module Data.List

/* This is a comment */
export datatype List a =
   NULL()
 | (:)(a,List a);

export (++)(a :: [a], b :: [a]) :: [a]
{
  case a of
    []       -> b
    (af:ar)  -> case b of
                  [] -> a
                  _  -> af:(ar ++ b)
};

export null(ls :: [a]) :: Bool
{
  case ls of
    [] -> True
    _  -> False
};

export length(ls :: [a]) :: Int
{
  case ls of
    []     -> 0
    _:rest -> length rest
};

export reverse(ls :: [a]) :: [a]
{
  helper(xs,acc) = {
    case xs of
      []       -> acc
      (a:rest) -> helper rest (a:acc)
  }

  helper ls [];
};

export restrict(Eq a) find(f :: a -> Bool, ls :: [a]) :: Maybe a
{
  case ls of
    []             -> False
    (a:_)    | f a -> a
    (_:rest)       -> find(f, rest)
};