datatype list t = Nil | Cons of t * list t fun length [t] (ls : list t) = let fun length' (ls : list t) (acc : int) = case ls of Nil => acc | Cons (_, ls') => length' ls' (acc + 1) in length' ls 0 end fun rev [t] (ls : list t) = let fun rev' (ls : list t) (acc : list t) = case ls of Nil => acc | Cons (x, ls') => rev' ls' (Cons (x, acc)) in rev' ls Nil end