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{-# LANGUAGE AllowAmbiguousTypes, FunctionalDependencies, ScopedTypeVariables, PolyKinds, TypeApplications, DataKinds #-}
{-# LANGUAGE RecordWildCards, OverloadedRecordDot, OverloadedRecordUpdate #-}
module RecordDotSyntaxA where
class HasField x r a | x r -> a where
hasField :: r -> (a -> r, a)
getField :: forall x r a . HasField x r a => r -> a
getField = snd . hasField @x -- Note: a.x = is getField @"x" a.
setField :: forall x r a . HasField x r a => r -> a -> r
setField = fst . hasField @x -- Note : a{x = b} is setField @"x" a b.
data Foo = Foo {foo :: Int}
instance HasField "foo" Foo Int where
hasField r = (\x -> case r of Foo { .. } -> Foo { foo = x, .. }, foo r)
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