Improve error messages for polymorphic tail parametricity violations #1320
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matthew-healy
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vkleen
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The general approach looks like a net improvement to me. I'm just not happy with the error message 😅 Values don't exist in a type, they have types or maybe they're compatible with types if subtyping is allowed. Maybe
"values of type {violating_type} are not guaranteed to be compatible with a polymorphic {tail_king} `{tail}`"
The record tail error message referencing the type { } also feels a bit weird to me, but I don't think that's introduced in this PR.
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@vkleen Totally happy to change the error message. I wasn't happy with it either, but figured it was better to just do something & then bikeshed it in the PR. 😁 Your suggestion is much better. |
Previously, if we had a type whose return value contained a polymorphic tail and we tried to return a value which was not statically known to be in the return type, we would see a type mismatch error with an "invented" type name (e.g. `a`) to represent "an arbitrary type in the tail". This made it unclear where the problem was, and didn't give any real direction on how to fix the problem. This commit raises a specific error in this case, which informs users that the type they've tried to return is not guaranteed to exist in the polymorphic tail. We also now pass around a var kind with type errors related to constant mismatches, so we can print a representation of the tail type (e.g. `[| ; x |]`) instead of inventing phantom type names for errors. Fixes #1316.
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Previously, if we had a type whose return value contained a polymorphic tail and we tried to return a value which was not statically known to be in the return type, we would see a type mismatch error with an "invented" type name (e.g.
a) to represent "an arbitrary type in the tail".This made it unclear where the problem was, and didn't give any real direction on how to fix the problem.
This commit raises a specific error in this case, which informs users that the type they've tried to return is not guaranteed to exist in the polymorphic tail. We also now pass around a var kind with type errors related to constant mismatches, so we can print a representation of the tail type (e.g.
[| ; x |]) instead of inventing phantom type names for errors.Fixes #1316.