user defined literals

[Steven Schveighoffer]

On Fri, 25 Mar 2011 11:14:26 -0400, Trass3r <un at known.com> wrote:

> Steven Schveighoffer Wrote:
>> What is the need for this?  Just to replace complex(0, 5) with 5i?  I
>> don't see the huge benefit.
> I didn't claim it was of "huge" benefit. It's just some slight syntactic  
> sugar.
> Furthermore I didn't mean to praise its usefulness. I was actually  
> asking about practical issues that could arise if it _was_ implemented  
> the way I presented.

OK, I understand.

>> I simply find the complication of the language is not worth the  
>> benefits.
> This is more towards my question. Please elaborate how this complicates  
> the language.

Aside from the parsing issues, which I think would be minor, it's more of  
a problem with anyone and his mother is allowed to define literal types.

That is, someone could have 1fud and I'm not sure what the hell that  
means, so I have to go find out where that xyz is defined.  It must be  
global, since literals can be used anywhere, so it can be anywhere.

I just find that I have very little need for custom literals with all the  
wealth of features that D has, so I would be hesitant to stray from the  
f(value) style which is easily understood and readily parsable.

>> Also, complex is the clear and obvious beneficiary
> Yep, though I think complex literals would still better be handled by  
> the compiler specifically so that 5 + 3i is transformed to Complex(5,3)  
> rather than 5 + Complex(0, 3)

We have quite powerful mixin abilities such that I think something like:

complex!("5 + 3i")

could be rewritten as Complex!double(5, 3) without compiler help.  And  
like you said, the feature as proposed does not do the ideal thing.

>> What are other use cases?  BigXXX for sure, but Rational
>> probably cannot be covered by this since you need two numbers.
> Units, I think.
> auto dist = 1km;
> sin(90deg);

These are good points, but of course, units are open to interpretation.   
Not every unit has a standard abbreviation, and not every abbreviation is  
unique.

These things are highly context sensitive.  For example, I could say:

auto mytime = 1h + 3m + 4s;

auto mydistance = 15m;

Notice that 3 minutes and 15 meters can't really be distinguished by the  
compiler.

I like the idea of having this be more context-sensitive and also allow  
for the feature without language changes:

auto mytime = tstamp!("1h + 3m + 4s");

Not only that, but it's compile-time, meaning there is no actual call to  
some operator processor to generate the timestamp.

I believe we already have this for octal.  If we could define a more  
standard way to create these things, then I think it would be a good step.

-Steve