Improvements to std.typecons.Nullable

[BLM768]

On Wednesday, 9 October 2013 at 17:07:18 UTC, monarch_dodra wrote:
> That was my point. Writting "Nullable!T == T" is the exact same 
> thing: Comparison of a value with the absence of a value. It's 
> neither equal nor different, it's an error.

Equality comparison is a bit different from properties such as 
emptiness. Virtually every language allows comparisons between 
null and non-null objects. In D and Java, this is the behavior 
for object references; in Ruby and Lua, this is the behavior for 
all values. Nullable!T was seemingly designed to emulate that 
sort of behavior, and overloading opEquals() would provide 
semantically correct behavior without any exceptions ever being 
thrown.

> My point though is that it's implicit nature makes it so that 
> you forget about said state.
>
> Adding a simple ".get()" is not that much verbose, and sticks 
> out that you are calling something that can be an error. You 
> should either be operating the handler (Nullable), or the Value 
> itself. Not a bit of "maybe the nullable, maybe the value"

Ultimately, it seems to boil down to a personal preference: 
should Nullable!T emulate the behavior of D's existing nullable 
types, or should it use a more explicit syntax? I personally lean 
toward consistency, in part because doing otherwise would be a 
breaking change that doesn't really seem justified unless we can 
solve the issue with _all_ nullable references. I do see the 
merits of your proposal, though.