File.byLine ought to return dups?

[Graham Fawcett]

Sorry for the double-post to .announce -- I had deleted my .announce 
post, but obviously not thoroughly enough. I'll follow up on the list.

Graham


On Fri, 04 Jun 2010 15:41:43 -0400, Steven Schveighoffer wrote:

> On Fri, 04 Jun 2010 15:27:03 -0400, Graham Fawcett <fawcett at uwindsor.ca>
> wrote:
> 
>> Hi folks,
>>
>> I expected the following program to print the lines of a file in
>> reverse:
>>
>>     // bad.d
>>     import std.stdio;
>>     import std.range;
>>    void main() {
>>       auto f = File("bad.d");
>>       foreach(char[] line; retro(array(f.byLine())))
>>         writeln(line);
>>     }
>>
>> However, this produces very unusual output: fragments of the same lines
>> are printed repeatedly. I suspect it's because the byLine() 'generator'
>> is not dup'ing the arrays it reads from the file.
>>
>> This works as expected:
>>
>>     // good.d
>>     import std.stdio;
>>     import std.range;
>>    Retro!(char[][]) retroLines(File f) {
>>      char[][] lines;
>>       foreach(line; f.byLine())
>>         lines ~= line.dup;         // note the .dup
>>       return retro(lines);
>>      }
>>    void main() {
>>       auto f = File("good.d");
>>       foreach(line; retroLines(f))
>>         writeln(line);
>>     }
>>
>> If you remove the '.dup', then this behaves badly as well.
>>
>> So is this a bug in File.byLine, or am I just using it badly? :)
> 
> The latter.  File is re-using the buffer for each line, so you are
> seeing the data get overwritten.  This is for performance reasons.  Not
> everyone wants incur heap allocations for every line of a file ;)  As
> you showed, it's possible to get the desired behavior if you need it. 
> The reverse would be impossible.
> 
> Now, that being said, a nice addition would be to create a duper range
> that allows you to do one expression:
> 
> foreach(char[] line; retro(array(duper(f.byLine()))))
> 
> -Steve