Adding the ?. operator to D

[ddcovery]

On Saturday, 12 September 2020 at 19:26:30 UTC, Stefan Koch wrote:
> On Saturday, 12 September 2020 at 16:51:20 UTC, Per Nordlöw 
> wrote:
>> Has there been any discussions on adding the ?. operator to 
>> simplify structure matching on class trees typically found in 
>> compilers such as dmd.
>
> Hmm the problem with .? making implicit null checks is that it 
> encourages
> null checks.
> Rather than encouraging a state in which existence and logic 
> are statically asserted.

Absolutely.

Using a "functional" orientation with "only(p)" as alternative to 
  "MayBe" monad, an unwrapping with "fold( fun )(initialValue)" 
function with a default value:


> void main(){
>   Person p = new Person("Peter", new Person("John",null));
>   auto name =  only(p).
>                filter!"a !is null".map!"a.father".
>                filter!"a !is null".map!"a.father".
>                filter!"a !is null".map!"a.name".
>                fold!"b"("Unknown");
>   writeln( format!"The name of your great grandfather is 
> %s"(name) );
> }
> 
> class Person {
>   string name;
>   Person father;
>   this(string name, Person father) {
>     this.name=name;
>     this.father=father;
>   }
> }

We can unify

> .filter!"a !is null".map!(func)

as an alternative template

> .n!(func)

and unfold using

> .getOrElse(defaultValue)

that unifies

> .fold!"b"(initValue)

The result could be

> auto name =  
> only(p).n!"a.father".n!"a.father".n!"a.name".getOrElse("Unknown");

That is verbose, may be inneficient, but a nice experiment :-)