Re: Functional composition in ELisp: add `compose' function?

[Stefan Monnier]

> 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