Support empty record definitions in parser#91
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Allow RRFields production to match zero or more fields, enabling empty record definitions. Fixed applicativeE to wrap empty lists in return, enabling proper JSON parsing for empty record types. Added test cases for empty records, mixed empty and filled records. Fixes #83.
When a record has zero fields, the TH code generators for various tool instances would bind variables (e.g. lambda parameters) that were never referenced in the body. With -Wall/-Wunused-matches, GHC flags these as warnings in the generated splices. Fix by using wildcard patterns (wildP) instead of named bindings (varP) when srFields is empty, across all affected generators: - Tools/JSON.hs (ToJSON): \x -> object [] => \_ -> object [] - Tools/JSON.hs (FromJSONWithErrs): (Object x) => (Object _) - Tools/QuickCheck.hs (Arbitrary): sized (\x -> pure Con) => sized (\_ -> ...) - Tools/DeepSeq.hs (NFData): \x -> () => \_ -> () - Tools/Example.hs (Example): sized (\x -> pure Con) => sized (\_ -> ...) - Tools/Traversal.hs (traversal): \f r -> pure Con => \_ _ -> pure Con
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Fixes #83.
This PR extends the parser to support this syntax:
Under the hood, this generates a Haskell datatype with a single "tag", i.e:
data EmptyRecord = EmptyRecord.This is actively used in this PR and it does the trick. While using it in Atlas I've got GHC to warn about some unused variables; it turns out that in a few places, when we generated TH definitions in the tools for the various instances, we could be generating things like:
(Like it would be the case now for an
EmptyRecord). This PR amends the TH generation to usewildPunder the right circumstances.