Re: pcase and minus-sign

[Andreas Röhler]

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"))))