std.v2020.algorithm etc[ WAS: Is run.d going to be expand for runtime and the phobos library?]

[Paul Backus]

On Monday, 22 June 2020 at 16:42:55 UTC, Andrei Alexandrescu 
wrote:
> On 6/22/20 12:14 PM, Paul Backus wrote:
>> Making it work in general requires either subverting the type 
>> system (like Rebindable does) or adding more special cases to 
>> the language.
>
> The principled approach is to generalize the trick currently 
> used only for arrays. Consider the lowering:
>
> "Whenever something is passed to a function, .opOnCall is 
> appended."
>
> Then:
>
> auto ref T opOnCall(T)(auto ref T x) { return x; }
> const(T)[] opOnCall(T)(const(T[]) x) { return x; }
> immutable(T)[] opOnCall(T)(immutable(T[]) x) { return x; }
>
> This is just a sketch, haven't thought it through, but if 
> properly defined the scheme would work having built-in arrays 
> work as they are today, and would also allow user-defined types 
> to define their own opOnCall as they find fit.

The trick used for arrays does not only apply to function calls:

     const(int[]) a = [1, 2, 3];
     const(int)[] b = a; // compiles

IMHO the principled way to allow user-defined implicit 
conversions is...to allow user-defined implicit conversions. But 
iirc that's a can of worms Walter prefers not to open.