GHC/Records.hs

    {-# LANGUAGE AllowAmbiguousTypes
               , FunctionalDependencies
               , KindSignatures
               , MultiParamTypeClasses
               , PolyKinds
      #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Records
    -- Copyright   :  (c) Adam Gundry 2015-2016
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC extensions)
    --
    -- This module defines the 'HasField' class used by the
    -- @OverloadedRecordFields@ extension.  See the
    -- <https://gitlab.haskell.org/ghc/ghc/wikis/records/overloaded-record-fields
    -- wiki page> for more details.
    --
    -----------------------------------------------------------------------------

    module GHC.Records
           ( HasField(..)
           ) where

    -- | Constraint representing the fact that the field @x@ belongs to
    -- the record type @r@ and has field type @a@.  This will be solved
    -- automatically, but manual instances may be provided as well.

    --   HasField :: forall {k}. k -> * -> * -> Constraint
    --   getField :: forall {k} (x::k) r a. HasField x r a => r -> a
    -- NB: The {k} means that k is an 'inferred' type variable, and
    --     hence not provided in visible type applications.  Thus you
    --     say     getField @"foo"
    --     not     getField @Symbol @"foo"
    class HasField x r a | x r -> a where
      -- | Selector function to extract the field from the record.
      getField :: r -> a