Formal optional interfaces

[Paul Backus]

On Tuesday, 5 March 2019 at 20:03:30 UTC, Jacob Carlborg wrote:
> The next Objective-C integration feature that will be 
> implemented will probably be support for "protocols" (this is 
> what Objective-C calls "interfaces"). Objective-C protocols 
> supports optional methods. To call an optional method on a 
> Objective-C protocols you're supposed to do a runtime check to 
> see if the object actually implements the method.
>
> This got me thinking if this needs to be supported as well. 
> Regular D interfaces do not support optional methods. I asked 
> on Slack how people would think about this feature and I got a 
> reply mentioning Andrei's C++ talk "The Next Big Thing" [1]. 
> This talk includes informal optional interfaces.
>
> I see a couple of problems with informal optional and 
> non-optional interfaces:
>
> [...]

Atila Neves's `concepts` library on dub [1] already addresses 
most of these issues, without requiring new language features or 
invasive changes to existing code.

What you propose might have been a good idea if it were part of D 
from the start (or if you were designing "D 3.0"), but trying to 
retrofit it now, after so much code (e.g., all of Phobos) has 
already been written using the existing "informal" style, seems 
like the worst of both worlds to me. It won't save us from having 
to deal with the shortcomings of "informal" interfaces, because 
code that uses them will still exist, but it *will* make the 
language more complicated and difficult to learn, and force 
programmers to waste their time dealing with incompatibilities 
between "formal" and "informal" code instead of solving actual 
problems.

[1] https://code.dlang.org/packages/concepts