Should we add drop to Phobos?

[Jonathan M Davis]

On Wednesday, August 17, 2011 01:23:18 Daniel Murphy wrote:
> "Jonathan M Davis" <jmdavisProg at gmx.com> wrote in message
> news:mailman.2361.1313462679.14074.digitalmars-d at puremagic.com...
> 
> > They already exist:
> > 
> > dropUntil == find
> > dropWhile == !find (i.e. same thing, but the predicate must be negated)
> > takeUntil == until
> > takeWhile == !until
> 
> I'd prefer to have all of these available in phobos.
> 
> > Andrei is against adding functions which are functionally identical to
> > existing functions save for the negation of the predicate, and he's
> > definitely
> > against adding aliases. To some extent, I'd like to have functions which
> > are
> > functionally identical save for the negation of the predicate, because I
> > think
> > that it makes for clearer, neater code, but I think that he has a very
> > good
> > point in that we don't want to clutter Phobos with functions which are
> > nearly
> > identical. New functions must add something of definite value to be
> > worth
> > adding.
> 
> I think one argument that is often used to argue for more features in the
> language applies here - less functions makes for a cleaner phobos, but
> messier user code.
> 
> eg:
> dropWhile!predicate(r) vs find!(Not!predicate)(r)
> The second version is noisier, requires extra parentheses, and requires an
> import of std.functional whenever it's used.
> Won't these end up as simple aliases in phobos?
> 
> Disclaimer: I haven't read the github discussion.

Ultimately, Andrei's argument against having a function like dropWhile when we 
already have find is that the new function doesn't really add any new 
functionality. All you have to do is negate the predicate, and what we have 
already works. Adding new functions which do essentially the same thing is 
therefore just cluttering up the library. On some level, I agree, and on some 
level I don't.

Regardless of that, however, we've already agreed previously that Phobos won't 
have aliases simply to give alternate names for functions. So, even if you 
could talk Andrei into a function like dropWhile or takeWhile, you're never 
going to see something like dropUntil, because it would be a pure alias. 
_Maybe_ you could get it if you argued for replacing the version of find which 
takes just a predicate and a single range with dropUntil, but a simple alias 
is agaisnt the Phobos coding standards. Those are pretty much just used as 
part of the deprecation process.

If you want to argue that dropUntil, dropWhile, takeUntil, and takeWhile 
should be added to Phobos, feel free to start a new thread on it, but Andrei 
is against it, and it's already been discussed a fair bit in that pull request 
(so, I'd advise reading that before posting a new thread on the issue). And 
regardless, I don't think that it's a good idea to bring it up before the 
issue of drop has been addressed. Andrei is already against drop, and that 
should arguably be a shoe-in. Fortunately, assuming that there's actually 
community support for it, he's willing for it to be added.

So, drop may be added, but you're going to have to come up with some very 
well-written, objective arguments for why the others should be added if you're 
going to stand any chance of convincing Andrei, and I'd wait on discussing it 
until drop is in, since if we can't even get drop in, we're sure not getting 
the others in.

- Jonathan M Davis