swift’s type checker still haunts us to this day

My experience with Swift is we tried to make a really fancy bi-directional Hindley-Milner type checker and it’s really great because you can have very beautiful minimal syntax but the problem is that A) compile times are really bad (particularly if you have complicated expressions) and B) the error messages are awful because now you have global constraint systems and when something goes wrong you have to infer what happened and the user can’t know that something over there made it so something over here can’t type check. This might be a reasonable stopping point: teams that care about compile times and good error messages could use the flag, and everyone else doesn’t have to. As a result, we’d get faster compilation times and clearer error messages, but the extra verbosity might be too much for the swift community to swallow.

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