gistfile1.txt

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

data Mode = Field | Descr
type family Inner mode field where
  Inner Field f = f
  Inner Descr f = String


data ArgF (f :: Mode) = ArgF
  { field1 :: Inner f Char
  , field2 :: Inner f Int
  }

someArg :: ArgF Field
someArg = ArgF
  { field1 = 'c'
  , field2 = 42 }

argDescr :: ArgF Descr
argDescr = ArgF
  { field1 = "Some field 1"
  , field2 = "Another fancy field"
  }

type Sel f = forall mode. (ArgF mode -> Inner mode f)

get :: Sel f -> f
get sel = sel someArg

fun :: Sel Char -> Char
fun sel = get sel