Re: pcase and minus-sign

[Joost Kremers <joostkremers@fastmail.fm>]

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" _))



-- 
Joost Kremers
Life has its moments