Builtin Filter Algorithm

Question

In 25.1 there is (still) no builtin algorithm named filter. Why? What should I use instead? I think I've read somewhere that a standardized filter algorithm should be added to Emacs.

Answer 1

My crystal ball tells me that you want

(require 'seq)

and then

... (seq-filter ...) ...

The seq package is built-in, even though you have to say explicitly when you want to use it. Of course it's still not called filter, you may say, so you additionally need

(defalias 'filter #'seq-filter)

tho I'd advise against doing that.

Answer 2

Does it need to be "named filter"?

As @Stefan mentions, seq-filter is available in Emacs 25.1.

Earlier releases, going back to almost forever, have functions remove-if and remove-if-not, in library cl.el. Recent releases provide the same functions under the names cl-remove-if and cl-remove-if-not, in library cl-seq.el (so you need not load library cl.el at runtime, to get them).

All of these do essentially the same thing, and probably the same thing as what you are requesting (whose behavior is not really specified). You can of course name any of them "filter" using defalias, if you want.