{-# LANGUAGE RankNTypes #-}
module HaskellCI.List (
    ListBuilder,
    buildList,
    item,
    ) where

import HaskellCI.Prelude

newtype ListBuilder x a = LB { unLB :: forall r. (([x] -> [x]) -> a -> r) -> r }

instance Functor (ListBuilder x) where
    fmap f (LB k) = LB $ k $ \endo a k' -> k' endo (f a)

instance Applicative (ListBuilder x) where
    pure x = LB $ \f -> f id x
    (<*>)  = ap

instance Monad (ListBuilder x) where
    return = pure

    m >>= k =
        LB $ \r ->
        unLB m $ \endo1 a ->
        unLB (k a) $ \endo2 b ->
        r (endo1 . endo2) b

buildList :: ListBuilder x () -> [x]
buildList (LB f) = f $ \endo _ -> endo []

item :: x -> ListBuilder x ()
item x = LB $ \f -> f (x :) ()