Stacking policies

[ZombineDev via Digitalmars-d]

On Saturday, 2 July 2016 at 21:37:12 UTC, ZombineDev wrote:
> On Saturday, 2 July 2016 at 20:48:51 UTC, Andrei Alexandrescu 
> wrote:
>> So I'm working on this checked integral thing and starting to 
>> really get into the possibilities offered by DbI. One core 
>> operation is "stacking" two policies on top of each other, 
>> i.e. an operation is first offered to the first one, and then 
>> the second one. Here's an excerpt:
>>
>> struct HookStack(H1, H2)
>> {
>>     import std.experimental.allocator.common : stateSize;
>>     import std.traits : hasMember;
>>
>> private:
>>     static if (stateSize!H1) H1 h1;
>>     else alias h1 = H1;
>>     static if (stateSize!H2) H2 h2;
>>     else alias h2 = H2;
>>
>> public:
>>     static if (hasMember!(H1, "defaultValue"))
>>         alias defaultValue = H1.defaultValue;
>>     else static if (hasMember!(H2, "defaultValue"))
>>         alias defaultValue = H2.defaultValue;
>>
>>     static if (hasMember!(H1, "min"))
>>         alias min = H1.min;
>>     else static if (hasMember!(H2, "min"))
>>         alias min = H2.min;
>>
>>
>>     static if (hasMember!(H1, "max"))
>>         alias max = H1.max;
>>     else static if (hasMember!(H2, "max"))
>>         alias max = H2.max;
>>
>>     static if (hasMember!(H1, "hookOpCast"))
>>         Dst hookOpCast(Dst, Src)(Src value)
>>         {
>>             return h1.hookOpCast!Dst(value);
>>         }
>>     else static if (hasMember!(H2, "hookOpCast"))
>>         Dst hookOpCast(Dst, Src)(Src value)
>>         {
>>             return h2.hookOpCast!Dst(value);
>>         }
>>     ...
>> }
>>
>> There's a bunch more stuff that looks very similar. This 
>> should be automated, i.e. depending on the types, aliases, 
>> enums, methods etc. that H1 and/or H2 define, there should be 
>> appropriate definitions in the composition.
>>
>> This is the kind of stuff that makes D awesome. Is anyone 
>> interested in exploring this? I'm already in an interruption 
>> from RCStr working on the checked int. I'll continue with the 
>> manual implementation.
>>
>>
>> Thanks,
>>
>> Andrei
>
> [...]
>
> Anyway, here's a working solution using string mixins:
> https://dpaste.dzfl.pl/4b1c702c00b8

Here's a more complete version that handles access to stateful 
members correctly: https://dpaste.dzfl.pl/aa19a428adfe

Unfortunately, I bumped into a language limitation that allows 
aliases of the form:

alias newName = this.member.oldName;

... only if `oldName` is manifest constant (enum). In other 
words, unlike "alias this", "alias" can't refer to instance 
members, because you get: Error: need 'this' for 'oldName' of 
type '<typeof(oldName)>'. Look for Error 1) in the latest link 
for more details.

IMO, "alias" should be at least as powerful as "alias this" in 
this situation.
What's your opinion about this, Andrei?