* Functional composition in ELisp: add `compose' function?
@ 2021-02-15 1:39 Stefan Kangas
2021-02-15 3:02 ` Stefan Monnier
0 siblings, 1 reply; 2+ messages in thread
From: Stefan Kangas @ 2021-02-15 1:39 UTC (permalink / raw)
To: emacs-devel
I often miss functional composition when I code Emacs Lisp, to say
things like:
(seq-filter (compose #'not #'stringp) '("a" "b" 1 2))
=> (1 2)
Here's an example definition:
(defun compose (&rest fns)
(cl-destructuring-bind (fun . rest) (reverse fns)
(lambda (&rest args)
(seq-reduce (lambda (v f) (funcall f v))
rest
(apply fun args)))))
There is at least some demand for it: the popular third-party library
dash.el provides it[1]. It also exists in Scheme[2] and Common Lisp[3].
Could we add this to Emacs Lisp?
Footnotes:
[1] https://github.com/magnars/dash.el#-compose-rest-fns
[2] https://www.gnu.org/software/guile/manual/html_node/Higher_002dOrder-Functions.html
[3] https://common-lisp.net/project/cl-utilities/doc/compose.html
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Functional composition in ELisp: add `compose' function?
2021-02-15 1:39 Functional composition in ELisp: add `compose' function? Stefan Kangas
@ 2021-02-15 3:02 ` Stefan Monnier
0 siblings, 0 replies; 2+ messages in thread
From: Stefan Monnier @ 2021-02-15 3:02 UTC (permalink / raw)
To: Stefan Kangas; +Cc: emacs-devel
> Here's an example definition:
>
> (defun compose (&rest fns)
> (cl-destructuring-bind (fun . rest) (reverse fns)
> (lambda (&rest args)
> (seq-reduce (lambda (v f) (funcall f v))
> rest
> (apply fun args)))))
LGTM (tho I'd use `pcase-let` instead of `cl-destructuring-bind`, of
course).
Note that, sadly, you can use `nreverse` up there.
Note, also sadly, that
(compose #'not #'stringp)
will be significantly less efficient than
(lambda (x) (not (stringp x)))
So we definitely should accompany it with a compiler-macro, which should
be able to macro-expand it to something equivalent to
(lambda (&rest args) (not (apply #'stringp args)))
but note that it's still significantly less efficient than
(lambda (x) (not (stringp x)))
which I find personally quite readable. I wish the byte-compiler was
able to turn
(lambda (&rest args) (not (apply #'stringp args)))
into
(lambda (x) (not (stringp x)))
[ Among other things, I believe it could improve the efficiency of
`cl-generic.el` function calls/dispatch. ]
but it seems difficult/dangerous to do in general.
Stefan
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