Recursive function to pretty-print the elements of a list

Question

I need to write a function which takes input of type (int * int) list and prints pairs of integers. This function should leverage another function printGenList (takes a function f and a list l and applies f to every element of list recursively) whose code I have written like this -

fun printGenList f l =
if NULL l
then ()
else ( (f (HD l) );  printGenList (f) (TL l)  );

and provide an anonymous function (the fn … => … construct) that will do the appropriate pretty printing.

Answer 1

The type signature tells you to have a list of pairs of integers. An example of a pair of ints would be (4,1). A list of pairs would then be [(a,b),(c,d),...], and not, as you've tried, a pair of lists of integers.

I'm sure you're familiar with the (x::xs) notation, since you seem to understand lists to some degree. If we are to match into pairs we can do it like this: ((n,m)::xs). This pattern will bind n and m to the corresponding ints and xs to the rest of the list. Then recursion is straightforward:

fun pInts [] = ()
  | pInts ((n,m)::xs) = print ("(" ^ Int.toString n ^ ", " Int.toString m ^ ")";
                        pInts xs

The important thing to glean from this is that you can bind several variables in a single pattern and use them in your functions. You can even, if you're sure you have more than one element in a list, bind several elements at once:

fun pairs []         = []
  | pairs [x]        = []
  | pairs (x::y::xs) = (x,y) :: pairs xs

Answer 2

You should take a look at tuples You actually have to define function with following signature :

fn : (int * int) list -> unit

So the function will be something like:

fun pr(lst :(int * int) list) : unit =
    case lst of
         [] => ()
        |(a,b) :: xs => let 
                            val out = Int.toString a ^ Int.toString b
                        in 
                            print(out);
                            pr xs
                        end

I hope that you get basic idea. Try to improve print formatting!