1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
|
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE 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
|
