try/catch idiom in std.datetime

[Walter Bright]

On 11/18/2013 11:44 AM, Andrei Alexandrescu wrote:
> 1. Consider:
>
>      this(in DateTime dateTime, immutable TimeZone tz = null) nothrow
>      {
>          try
>              this(dateTime, FracSec.from!"hnsecs"(0), tz);
>          catch(Exception e)
>              assert(0, "FracSec's constructor threw when it shouldn't have.");
>      }
>
> That's because FracSec.from!"hnsecs"(n) may throw for some values of n. To me,
> this is overkill. Clearly there must be a way to construct a zero time interval
> without much code being executed at all, let alone checks and exceptions and
> whatnot. (In particular FracSec.zero looks like the ticket.) Furthermore, having
> a simpler constructor call a more complicated constructor reminds one of the
> mathematician who throws away the water in the kettle to regress to an already
> solved problem.

I agree. But I'll add that it would be even better to redesign FracSec.from so 
it never throws at all - have it assert(0) for invalid values being passed to it.

Exceptions should not be thrown for invalid arguments. Data validation should be 
handled separately.