Checking function parameters in Phobos

[Simen Kjærås]

On 26.11.2013 21:14, inout wrote:
> I find this to be too verbose to be useful.

This I understand. It is actually the best argument I can find in favor 
of doing constraints checking upon construction, rather than in a 
separate construction function. This allows you to use one alias instead 
of two.


 > And you also need to
> be very careful not to discard any existing qualifiers on input
> and carry them over. This will essentially make any function that
> uses them to be templated, while all the instances will be the
> same (yet have a different body since no D compiler merges
> identical functions).

Could you give an example of this? It's a bit unclear to me what you 
mean. Is it this sort of thing:

   auto doPrimeStuff(Validated!(int, isPrime) a){return a;}
   auto doLessThan42Stuff(Validated!(int, lessThan42) a){return a;}

   Validated!(int, isPrime, lessThan42) i = 13;

   i.doPrimeStuff().doLessThan42Stuff();

Where the second chained function call fails due to lessThan42 being 
removed from the constraints? (There's also the problem that this 
wouldn't work in the first place due to D's lack of implicit conversions)

> I still find wrapping int with some type to add a tag to it
> without adding any methods is not a great idea - it doesn't scale
> well with composition and tag propagation. Any operation that
> expects int will essentially discard all the qualifiers.

And any operation of the kind you describe is likely to change the value 
so the constraints need to be checked again. abs(Validated!(int, 
isNegative)) cannot possibly return the same type it received.

-- 
   Simen