Why does `filterBidirectional` exist, and why isn't it called `filter`?

[Steven Schveighoffer]

On 3/9/23 3:06 AM, FeepingCreature wrote:
> Yes I know the stated reason, to save time initializing the range in 
> `filter`.
> 
> You know what I did because we didn't know `filterBidirectional` existed?
> 
> I literally *walked the entire range* returned by `filter` to find the 
> last matching element.

Why not `rng.retro.filter`?

> `filter` should expose `back`. By all means, let `back` lazily 
> initialize, but I don't understand how `filterBidirectional` was ever 
> considered acceptable. Since when do we sacrifice ease of use for 
> performance? I mean, since 2011 apparently. - This is bad ergonomics, 
> bad discoverability and bad API design. `filterBidirectional` delenda est.

This has been a constant debate -- should ranges be allowed to lazily 
initialize on the first call to front (or back)?

I'd say no. back/front shouldn't be modifying operations. I dislike the 
idea of storing a "was initialized" flag and having to check that on 
every call.

That being said, there's no formal requirement for it. It's just a 
convention I think Phobos should stick to for its included ranges.

-Steve