GHC/OverloadedLabels.hs

    {-# LANGUAGE NoImplicitPrelude
               , MultiParamTypeClasses
               , MagicHash
               , KindSignatures
               , DataKinds
      #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.OverloadedLabels
    -- Copyright   :  (c) Adam Gundry 2015
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC extensions)
    --
    -- This module defines the `IsLabel` class is used by the
    -- OverloadedLabels extension.  See the
    -- <https://ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/OverloadedLabels wiki page>
    -- for more details.
    --
    -- The key idea is that when GHC sees an occurrence of the new
    -- overloaded label syntax @#foo@, it is replaced with
    --
    -- > fromLabel (proxy# :: Proxy# "foo") :: alpha
    --
    -- plus a wanted constraint @IsLabel "foo" alpha@.
    --
    -----------------------------------------------------------------------------

    -- Note [Overloaded labels]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~
    -- An overloaded label is represented by the 'HsOverLabel' constructor
    -- of 'HsExpr', which stores a 'FastString'.  It is passed through
    -- unchanged by the renamer, and the type-checker transforms it into a
    -- call to 'fromLabel'.  See Note [Type-checking overloaded labels] in
    -- TcExpr for more details in how type-checking works.

    module GHC.OverloadedLabels
           ( IsLabel(..)
           ) where

    import GHC.Base ( Symbol )
    import GHC.Exts ( Proxy# )

    class IsLabel (x :: Symbol) a where
      fromLabel :: Proxy# x -> a