gistfile1.hs

{-# LANGUAGE ScopedTypeVariables #-}



data Foo a = Foo {
    fooInt :: Int,
    fooA :: a
}

-- Foo has its own indexes as well as all the indexes of a.
instance Indexable a => Indexable (Foo a) where
    empty = [ ixFun (return . fooInt) ] ++ getIndexes (IX.empty :: IX.IxSet a)
    where
        getIndexes (IxSet xs) = map getIndex xs -- The IxSet constructor is not exported so this isnt possible.
        getIndex (Ix _ f) = ixFun (f . fooA)