Principled method of lookup-or-insert in associative arrays?

[spir]

On Sat, 20 Nov 2010 00:07:57 -0800
Andrei Alexandrescu <SeeWebsiteForEmail at erdani.org> wrote:

> TDPL has an example that can be reduced as follows:
> 
> void main() {
>    uint[string] map;
>    foreach (line; stdin.byLine()) {
>      ++map[line];
>    }
> }
> 
> byLine reuses its buffer so it exposes it as char[]. Therefore, 
> attempting to use map[line] will fail. The program compiled and did the 
> wrong thing because of a bug.
> 
> The challenge is devising a means to make the program compile and work 
> as expected. Looking up an uint[string] with a char[] should work, and 
> if the char[] is to be inserted, a copy should be made (via .idup or 
> to!string). The rule needs to be well defined and reasonably general.
> 
> The effect is something like this:
> 
> void main() {
>    uint[string] map;
>    foreach (line; stdin.byLine()) {
>      auto p = line in map;
>      if (p) ++*p;
>      else map[line.idup] = 1;
>    }
> }
> 
> Ideally the programmer would write the simple code (first snippet) and 
> achieve the semantics of the more complex code (second snippet). Any ideas?

I find this proposal really necessary. But aren't there two issues here?
* Comparison (for lookup) by value equality should not care about qualifiers (ie compare raw content, here plain array memory areas).
* Assignment should perform "qualification conversion" automatically, eg
	char[] chars = "abc";
	string s = chars;
This involves no implicit magic here, as target qualification is explicit. So, why not?

There is a repetitive programming pattern in D:
* play around with *string's in general
* as soon as text processing is needed, convert to *char[]
* when finished, convert back to *string
This should not be implicit, but automatic.
Is there anything risky here?


Denis
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vit esse estrany ☣

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