GHC/OverloadedLabels.hs

    {-# LANGUAGE AllowAmbiguousTypes
               , DataKinds
               , FlexibleInstances
               , KindSignatures
               , MultiParamTypeClasses
               , ScopedTypeVariables
               , TypeApplications
      #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.OverloadedLabels
    -- 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 '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.
    --
    -- When @OverloadedLabels@ is enabled, if GHC sees an occurrence of
    -- the overloaded label syntax @#foo@, it is replaced with
    --
    -- > fromLabel @"foo" :: alpha
    --
    -- plus a wanted constraint @IsLabel "foo" alpha@.
    --
    -- Note that if @RebindableSyntax@ is enabled, the desugaring of
    -- overloaded label syntax will make use of whatever @fromLabel@ is in
    -- scope.
    --
    -----------------------------------------------------------------------------

    -- Note [Overloaded labels]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~
    -- An overloaded label is represented by the 'HsOverLabel' constructor
    -- of 'HsExpr', which stores the 'FastString' text of the label and an
    -- optional id for the 'fromLabel' function to use (if
    -- RebindableSyntax is enabled) .  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 )

    class IsLabel (x :: Symbol) a where
      fromLabel :: a