Just Started Learning SML: How to take two ints and return the list of all ints between

Question

Pretty much what the title says. I just started learning SML and am having trouble with my first project. I understand this is a very basic question and it would be incredibly easy for me to do in something like java, but I'm having difficulty understanding SML. I need to write a function intsFromTo. It should take a pair of integers, low and high, and return a list of consecutive integers in ascending order, namely all those that are greater than or equal to low and less than or equal to high.

Here's what I have so far

fun intsFromTo (low,high) = val intList = []
if low >= high then nill
else intList::low intsFromTo(low+1,high);

and I'm getting the following errors which I'm also having difficulty understanding.

https://i.sstatic.net/hMRRR.png

Any and all help to get me going would be greatly appreciated.

Answer 1

Note: The code could be mode more compact for style but might not so clearly illustrate the basic concepts.

Typical Recursion

fun intsFromTo(low, high) =
  let
      val out = []
  in
      if 
          low <= high
      then
          low :: intsFromTo2(low + 1, high)
      else
          out
  end

Alternative Approach

An alternative way to produce the list is using map-reduce style semantics on a list.

fun intsFromTo(low, high) =
  if high > low         
  then     (* the list will contain at least one value *)
     if
       high >= 0
     then
       List.tabulate((high + 1) - low,
                fn k => k + low)
     else
       List.rev(
          List.tabulate((~low + 1) + high,
                fn k => ~k + high))
  else
      []    (* list empty: low >= high *)

Why an Alternative? Just out of curiosity.

Performance

Anecdotal timing indicates that the first version is about twice as fast as the second version using:

fun time() =
  let 
      val t = Timer.startCPUTimer();
  in
      intsFromTo(~1000000,1000000);
      Time.toReal(#usr(Timer.checkCPUTimer(t)))
  end

At least on my computer while it is being used for other things.