Why string alias is invariant ?

[Sergey Gromov]

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.

SnakE