Null references

[Nick Treleaven]

On 21/08/2012 14:30, Simen Kjaeraas wrote:
> On Tue, 21 Aug 2012 13:55:58 +0200, Regan Heath <regan at netmail.co.nz>
> wrote:
>
>> I don't buy the argument that having to explicitly type ".get" will
>> stop anyone from automatically coding statements like:
>>
>> val g: Option[T] = f.doSomething()
>> g.get.doSomethingElse()
>
> So don't give them that access?

Yes, or force conversion of the Option, handling the invalid case:

g.valueOrElse(null).doSomethingElse();

The above being explicit that null might be returned. The code is no 
safer, but it is much clearer. Also sometimes the caller has a better 
alternative value than null to use which is actually valid.

> I have an Option struct on my home computer that only allows access to
> the held value if the error case is simultaneously handled:
>
> Option!int a = foo();
>
> int n = a.match!(
>      (int x) => x,
>      (None n) => 0 // or throw, or what have you.
>      );
>
> The same solution is perfectly adaptable to MaybeNull, MaybeNan, and
> probably
> a host of others of the same family.
>
> With some clever use of opDispatch, one could write an Option struct that
> lets you call methods and acces members of the wrapped type, and return
> Option!ReturnType.

Apart from the opDispatch bit, I've been slowly working on something 
similar (Boost license):

https://github.com/ntrel/d-maybe/blob/master/maybe.d

I'm still working on it, but the basic ideas are there. I think the best 
bit is apply(), which supports passing multiple Maybe values, calling 
the delegate only if all Maybes are valid. In fact Maybe.map() is 
unnecessary due to apply().

Nick