Interpolated strings

[Jonathan Marler via Digitalmars-d]

On Wednesday, 19 April 2017 at 12:03:47 UTC, Stefan Koch wrote:
> On Wednesday, 19 April 2017 at 11:59:51 UTC, Jonas Drewsen 
> wrote:
>
>> What about supporting an optional prefix inside the {} like:
>>
>> int year = 2017;
>> format($"The date is {%04d year}");
>>
>> so if there is a % immediately following the { then the chars 
>> until next whitespace is format specifier. You can of course 
>> leave out the format specifier and it will default to %s.
> I really don't see how string interpolation is better then
> ` "The date is " ~ format("%04d", year)); `

I can think of 3 reasons.

1. Requires GC.

NOTE: I believe that most applcations should use GC memory, that 
being said, library code has to be nogc wherever it can if it 
wants to be used by nogc apps, so this solution is unusable in 
alot of library code.

2. It's verbose.  Jonas provided a good example of why and he 
makes a good point that your example only has 1 formatted 
argument and this problem gets much worse with multiple.

3. The biggest reason I wouldn't use this solution because it 
uses string composition.  String composition wastes memory and 
memory management cycles.  And when I say "waste" what I mean is, 
the use is unnecessary.  In general composition is a powerful 
tool because it allows you to easily overload and abstract and 
add new functionality, however, it requires runtime overhead.  
This is the reason that the toString(delegate) was created to 
replace the composable toString paradigm.  It's also the reason 
that string formatting exists at all.

To summarize:

// requires GC, too verbose, uses string composition which wastes 
heap resources
writeln("The date is " ~ format("%04d", year));

// efficient, but format string and args can get out of sync.
writefln("the date is %04d", year);

//
writefln($"the date is {year:04d}");

If string interpolation gets reduced to the 2nd case, then it 
will have the same efficiency and solve a problem.  Whether that 
problem justifies the change is up for debate, but I think it 
*might be*.