Feature request: __traits(canInstantiate), like __traits(compiles) but without suppressing syntax errors

Petar Petar
Fri Jan 17 15:34:44 UTC 2020

On Friday, 17 January 2020 at 14:57:27 UTC, Steven Schveighoffer 
> On 1/17/20 12:54 AM, FeepingCreature wrote:
>> Would be nice, but would be a hard spec change. This is the 
>> sort of thing that D should have been doing from the 
>> beginning, but we may be too far in now for __traits(compiles) 
>> to be changed to do this. However, this is the behavior I 
>> would ask for for __traits(canInstantiateWith).
> Just to point you at my previous thread on this: 
> https://forum.dlang.org/post/q1jequ$r63$1@digitalmars.com
> I think more tools for diagnosing why something doesn't happen 
> would be good, but the request of "tell me if I was stupid" is 
> I think impossible for the compiler to figure out.
> -Steve

In other languages, such as C#, TypeScript and Rust, generic 
functions can use only properties of generic types iff such 
properties were required in the function generic type 
constraints. AFAIK Nim's Concepts [1] are also moving in that 
direction, even though their current way is more similar to D (I 
chatted with one of their core developers, a couple of months ago 
at a conference, but I may be misremembering some details).

In other words:

void use(T)(T obj)
where (hasMethod!(T, "callMe", int function(string, bool)))
     int x = obj.callMe("hello", true); // this works

     // the following would cause the template definition to
     // fail to type check (before even being instantiated):
     // obj.someThingElse = 3;

Of course, requiring complete constraints for D templates would 
be quite controversial, (although many languages are able to get 
away with that), but I think it could be a good if we could 
opt-into that on a per function basis (e.g. using `where`, 
instead of `if`).

If D has such a feature, I believe that libraries like phobos 
would use it extensively, while user (app) code would use current 
template duck typing model and only opt-into a stronger model 
when a developer wants to debug why his template code doesn't 
work as expected.

[1]: https://nim-lang.org/docs/manual_experimental.html#concepts

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