• TheGag96 (12/12) Feb 13 2017 Tonight I stumbled upon Andrei's concept of policy-based design
• Daniel Kozak via Digitalmars-d-learn (7/9) Feb 13 2017 Because certificate is expired, dpaste use certs from Lets Encrypt which...
• TheFlyingFiddle (33/45) Feb 14 2017 Something like this can be used to check if the mixin has a
• TheGag96 (4/5) Feb 14 2017 Oh, I didn't know you could name mixin template instantiations

TheGag96 <thegag96 gmail.com> writes:

Tonight I stumbled upon Andrei's concept of policy-based design 
(https://en.wikipedia.org/wiki/Policy-based_design) and tried to 
implement their example in D with the lack of multiple 
inheritance in mind.

https://dpaste.dzfl.pl/adc05892344f (btw, any reason why 
certificate validation on dpaste fails right now?)

The implementation isn't perfect, as I'm not sure how to check 
members of mixin templates so that you  could verify whether 
print() and message() are actually where they should be. How 
would you do that? Is there any use for this kind of thing in D, 
and if so, what would it be? I've hardly dabbled in OOP patterns, 
but the abstraction seems kinda interesting.

Daniel Kozak via Digitalmars-d-learn <digitalmars-d-learn puremagic.com> writes:

Dne 14.2.2017 v 07:48 TheGag96 via Digitalmars-d-learn napsal(a):
 https://dpaste.dzfl.pl/adc05892344f (btw, any reason why certificate 
 validation on dpaste fails right now?)

Because certificate is expired, dpaste use certs from Lets Encrypt which 
has short time of validity
I have same issue in the past with my websites, now I am using cron to 
keep it updated
1      0       *       *       *       certbot renew --nginx 
--keep-until-expiring

TheFlyingFiddle <kurtyan student.chalmers.se> writes:

On Tuesday, 14 February 2017 at 06:48:33 UTC, TheGag96 wrote:
 Tonight I stumbled upon Andrei's concept of policy-based design 
 (https://en.wikipedia.org/wiki/Policy-based_design) and tried 
 to implement their example in D with the lack of multiple 
 inheritance in mind.

 https://dpaste.dzfl.pl/adc05892344f (btw, any reason why 
 certificate validation on dpaste fails right now?)

 The implementation isn't perfect, as I'm not sure how to check 
 members of mixin templates so that you  could verify whether 
 print() and message() are actually where they should be. How 
 would you do that? Is there any use for this kind of thing in 
 D, and if so, what would it be? I've hardly dabbled in OOP 
 patterns, but the abstraction seems kinda interesting.

Something like this can be used to check if the mixin has a 
specific member:

template hasMixinMember(alias mixin_, string member) {
   enum hasMixinMember = __traits(compiles, () {
       mixin mixin_ mix;
       static assert(__traits(hasMember, mix, member));
   });
}

struct HelloWorld(alias OutputPolicy, alias LanguagePolicy)
   if(hasMixinMember!(OutputPolicy, "print") &&
      hasMixinMember!(LanguagePolicy, "message"))
{
   mixin OutputPolicy;
   mixin LanguagePolicy;

   void run() {
     print(message());
   }
}

Note: This method could fail if you do some compile-time 
reflection black magic inside the mixins.



Could also do this:

struct HelloWorld(alias OutputPolicy, alias LanguagePolicy)
{
   mixin OutputPolicy output;
   mixin LanguagePolicy lang;

   void run() {
     output.print(lang.message());
   }
}

If "output" / "lang" does not contain a particular member you 
will get a compile time error at the usage point (although it's 
not the best message).

TheGag96 <thegag96 gmail.com> writes:

On Tuesday, 14 February 2017 at 10:05:19 UTC, TheFlyingFiddle 
wrote:
 (snip)

Oh, I didn't know you could name mixin template instantiations 
like that! Thanks for the tip, that makes things work nicely!