{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}


import Data.List (partition)

main :: IO ()
main = do
  let foo = Foo "foo" 0
      Foo' msg l = foo
  print msg
  print l

  
data Foo = Foo String Int

pattern Foo' :: String -> Int -> Foo
pattern Foo' s i <- (f -> (s, i))
   
f :: Foo -> (String, Int)
f (Foo s i) = (s, i + length s)

-- can I simplify this pattern synonym and avoid f being a top level function? I would like to write something like
-- pattern Foo' :: String -> Int -> Foo
-- pattern Foo' s i <- (f -> (s, i))
--  where
--    f (Foo s i) = (s, i + length s)