GetFieldOfType.hs

{-# LANGUAGE AllowAmbiguousTypes   #-}
{-# LANGUAGE TypeApplications      #-}
{-# LANGUAGE ScopedTypeVariables   #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleContexts      #-}

import Control.Monad

data MobileField = MobileField String
data EmailField = EmailField String

data Field = MF MobileField | EF EmailField

class HasFieldOfType f s where
  getFieldOfType :: s -> Maybe f

instance HasFieldOfType MobileField Field where
  getFieldOfType (MF f) = Just f
  getFieldOfType _      = Nothing

instance HasFieldOfType EmailField Field where
  getFieldOfType (EF f) = Just f
  getFieldOfType _      = Nothing

mobileNumber :: MobileField -> String
mobileNumber (MobileField x) = x

emailAddress :: EmailField -> String
emailAddress (EmailField x) = x

foo :: forall f . HasFieldOfType f Field => [Field] -> Maybe f
foo = msum . map (getFieldOfType @ f)

fields :: [Field]
fields = [MF (MobileField "+4412345678"), EF (EmailField "foo@bar.baz")]

main :: IO ()
main = do
  print $ mobileNumber <$> foo @MobileField fields
  print $ emailAddress <$> foo @EmailField fields