module StrictList (
    StrictList(..)
  , foldl
  , reverse
  , head
  , tail
  ) where

import           Prelude hiding (foldl, head, reverse, tail)

data StrictList a = !a :$ !(StrictList a) | Empty
                  deriving (Show, Eq)

infixr 5 :$

foldl :: (b -> a -> b) -> b -> StrictList a -> b
foldl f = lgo
    where lgo z Empty     = z
          lgo z (x :$ xs) = let z' = f z x in z' `seq` lgo z' xs

reverse :: StrictList a -> StrictList a
reverse = foldl (flip (:$)) Empty

head :: StrictList a -> a
head (x :$ _ ) = x
head Empty     = error "Empty StrictList has no head"

tail :: StrictList a -> StrictList a
tail (_ :$ xs) = xs
tail Empty     = error "Empty StrictList has no tail"