module Data.List

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

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

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

export length(ls :: [a]) :: Int
{
  case ls {
    []       -> 0;
    (_:rest) -> 1 + length(rest);
  }
};

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

  helper(ls, []);
};

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