Checking conditions unrelated to expression

Checking conditions unrelated to expression

[Heime via Users list for the GNU Emacs text editor]

It looks like pcase can only be used with some variable condition

This will check condition of featr

  (pcase featr
     ('this (do-this))
     ('that (do-that))
     (_ (message "some message"))

What if I want to check a number of conditions, not all dependent upon
featr, what should one do? 

Suppose I want to test (eq duck 'quack), cannot use a pcase because tests
are dependent upon featr.

  (pcase featr
     ('this (do-this))
     ('that (do-that))
     (_ (message "some message"))

Re: Checking conditions unrelated to expression

[Thibaut Verron]

Le dim. 8 déc. 2024 à 13:06, Heime via Users list for the GNU Emacs text
editor <help-gnu-emacs@gnu.org> a écrit :

> It looks like pcase can only be used with some variable condition
>
> This will check condition of featr
>
>   (pcase featr
>      ('this (do-this))
>      ('that (do-that))
>      (_ (message "some message"))
>
> What if I want to check a number of conditions, not all dependent upon
> featr, what should one do?
>
> Suppose I want to test (eq duck 'quack), cannot use a pcase because tests
> are dependent upon featr.
>
>   (pcase featr
>      ('this (do-this))
>      ('that (do-that))
>      (_ (message "some message"))
>

You can use a plain case-switch, rather than a pattern-matching one:

(cond
  ((eq featr 'this) (do-this))
  ((eq featr 'that) (do-that))
  ((eq duck 'quack) (do-duck))
  (t (message "blabla")))

https://www.gnu.org/s/emacs/manual/html_node/elisp/Conditionals.html

You can also replace the first two cases by a pattern-matching case, if
you'd like:

(cond
  (pcase featr
      ('this (do-this) t) ;; important to return non-nil in each branch
      ('that (do-that) t)
      (_ nil))
  ((eq duck 'quack) (do-duck))
   (t (message "blabla")))

Re: Checking conditions unrelated to expression

[Heime via Users list for the GNU Emacs text editor]

Sent with Proton Mail secure email.

On Monday, December 9th, 2024 at 12:31 AM, Thibaut Verron <thibaut.verron@gmail.com> wrote:

> Le dim. 8 déc. 2024 à 13:06, Heime via Users list for the GNU Emacs text
> editor help-gnu-emacs@gnu.org a écrit :
> 
> > It looks like pcase can only be used with some variable condition
> > 
> > This will check condition of featr
> > 
> > (pcase featr
> > ('this (do-this))
> > ('that (do-that))
> > (_ (message "some message"))
> > 
> > What if I want to check a number of conditions, not all dependent upon
> > featr, what should one do?
> > 
> > Suppose I want to test (eq duck 'quack), cannot use a pcase because tests
> > are dependent upon featr.
> > 
> > (pcase featr
> > ('this (do-this))
> > ('that (do-that))
> > (_ (message "some message"))
> 
> 
> You can use a plain case-switch, rather than a pattern-matching one:
> 
> (cond
> ((eq featr 'this) (do-this))
> ((eq featr 'that) (do-that))
> ((eq duck 'quack) (do-duck))
> (t (message "blabla")))
> 
> https://www.gnu.org/s/emacs/manual/html_node/elisp/Conditionals.html
> 
> You can also replace the first two cases by a pattern-matching case, if
> you'd like:
> 
> (cond
> (pcase featr
> ('this (do-this) t) ;; important to return non-nil in each branch
> ('that (do-that) t)
> (_ nil))
> ((eq duck 'quack) (do-duck))
> (t (message "blabla")))


But I cannot do the other way round, right?  Cannot have a pcase using featr
as EXP, but checking something else.

Re: Checking conditions unrelated to expression

[Stefan Monnier via Users list for the GNU Emacs text editor]

> Suppose I want to test (eq duck 'quack), cannot use a pcase because tests
> are dependent upon featr.
>
>   (pcase featr
>      ('this (do-this))
>      ('that (do-that))
>      (_ (message "some message"))

Like Thibaut said, for this example `cond` would probably work best.
But if you have good reasons to use `pcase`, you *can* do what
you suggest.
E.g.:

   (pcase featr
      ('this (do-this))
      ('that (do-that))
      (_ (if (eq duck 'quack) (do-not) (message "some message"))))

or

   (pcase featr
      ('this (do-this))
      ('that (do-that))
      ((guard (eq duck 'quack)) (do-not))
      (_ (message "some message")))

or

   (pcase featr
      ('this (do-this))
      ('that (do-that))
      ((let 'quack duck) (do-not))
      (_ (message "some message")))

...


        Stefan

Re: Checking conditions unrelated to expression

[Michael Heerdegen via Users list for the GNU Emacs text editor]

Hello,

the only thing I want to add to Stefan's answer: the 'let' pattern can
be (despite the name) used as a full-featured "sub-pcase".  But also see
'app' etc. - just see the pattern descriptions in the fine docs.

Michael.