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

[Derek Parnell]

On Sat, 09 May 2009 11:43:09 -0500, Andrei Alexandrescu wrote:

> Consider:
> 
> 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?

I'm sure about 'best', but I'd prefer the Python method.

The example is similar to ...

    array = array ~ array.length;

in as much as the result of the assignment is that the array length
changes, but here it more easy to see that the pre-assignment length is
being used by the RHS.

In COBOL-like syntax ...

   move dic.length to dic[word].

it is also more obvious what the coder's intentions were.

In assembler-like syntax (which is what eventually gets run, of course) ...

   mov regA, dic.length
   mov dic[word], regA

It just seems counter-intuitive that the target expression's side-effects
should influence the source expression.

-- 
Derek Parnell
Melbourne, Australia
skype: derek.j.parnell