Question

This assignment is from Induction to Programming using sml 5.14

Here is my code but I get this message pattern matching is not exhaustive in in base case..

fun revrev [] = [[], []]
  | revrev [x::xs,y::ys] = revrev[ys@[y], xs @ [x]];


val test1revrev = revrev [[1, 2],[3, 4, 5]];

From valtest1 I want the output [[5, 4, 3], [2, 1]]

I fail to see why my function doesn’t work and need a little insight maybe.

For instance this works…

fun rev [] = []
  | rev (x::xs) = rev1 xs @ [x];

val test1rev = rev [1, 2, 3];
I get [3, 2, 1]

Answer 1

Pattern matching isn’t exhaustive because you only match either empty list or list with two elements.

I think the intention of the exercise is to implement revrev based on rev which has been introduced before. Here are a few tips:

• The base case is empty list. You simply return an empty list.
• The inductive case matches a non-empty list x::xs. Similar to implementation of rev, you can call revrev on xs and put a transformation of x at the end. This time you need to use rev so that x itself is also reversed.

EDIT:

Your function doesn’t work in general cases. What I meant is the following skeleton:

fun revrev [] = []
  | revrev (x::xs) = ...

where x is also a list. Since this function is very similar to rev, I hope that you can fill in ... by yourself.