Support pattern matching on empty types #5298
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It currently fails.
Previously, `match` and `cases` expressions needed to have at least one pattern to match on. This allows them to work with zero patterns, which is useful for matching on empty types. Since `EmptyMatch` is no longer a failure case, errors that previously said “I expected some patterns after a match / with or cases but I didn't find any,” now say “Pattern match doesn't cover all possible cases”. Fixes unisonweb#4731.
pchiusano
approved these changes
Aug 23, 2024
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Overview
Previously,
matchandcasesexpressions needed to have at least one pattern to match on. This allows them to work with zero patterns, which is useful for matching on empty types.Since
EmptyMatchis no longer a failure case, errors that previously said “I expected some patterns after a match / with or cases but I didn't find any,” now say “Pattern match doesn't cover all possible cases”.Fixes #4731.
Test coverage
There is a new transcript that covers the new cases. The existing error-messages transcript captures the change in error message with empty
casesthat are applied to non-empty types.