record update pain

RecordUpdatePain.hs

{-# LANGUAGE DuplicateRecordFields #-}
{-# LANGUAGE NoFieldSelectors #-}
{-# LANGUAGE OverloadedRecordDot #-}
{-# LANGUAGE RecordWildCards #-}
module Main where
import Distribution.Simple (KnownExtension(RecordWildCards))

data Foo = Foo {
    aaa :: Int,
    bbb :: Bool
}

data Bar = Bar {
    aaa :: Int,
    bbb :: Bool
}

foo :: Foo
foo = Foo 5 True

bar :: Bar
bar = Bar 3 False

wee :: Foo -> Bar
wee Foo {aaa,bbb} = Bar {aaa,bbb}

-- | Ambiguous
-- boo' :: Foo -> Bar -> Bar
-- boo' foo bar = bar { aaa = foo.aaa }

-- | Using the explicit constructor + RecordWildCards is not ambiguous
-- 
--  https://downloads.haskell.org/ghc/latest/docs/users_guide/exts/duplicate_record_fields.html
--  > Uses of fields that are always unambiguous because they mention the
--  constructor, including construction and pattern-matching, may freely use
--  duplicated field names.
--
boo :: Foo -> Bar -> Bar
boo foo Bar {aaa,..} = Bar { aaa = foo.aaa, .. }

main :: IO ()
main = pure ()