Rework type checker using "propagate types inward" trick #1283
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Co-authored-by: Restyled.io <commits@restyled.io>
kostmo
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May 27, 2023
| -- | Add a source location to a type error and re-throw it. | ||
| addLocToTypeErr :: Syntax' ty -> TypeErr -> TC a | ||
| addLocToTypeErr s te = case te of | ||
| UnifyErr NoLoc a b -> throwError $ UnifyErr (s ^. sLoc) a b |
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can throwError be moved outside of the case expression?
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Good call, but I actually want to completely rewrite addLocToTypeErr (it should actually have a type like SrcLoc -> TC a -> TC a, and maybe a different name to go with it; it should handle all the different kinds of type errors, to facilitate which we should factor the SrcLoc out so it's not duplicated in every constructor, etc.) so I'd prefer to just wait and do all the refactoring at once.
Co-authored-by: Karl Ostmo <kostmo@gmail.com>
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Improvements to type inference and type error messages. Includes: - More informative error when record literal fields don't match the expected set of fields - More informative type mismatch error which includes the term where the mismatch occurred. - Reinstates a case for lambdas with explicit type annotations in inference mode (which was removed in #1283), which helps preserve better error messages in some cases.
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The basic idea is that we try as hard as possible to stay in "checking mode", where we have an "expected type" from the context and "push it down" through a term. We also try hard to avoid generating fresh unification type variables unless absolutely necessary. That way, when we come to something that doesn't match the expected type, we can immediately flag a specific error about that part of the code, rather than later having a generic unification error.
This is mostly just refactoring, but it already improves the type error messages somewhat:
There is still a lot more I want to do to improve the type error messages but this is a start.
Closes #99.