Derive lawful Applicative and Alternative for PgMsgParser#34
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The hand-written instances were the pointwise lifting of Maybe through two readers, which is exactly what ReaderT Char (ReaderT LBS.ByteString Maybe) provides. Deriving via that type makes the instances lawful by construction and removes the TODO questioning the Applicative laws.
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This removes the
TODO: Is this Applicative correct? Double-check lawsinHpgsql.MsgsBehavior is unchanged: the derived methods coerce to
the same functions the manual instances defined.
One note for reviewers: ReaderT must be imported
with its constructor (ReaderT (..)) for GHC to build the coercion.
No new dependencies — transformers was already a direct dependency.
Tested with run tests (131 examples, 0 failures) and run format-hs; hlint has no hints.