Expressing range constraints in CNF form

[Stefan Koch via Digitalmars-d]

On Sunday, 11 June 2017 at 15:25:11 UTC, Andrei Alexandrescu 
wrote:
> On 6/11/17 11:11 AM, Nick Treleaven wrote:
>> On Sunday, 11 June 2017 at 00:28:58 UTC, Andrei Alexandrescu 
>> wrote:
>>> https://github.com/dlang/phobos/pull/5461
>>>
>>> There's many good advantages to this. The immediate one is 
>>> the constraint is better structured and easier to understand. 
>>> Then, the compiler can print the exact clause that failed, 
>>> which improves the precision and quality of the error message.
>> 
>> Great!
>
> Thanks.
>
>>> // Also possible (no change to the language)
>>> enum bool isInputRange(R) =
>>>     is(typeof((ref R r) => r)) && msg("must be copyable")
>>>     && is(ReturnType!((R r) => r.empty) == bool) && msg("must 
>>> support bool empty")
>>>     && is(typeof(lvalueOf!R.front)) && msg("must support 
>>> front")
>>>     && is(typeof(lvalueOf!R.popFront)) && msg("must support 
>>> back");
>> 
>> I'm not getting how this works.
>
> Ostensibly the function is trivial:
>
> bool msg(string) { return true; }
>
> It doesn't change the semantics. The compiler would recognize 
> it as an intrinsic and would print the message if the clause to 
> its left has failed.
>
>
> Andrei

Exposing magic functions which are recognized by name strikes me 
as being worse then proper compiler intrinsics.