SML Function to make a list from 1 to n-CodePudding

I am new to SML and was a bit confused. I am trying to write a function called makeList of type int -> int list that takes as input a positive integer n and returns the list [1, 2, ..., n]

I want the only input to be the ending number, but I am struggling on doing this without specifying a starting point as well.

fun createList(start:int, ending:int) = 
if(start = ending) 
    then []
    else
            start::createList(start   1, ending);

I want to be able to input just one number, such as 6 and get a list such as: [1,2,3,4,5,6] and I want to do this without using an external library like list.something

This is the only way I was taught, and I would appreciate tips on how to improve.

CodePudding user response：

You're 99% of the way there. With your current function, createList(1, 7) yields [1, 2, 3, 4, 5, 6]. You just need another function that automatically fills in one of the parameters.

fun createList(start, ending) = 
  if start = ending then []
  else start :: createList(start   1, ending);

fun createList'(ending) =
  createList(1, ending   1);

Try this and createList'(6) will yield [1, 2, 3, 4, 5, 6].

Now, let's say you don't want both to be visible. You can "hide" one function as a local binding.

fun createList(ending) =
  let 
    fun aux(start, ending) =
      if start = ending then []
      else start :: aux(start   1, ending)
  in
    aux(1, ending   1)
  end;