Remove the transformers dependency - #42
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Kleisli m a is a newtype over `a -> m b`, so two nested Kleislis have exactly this parser's shape, and their Applicative and Alternative instances are the same pointwise lifting of Maybe that ReaderT provides. Kleisli lives in base, so this drops the only use of transformers in Hpgsql.Msgs while keeping both instances lawful by construction.
Both connection string parsers ran in `ExceptT String Identity`, which is isomorphic to `Either String` - the type they already returned. Using Either directly removes the last use of transformers in the library, so the dependency goes away. Behavior is unchanged: ExceptT's bind and applicative short-circuit on the first error exactly like Either's.
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Follow-up to the discussion in #34.
Two changes, one per commit:
Hpgsql.Msgs: derivePgMsgParser'sApplicative/AlternativeviaKleisli (Kleisli Maybe LBS.ByteString) Charinstead ofReaderT.Kleisliis thea -> m bnewtype from base, and its instances are the same pointwise lift intoMaybethatReaderTprovides, so the instances stay lawful by construction.Hpgsql.Connection: both connection string parsers ran inExceptT String Identity, which is isomorphic to theEither Stringthey already return. UsingEitherdirectly removes the last use oftransformers, so the dependency is dropped frombuild-depends.Behavior is unchanged in both cases:
Kleisli's<|>keepsMaybe's left bias (and carries the same INLINE pragmaReaderT's does), andExceptT's bind and applicative short-circuit on the first error exactly likeEither's.Verified with
nix-build -A hpgsqlon ghc967, ghc984, ghc9103 and ghc9122, and-A testsPg18/-A hpgsqlSimpleCompatTestsPg18-133 examples, 0 failures, including the
LibPqConnStringParsingSpecyou added in #41, which covers the parsers this touches. hlint and fourmolu are clean.