DIP9 -- Redo toString API

[Don]

spir wrote:
> On Mon, 22 Nov 2010 10:14:03 +0100
> Don <nospam at nospam.com> wrote:
> 
>> Tomek Sowiński wrote:
>>> Don <nospam at nospam.com> napisał(a):
>>>
>>>> The efficiency issues are important, but are not the primary motivation.
>>>> toString() is just wrong. The idea that there is ONE AND ONLY ONE 
>>>> textual representation of an object, is, frankly, idiotic.
>>> I always thought of toString() as an aid in debugging, where having one 
>>> and only way to print out an object makes sense. 
>> It isn't just used in debugging. It's used for writefln.
>> But even for debugging, it doesn't work.
>>
>> If I have a struct:
>>
>> struct Foo
>> {
>>     Complex!(double) val;
>> }
>>
>> and, for debugging purposes, I want to write 'val' to 10 decimal places.
>> How do I do that?
> 
> I agree with you. toString lacks flexibility in some (not very common but relevant) cases, that would be brought by an optional format specifier. I would enjoy beeing able to write this with toString, instead of a custom text() method:
> 
> struct Complex {
>     float r,i;
>     string text(string numberFormat="%.3f") {
>         string outFormat = format("%s+%sj", numberFormat,numberFormat);
>         return format(outFormat, this.r,this.i);
>     }
> }
> void main () {
>     auto c = Complex(1.11111,3.333333333);
>     writeln(c.text());
>     writeln(c.text("%.7f"));
> }
> ==>
> 1.111+3.333j
> 1.1111100+3.3333333j
> 
> 
> But, to be fair, does this have anything to do with the current <writeTo vs toString> debate?
> The only actual point of said debate is efficiency.

The debate was triggered by me saying (in Bugzilla) that BigInt would 
not provide a parameterless toString(), because I can't implement it.
Instead, in BigInt, I provided a function:
void toString( void delegate(const(char)[]) sink, string format);

which solves the formatting issue, as well as the performance issue.

I'm certainly open to other ways of solving the problem. But
string toString();
doesn't seem adequate for the task.