apply function

apply function

[Heime]

I have seem code that calls a function in the following way

  (apply 'nemboss-estring (pp-to-string object) bfname))

than the usual

  (emboss-estring (pp-to-string object) bfname)

What might be the reasons for doing so ?

Re: apply function

[Karan Ahlawat]

On 07/08/23 11:15, Heime wrote:

> I have seem code that calls a function in the following way
>
>    (apply 'nemboss-estring (pp-to-string object) bfname))
>
> than the usual
>
>    (emboss-estring (pp-to-string object) bfname)
>
> What might be the reasons for doing so ?

The most common use case I've seen for this is that apply can take in a 
list as the arguments to the function, where a list would not work, and 
then spread the elements of the list as individual arguments. So

     (+ (list 1 2 3))

doesn't work, but doing

     (apply #'+ (list 1 2 3)) ; outputs 6

does work, since it essentially reduces it to (+ 1 2 3)

Re: apply function

[Heime]

Sent with Proton Mail secure email.

------- Original Message -------
On Monday, August 7th, 2023 at 6:12 PM, Karan Ahlawat <ahlawatkaran12@gmail.com> wrote:


> On 07/08/23 11:15, Heime wrote:
> 
> > I have seem code that calls a function in the following way
> > 
> > (apply 'nemboss-estring (pp-to-string object) bfname))
> > 
> > than the usual
> > 
> > (emboss-estring (pp-to-string object) bfname)
> > 
> > What might be the reasons for doing so ?
> 
> 
> The most common use case I've seen for this is that apply can take in a
> list as the arguments to the function, where a list would not work, and
> then spread the elements of the list as individual arguments. So
> 
> (+ (list 1 2 3))
> 
> doesn't work, but doing
> 
> (apply #'+ (list 1 2 3)) ; outputs 6
> 
> does work, since it essentially reduces it to (+ 1 2 3)

I want to print the output from a macro by passing its output to a function.
Would use of apply be more appropriate ?

(show-mcode '(thismacro (* 3 5) (* 5 7)))

Or should I use something else ?  Should the expansion of a macro be turned 
to a list before passing it to a function ?

Re: apply function

[Emanuel Berg]

Karan Ahlawat wrote:

>> What might be the reasons for doing so ?
>
> The most common use case I've seen for this is that apply
> can take in a list as the arguments to the function, where
> a list would not work, and then spread the elements of the
> list as individual arguments. So
>
>     (+ (list 1 2 3))
>
> doesn't work, but doing
>
>     (apply #'+ (list 1 2 3)) ; outputs 6
>
> does work

The other use case is when one has the function stored in
a variable or provided as an argument, i.e. it is not known
from the static code perspective what function will be
executed. Observe:

(require 'cl-lib)

(defun test-final-line-f (fun pos-list)
  (cl-loop for p in pos-list do
    (goto-char p)
    (apply fun nil) ))

Here is a better example, maybe, where the function is either
`re-search-backward' or `re-search-forward' depending on the
argument REV (as in "reverse").

;;; -*- lexical-binding: t -*-
;;
;; this file:
;;   https://dataswamp.org/~incal/emacs-init/regexp.el

(require 'cl-lib)
(require 'dwim)

(defun replace-regexp-1 (re rep &optional beg end rev)
  (interactive
   `(,(read-string "regexp: ")
     ,(read-string "replace: ")
     ,@(use-region t)
     ,current-prefix-arg))
  (or beg (setq beg (point-min)))
  (or end (setq end (point-max)))
  (pcase-let ((`(,start ,sfun ,stop)
               (if rev
                   (list end #'re-search-backward beg)
                 (list beg #'re-search-forward end) )))
    (save-excursion
      (goto-char start)
      (let ((c 0))
        (while (apply sfun re stop '(t))
          (cl-incf c)
          (replace-match rep) )
        (message "matches replaced: %d" c) ))))

(defalias 'rr #'replace-regexp-1)

(provide 'regexp)

Source:
  https://dataswamp.org/~incal/emacs-init/dwim.el
  https://dataswamp.org/~incal/emacs-init/measure.el
  https://dataswamp.org/~incal/emacs-init/regexp.el

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