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|
{-# 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
|
