extractor.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE TypeFamilies #-}

import GHC.Types

class Foo f where
    type Ctx f i :: Constraint
    extract :: Ctx f i => i -> f i -> String

newtype Los i = Los [String] deriving (Eq, Ord, Read, Show)
instance Foo Los where
    type Ctx Los i = i ~ Int
    extract i (Los ss) = ss !! i

data Extractor f i = Extractor
    { idx :: i
    , unrelated :: f i
    } deriving (Eq, Ord, Read, Show)