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Comment by seanw444 | Hacker News Reader

I've never run into a generic "filter" function which keeps only the non-matching elements.

Common Lisp's filter is remove-if which works this way.

(It also has remove-if-not but that's deprecated and if you use it your code smells.)

`remove-if-not` was deprecated before the Common Lisp standard was approved and yet it remained (and will never be removed because the standard will never be updated). That's not deprecated for any practical purpose. And it's more convenient than using `(remove-if (complement #'some-predicate) sequence)`

Scheme has `filter` and `filter-not` in the SRFI-1 list library. Both of which can easily be written using a fold to bring this vaguely on topic.

Nope! Common Lisp's filter is in fact remove-if-not, which is exactly the same thing as "keep if": keep all items which match the predicate (removing those that do not).

I suspect the reason the function was deprecated was its naming, nothing more; had it been called retain-if or keep-if, it would not have attracted deprecating attention.

The smell added to your code is just the double-negative name of that function, not what it's doing for you.

The name filter smells even more. Is that filtering for items that match? Or filtering out?

In physical filters, sometimes the filtrate is considered the payload output (that which passes through the filter) and sometimes the retentate (that which is caught in the filter).

keep-if is readable.

I agree that keep-if is a better name. But you reiterated my point: remove-if works in the counterintuitive way the original comment noted, i.e. not like the common connotation of 'filter.'

As for deprecation, IIRC the '-if-not' functions were deprecated because the committee felt the 'complement' function accomplished that task better.

Edit: My IIRC seems largely correct. More detail at

https://www.lispworks.com/documentation/HyperSpec/Issues/iss...

Nonetheless, they were fooled by that function, because it's just keep-if by a funny name that includes "not" suggesting that it contains a complement that might be factored out.

If you want keep-if, you don't want to use a different function, which forces you to complement your predicate. If you want (keep-if #'redp jellybean-list) to keep the red jelly beans, you don't want to write (remove-if (complement #'not-red-p) jellybean-list). If keep-if has a silly name remoe-if-not, you might nonetheless prefer (remove-if-not #'redp jelly-bean-list).

Shims like complements are ugly, and compilers won't optimize through them for arbitrary function definitions (whose source code is not even in scope), so it is good to have both keepers and removers. Heck, it's useful to have a function which does both in one pass returning two values: the filtrate and the retentate.

Smalltalk has #reject: which does that. You could, of course, just wrap a not around the test in the closure, but sometimes reject with a well-named predicate is easier to read.

bsnpApproved := tvShows reject: [ :eachShow | eachShow hasNaughtyContent ].

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