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-- Test that DuplicateRecordFields doesn't affect the metadata
-- generated by GHC.Generics or Data.Data
-- Based on a Stack Overflow post by bennofs
-- (http://stackoverflow.com/questions/24474581)
-- licensed under cc by-sa 3.0
{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
import GHC.Generics
import Data.Data
import Data.Proxy
type family FirstSelector (f :: * -> *) :: *
type instance FirstSelector (M1 D x f) = FirstSelector f
type instance FirstSelector (M1 C x f) = FirstSelector f
type instance FirstSelector (a :*: b) = FirstSelector a
type instance FirstSelector (M1 S s f) = s
data SelectorProxy s (f :: * -> *) a = SelectorProxy
type SelectorProxy' s = SelectorProxy s Proxy ()
-- Extract the first selector name using GHC.Generics
firstSelectorName :: forall a. Selector (FirstSelector (Rep a))
=> Proxy a -> String
firstSelectorName _ =
selName (SelectorProxy :: SelectorProxy' (FirstSelector (Rep a)))
-- Extract the list of selector names for a constructor using Data.Data
selectorNames :: Data a => a -> [String]
selectorNames = constrFields . toConstr
data T = MkT { foo :: Int } deriving (Data, Generic)
data U = MkU { foo :: Int, bar :: Bool } deriving (Data, Generic)
main = do -- This should yield "foo", not "$sel:foo:MkT"
print (firstSelectorName (Proxy :: Proxy T))
-- Similarly this should yield "foo"
print (firstSelectorName (Proxy :: Proxy U))
-- This should yield ["foo"]
print (selectorNames (MkT 3))
-- And this should yield ["foo","bar"]
print (selectorNames (MkU 3 True))
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