Semantics of toString

[Don]

Bill Baxter wrote:
> On Tue, Nov 10, 2009 at 2:51 AM, Don <nospam at nospam.com> wrote:
>> Lutger wrote:
>>> Justin Johansson wrote:
>>>
>>>> Lutger Wrote:
>>>>
>>>>> Justin Johansson wrote:
>>>>>
>>>>>> I assert that the semantics of "toString" or similarly named/purposed
>>>>>> methods/functions in many PL's (including and not limited to D) is
>>>>>> ill-defined.
>>>>>>
>>>>>> To put this statement into perspective, I would be most appreciative of
>>>>>> D NG readers responding with their own idea(s) of what the semantics of
>>>>>> "toString" are (or should be) in a language agnostic ideology.
>>>>>>
>>>>> My other reply didn't take the language agnostic into account, sorry.
>>>>>
>>>>> Semantics of toString would depend on the object, I would think there
>>>>> are
>>>>> three general types of objects:
>>>>>
>>>>> 1. objects with only one sensible or one clear default string
>>>>> representations, like integers. Maybe even none of these exist (except
>>>>> strings themselves?)
>>>>>
>>>>> 2. objects that, given some formatting options or locale have a clear
>>>>> string representation. floating points, dates, curreny and the like.
>>>>>
>>>>> 3. objects that have no sensible default representation.
>>>>>
>>>>> toString() would not make sense for 3) type objects and only for 2) type
>>>>> objects as part of a formatting / localization package.
>>>>>
>>>>> toString() as a debugging aid sometimes doubles as a formatter for 1)
>>>>> and
>>>>> 2) class objects, but that may be more confusing than it's worth.
>>>>>
>>>> Thanks for that Lutger.
>>>>
>>>> Do you think it would make better sense if programming languages/their
>>>> libraries separated functions/methods which are currently loosely
>>>> purposed
>>>> as "toString" into methods which are more specific to the types you
>>>> suggest (leaving only the types/classifications and number thereof to
>>>> argue about)?
>>>>
>>>> In my own D project, I've introduced a toDebugString method and left
>>>> toString alone. There are times when I like D's default toString printing
>>>> out the name of the object
>>>> class.  For debug purposes there are times also when I like to see a
>>>> string printed
>>>> out in quotes so you can tell the difference between "123" and 123.  Then
>>>> again, and since I'm working on a scripting language, sometimes I like to
>>>> see debug output distinguish between different numeric types.
>>>>
>>>> Anyway going by the replies on this topic, looks like most people view
>>>> toString as being good for debug purposes and that about it.
>>>>
>>>> Cheers
>>>> Justin
>>>>
>>> Your design makes better sense (to me at least) because it is based on why
>>> you want a string from some object.
>>> Take .NET for example: it does provide very elaborate and nice formatting
>>> options based and toString() with parameters. For some types however, the
>>> default toString() gives you the name of the type itself which is in no way
>>> related to formatting an object. You learn to work with it, but I find it a
>>> bit muddled.
>>> As a last note, I think people view toString as a debug thing mostly
>>> because it is very underpowered.
>> There is a definite use for such as thing. But the existing toString() is
>> much, much worse than useless. People think you can do something with it,
>> but you can't.
>> eg, people have asked for BigInt to support toString(). That is an
>> over-my-dead-body.
> 
> You can definitely do something with it -- printf debugging.  And if I
> were using BigInt, that's exactly why I'd want BigInt to have a
> toString. 

I almost always want to print the value out in hex. And with some kind 
of digit separators, so that I can see how many digits it has.

  Just out of curiousity, how does someone print out the
> value of a BigInt right now?

In Tango, there's just .toHex() and .toDecimalString(). Needs proper 
formatting options, it's the biggest thing which isn't done. I hit one 
too many compiler segfaults and starting patching the compiler instead 
<g>. But I really want a decent toString().

Given a BigInt n, you should be able to just do

writefln("%s %x", n, n);  // Phobos
formatln("{0} {0:X}", n); // Tango

To solve this part of the issue, it would be enough to have toString() 
take a string parameter. (it would be "x" or "X" in this case).

string toString(string fmt);
But the performance would still be very poor, and that's much more 
difficult to solve.