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Re: Does pcase need progn in if condition
From: |
Jean Louis |
Subject: |
Re: Does pcase need progn in if condition |
Date: |
Sat, 22 May 2021 12:09:28 +0300 |
User-agent: |
Mutt/2.0.6 (2021-03-06) |
* 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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Re: Does pcase need progn in if condition, Emanuel Berg, 2021/05/21