[Request] A way to extract all instance of X from a range

[Jakob Ovrum via Digitalmars-d]

On Monday, 21 March 2016 at 11:34:15 UTC, Nick Treleaven wrote:
> There is a pull for Option:
> https://github.com/D-Programming-Language/phobos/pull/3915
>
> We could have:
>
> // fun takes r.front and produces an Option of that type
> auto mapFilter(alias fun, R)(R r);
>
> // turn a possibly null value into an Option
> Option!T nullFilter(T)(T v) if (isNullable!T);
>
> auto src = [new Object(), new T(), null];
> auto res = mapFilter!(e => nullFilter(cast(T)e));
> assert(res.equal([src[1]]));

Using the interface proposed in the PR:

auto src = [new Object(), new T(), null];
assert(src.map!(e => 
option(cast(T)e)).joiner.equal(only(src[1])));