Writing multiple functions in SML - Sequential Composition

问题

I would like to understand how sequential composition works much better than I do now in SML. I have to write a program that takes a list of integers and moves the integer at index zero to the last index in the list. ie. [4, 5, 6] -> [5, 6, 4].

The code I have right now is:

- fun cycle3 x =
= if length(x) = 1 then x
= else (List.drop(x, 1);
= x @ [hd(x)]);
val cycle3 = fn : 'a list -> 'a list

The question lies in my else statement, what I want to happen is first concatenate the first term to the end, and then second drop the first term. It seems simple enough, I just don't understand how to perform multiple functions in a particular order using SML. My understanding was that the first function called has the scope of the second function that would have the scope of the third function.. etc etc.. What am I doing wrong here?

回答1:

Most things in SML are immutable -- your function, rather than modifying the list, is building a new list. List.drop(x,1) evaluates to a new list consisting of all but the first element of x, but does not modify x.

To use your method, you would bind the result of List.drop(x,1) to a variable, as in the following:

fun cycle3 x = if length x = 1
               then x
               else let
                      val y = List.drop(x,1)
                    in
                      y @ [hd(x)]
                    end

Alternately, a cleaner way of doing this same thing, that also handles the possibility of an empty list:

fun cycle3 [] = []
  | cycle3 (x::xs) = xs @ [x]

来源：https://stackoverflow.com/questions/22137815/writing-multiple-functions-in-sml-sequential-composition