Optional and orElse: design feedback/critique?

[aliak]

On Saturday, 27 July 2019 at 14:46:55 UTC, Alexandru Ermicioi 
wrote:
> On Saturday, 27 July 2019 at 13:17:32 UTC, aliak wrote:
>> Hi,
>>
>> Can I ask for feedback on what people expect an optional/maybe 
>> type to have, and how to behave. There have been a number of 
>> discussions on the forums about it so far, and attempted PRs 
>> in phobos.
>
> Would be nice to have functionality similar to Optional from 
> java 8+, such as orElseThrow, orElseGet, ifPresent etc. Also

A lot of the java stuff is covered with std.algorithm and orElse

auto a = some(3);
auto gotValue = a.orElse(7); // orElseGet
a.each!(a => writeln(a)); // ifPresent

orElseThrow on the other hand would have to be added separately. 
I use that quite often as well but I can't work it in to the 
provided orElse because "throw blah" is not an expression. I have 
another experimental thing where that's available though -> 
https://aliak00.github.io/ddash/ddash/utils/orelse/orElseThrow.html (but that library is not the most stable for now)

One other way would be to add an expression throw:

off top of me head:

auto throw_(T : Throwable)(lazy T e) { throw e(); }

Then this would work:

a.orElse(throw_(new Exception("")));

> would be nice if it worked perfectly with immutable/const data, 
> and when Optional itself is immutable/const. Another nice

You can currently wrap cost/immutable data. I try to test to 
ensure things work with qualified optionals [0], but it's tricky 
with the const system in D, and with how inout behaves with 
wrapper objects [1, 2]. The other problem here is that const 
ranges are not ranges. So maybe a decay function on an optional 
type?

const a = some(3);
a.decay.map...

[0]: 
https://github.com/aliak00/optional/blob/de223a7d63d346386b0d02d119cf4bdd79366299/tests/optional.d#L11
[1]: https://issues.dlang.org/show_bug.cgi?id=19126
[2]: https://issues.dlang.org/show_bug.cgi?id=19125

> feature would be conversion from immutable to const optional, 
> and from immutable/const optional to mutable Optional with 
> const payload through probably copy constructor and opCast 
> methods.

Hmm. Yes. The latter can probably be handled with the 
"decay-like" function. Isn't the former handled by D anyway? 
immutable is implicitly convertible to const?

immutable a = some(3);
const Optional!int b = a;

>
> Best regards,
> Alexandru

Thanks for the feedback!