Proposing std.typecons : Optional (with PR)

[FeepingCreature]

On Tuesday, 11 June 2019 at 12:30:10 UTC, aliak wrote:
> Your argument against it being a range sounds a bit too 
> philosophical. I think we should  consider the actual technical 
> advantages/disadvantages of having it as a range or not. And if 
> not, at least getting functions like map/flatmap/each/whatever 
> as usable with optional.
>

We have "map for Optional/Nullable", that's `apply`.

> I basically think a bit of thought should be put in to putting 
> an optional type in the standard library instead of copying 
> nullable (as mentioned, the range thing is not the only problem 
> - another big one is treating null as a valid value for 
> classes).

That's a fundamental problem with D though. Null *is* a valid 
value for classes. Making a class type optional just means that 
you're adding *another* type that also means "absent" at a 
different level of abstraction.

Was it the right decision to allow null in class types? Who 
knows. (No.) But I don't think that's a decision that should be 
patched in library types.