How to pretty-print tree-like value step by step?

Question

So I've got a datatype:

datatype ex = A of int | B of ex * ex;

and an example variable:

val x = (B (A 1, B (A 2, A 3)));

I want to be printing through it like this:

"(1, (2, 3))"

Any help would be appreciated! Thanks.

Answer 1

Assuming this pretty-printer is going to be a pp : ex → string, here is a template:

fun pp (A i) = ...
  | pp (B (ex1, ex2)) = ...

This template pattern matches against every possible value constructor for the type ex. The patterns go one level deep, so for your example value, ex1 is bound to A 2, and ex2 is bound to A 3 in the first iteration of the call. You want to handle ex1 and ex2 recursively in the same way as you handled ex0 being B (A 1, B (A 2, A 3)).

For concatenating strings, you might consider op ^ : string × string → string or String.concat : string list → string:

- val test1 = "hello" ^ "world";
> val test1 = "helloworld" : string
- val test2 = String.concat [ "if ", "you've ", "got ", "many ", "strings!" ];
> val test2 = "if you've got many strings!" : string

For pretty-printing integers, you may consider Int.toString : int → string. You may want to wrap this function, since Standard ML presents negative integers a little funky:

- val test3 = Int.toString ~5;
> val test3 = "~5" : string