Why string alias is invariant ?

[Sean Kelly]

Sergey Gromov wrote:
> Sean Kelly Wrote:
>> Janice Caron wrote:
>>> On Jan 30, 2008 11:03 PM, Sergey Gromov <snake.scaly at gmail.com> wrote:
>>>
>>>> P.S.  Many thought mus be put into choosing a return type for a library
>>>> function.  Because if it returns a unique copy of data it must be char[]
>>>> so that i'm free to modify it.
>>> Well, consider again the example of lowercasing a string to see why
>>> that is not so. If I return the original string (not a copy of it),
>>> then you are /not/ free to modify it, because there might be other
>>> pointers to that data. So you must first copy it (using dup) and then
>>> This means that the copy need be done /only
>>> when it is required/, instead of every single time you call the
>>> function - so yes, it is an improvement in efficiency.
>> I'd say that it is an improvement in safety rather than efficiency
>> because the model assumes the string may be shared and thus enforces
>> copy on write.  But consider something like this:
>>
>>     char[] data = cast(char[]) read( "myfile.txt" );
>>     char[][] lines = splitlines( data );
>>
>>     foreach( line; lines )
>>     {
>>         writefln( tolower( line.idup ) );
>>     }
>>
>> In this routine, the programmer knows he is the sole owner of the data
>> and simply wants to print the contents of a file in lower case
>> line-by-line.  And to do so the contents of data must be duplicated,
>> which causes GC churn and may slow the app considerably.
> 
> If I were doing this in C by loading an entire file in memory, I would
> lowercase it in-place which is both faster and takes less memory.
> D should support such efficient solutions.

The D language does.  It simply isn't a feature provided by Phobos.


Sean