pcase with progn

pcase with progn

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

Does pcase require progn for multiple commands?

(defun pola-symbol-name (sbl &optional sel)
  "Get the string representation of symbol SBL.  If it is
already a string, return it unchanged."

  (cond
    ((symbolp sbl) (symbol-name sbl))
    ((stringp sbl) sbl)
    (t (pcase sel
         ('nil (error "SBL must be a symbol or a string: %s" sbl))
         ('nil-output nil)
         ('default "unknown")
         ('warn (progn
                  (message "SBL not symbol nor string: %s" sbl)
                  nil))
         ('pass sbl)
         (_ (error "SEL Invalid: %s" sel))))))



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Re: pcase with progn

[Scott Bell]

Heime via Users list for the GNU Emacs text editor
<help-gnu-emacs@gnu.org> writes:

> Does pcase require progn for multiple commands?

Nope.  Each clause is of the form ‘(PATTERN BODY-FORMS...)’, where the
body forms of the successful clause are evaluated and the final value is
returned.  In this sense it behaves similarly to ‘cond’.

Re: pcase with progn

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

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On Thursday, November 28th, 2024 at 7:39 AM, Scott Bell <sctb@me.com> wrote:

> Heime via Users list for the GNU Emacs text editor
> help-gnu-emacs@gnu.org writes:
> 
> > Does pcase require progn for multiple commands?
> 
> 
> Nope. Each clause is of the form ‘(PATTERN BODY-FORMS...)’, where the
> body forms of the successful clause are evaluated and the final value is
> returned. In this sense it behaves similarly to ‘cond’.

The command should return the string representation of symbol if
it exists.  When the command fails (symbol does not exist), are commands
customarily return nil or an error?

Re: pcase with progn

[Scott Bell]

Heime <heimeborgia@protonmail.com> writes:

> (defun pola-symbol-name (sbl &optional sel)
>   "Get the string representation of symbol SBL.  If it is
> already a string, return it unchanged."
>   ...
> The command should return the string representation of symbol if
> it exists.  When the command fails (symbol does not exist), are commands
> customarily return nil or an error? 

This depends entirely on the meaning of your function and its intended
usage. Generally speaking, it’s preferable to return nil to represent
failure unless nil is a valid value in the successful case. Since your
function always returns a string if it’s successful, it would probably
be good to return nil on failure.

Errors are most often employed when the program has encountered some
value or condition under which it’s not possible to proceed normally. In
that case, signalling an error stops execution and returns to the
nearest enclosing error handler which will decide what to do about it.

Re: pcase with progn

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

> Does pcase require progn for multiple commands?

You can find the answer in `C-h f pcase RET`.


        Stefan