assignment: left-to-right or right-to-left evaluation?

Andrei Alexandrescu SeeWebsiteForEmail at
Sat May 9 09:43:09 PDT 2009


uint fun();
int gun();
int[] a = new int[5];
a[fun] = gun;

Which should be evaluated first, fun() or gun()? It's a rather arbitrary 
decision. C/C++ don't even define an order. Python chooses 
left-to-right, EXCEPT for assignment, which is right-hand side first. 
Lisp and C# choose consistent left-to-right. I don't like exceptions and 
I'd like everything to be left-to-right. However, this leads to some odd 
cases. Consider this example in TDPL:

import std.stdio, std.string;

void main() {
   uint[string] dic;
   foreach (line; stdin.byLine) {
     string[] words = split(strip(line));
     foreach (word; words) {
       if (word in dic) continue; // nothing to do
       uint newID = dic.length;
       dic[word] = newID;
       writeln(newID, '\t', word);

If we want to get rid of newID, we'd write:

       writeln(dic.length, '\t', word);
       dic[word] = dic.length;

by the Python rule, and

       writeln(dic.length, '\t', word);
       dic[word] = dic.length - 1;

by the C# rule.

What's best?


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