Expressing range constraints in CNF form

[Andrei Alexandrescu via Digitalmars-d]

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