Re: Does pcase need progn in if condition

[Jean Louis <bugs@gnu.support>]

* Emanuel Berg via Users list for the GNU Emacs text editor <help-gnu-emacs@gnu.org> [2021-05-22 07:40]:
> sometimes one can put useful things there, e.g. `let', and
> avoid using progn for this reason
> 
> Yeah, progn is imbecile, almost. only, for some reason, prog1
> is cool, and prog2 is weird.
> 
> (if t 1 2 3 4) ; 1

`progn` returns the the last value:

(progn
  '(do-something)
  '(do-more)
  emacs-build-number) ⇒ 1

and is better looking then `let` when empty:

(let ()
  (message "hello")
  '(do-more)
  emacs-build-number) ⇒ 1

`prog1' is very handy as it will not return the last value but the value from 1st form:

Example is when you wish to return a value and after that informa a user like here below:

(cond ((= a 1) (do-something))
      (t (prog1 nil (message "Query shall be longer..."))))

After returning `report' it will message user about it. Maybe you
wish to return the status of a function and continue informing
user?

(defun function ()
  "Return REPORT"
  (prog1
      report
    (message report))) 

And `prog2' is more than this, it will return the value from 2nd
form, not third or first.

(prog2 
    (do-this-but-not-keep-return-value)
    (do-this-and-keep-and-return-value)
  (do-more-something-but-return-value-is-from-above))

-- 
Jean

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