hlist.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}

data HList :: [*] -> * where
    HNil :: HList '[]
    HCons :: a -> HList k -> HList (a ': k)

hlen :: HList k -> Int
hlen HNil = 0
hlen (HCons _ hs) = 1 + hlen hs

withHList :: forall a b r. [Either a b] -> (forall k. HList k -> r) -> r
withHList xs f = go (reverse xs) HNil
    where
        go :: [Either a b] -> HList k -> r
        go [] hl = f hl
        go (Right x : xs) hl = go xs (HCons x hl)
        go (Left x : xs) hl = go xs (HCons x hl)

main = print (withHList [Left "l", Right 'r', Right '2', Left "foo"] hlen)