Re: pcase and minus-sign

["Andreas Röhler" <andreas.roehler@easy-emacs.de>]

On 30.11.2016 14:56, Joost Kremers wrote:
>
> On Wed, Nov 30 2016, Andreas Röhler wrote:
>> Thanks, that helps. Seems it relates to the following in docstring:
>>
>> SYMBOL    matches anything and binds it to SYMBOL.
>
> Yup.
>
>> Now if I use some arbitrary char, like "a",
>>
>> (defun foo (arg)
>>    (interactive "P")
>>    (pcase arg
>>      (a (message "%s" "ARG was `a'"))
>>      (1 (message "%s" "ARG was `1'"))
>>      ('- (message "%s" "ARG was minus-sign"))
>>      (_ (message "%s" "ARG not minus-sign"))))
>>
>> It picks that a-branch at any case - as documented but strange.
>
> I don't think it's strange, TBH. `a' is essentially a let-bound 
> variable. Having the ability to bind values is extremely useful in 
> pcase, I'd say.
>
> It may be a bit surprising that - functions the same way, but it's not 
> strange once you realise that - is not a special character in Lisp 
> (unlike most other programming languages). For example, the following 
> code works fine:
>
> (let ((- "hi"))
>  (message "%s" -))
>
> Note BTW that the same if true for the underscore: in your example, _ 
> is also just a symbol used to let-bind some value. The only special 
> thing about _ is that the byte-compiler doesn't complain about an 
> unused variable if its name starts with an underscore. But again, the 
> following code works just fine:
>
> (let ((_ "hi"))
>  (message "%s" _))
>
>
>

Hmm, thanks all.

But what make the char `a' so special WRT char `1'?:

(defun foo (arg)
   (interactive "P")
   (pcase arg
     (1 (message "%s" "ARG was `1'"))
     (a (message "%s" "ARG was `a'"))
     ('- (message "%s" "ARG was minus-sign"))
     (_ (message "%s" "ARG not minus-sign"))))