Data.Record.QQ.CodeGen

Quasi-quoter support for large records

NOTE: The only reason for the existence of this module is that record pattern syonyms in ghc are currently not useable: when we declare a record pattern synonym, ghc automatically derives field accessors for every field in the record. We don't want those accessors: they result in name clashes (DuplicateRecordFields does not apply to record pattern synonyms) and, more importantly, they result in quadratic code size again. Once the NoFieldSelectors language extension is merged (probably ghc 9.2), we can reconsider whether this module is still required.

See also:

• https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0160-no-toplevel-field-selectors.rst
• https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4743

Documentation

lr :: QuasiQuoter

Construct or match on large-records-style records

Example construction usage:

inOrder :: R Bool
inOrder = [lr| MkR { x = 1234, y = [True] } |]

or:

constructorApp :: R Bool
constructorApp = [lr| MkR |] 1234 [True]

Example matching usage:

projectOne :: T Bool -> Int
projectOne [lr| MkT { x = a } |] = a

Exported for the benefit of tests

lrExp :: forall m. Quasi m => String -> m Exp

lrPat :: forall m. Quasi m => String -> m Pat