Re: The poor state of documentation of pcase like things.

[phillip.lord@russet.org.uk (Phillip Lord)]

Richard Stallman <rms@gnu.org> writes:

> [[[ To any NSA and FBI agents reading my email: please consider    ]]]
> [[[ whether defending the US Constitution against all enemies,     ]]]
> [[[ foreign or domestic, requires you to follow Snowden's example. ]]]
>
>   > There are many special UPatterns, and their variety makes this the hardest
>   > aspect to master. Let's consider them one by one.
>
>   > ## Underscore `_'
>
>   > To match against anything whatsoever, no matter its type or value, use
>   > underscore. Thus to match against a list containing anything at all at its
>   > head, we'd use:
>
>   >     (pcase value
>   >       (`(_ 1 2)
>   >        (message "Matched a list of anything followed by (2 3)")))
>
> I don't follow this part.  (_ 1 2) seems to be a QPattern.
> Is that right?  So how is it that an element can be a UPattern?


I think the example is wrong. This....

(pcase '(3 1 2)
   (`(_ 1 2)
   (message "Matched a list of anything followed by (2 3)")))

prints nothing.

while this...

(pcase '(3 1 2)
   (`(,_ 1 2)
   (message "Matched a list of anything followed by (2 3)")))

prints the message (which should be "followed by (2 3)").

I am a bit surprised to find that _ needs , in these examples, and I
think that it's a bug.

I would anyway change it to:

(pcase '(3 1 2)
   (`(,_ 1 2)
      "Matched a list of anything followed by (1 2)"))

As I think it makes the point that `pcase' returns something, as opposed
to working by side effect. I think the example in the manual needs the
same change.

> It would help if some of the examples used symbols inside a QPattern,
> without comma, so we can see what that does.


A symbol IS a QPattern

(pcase '(a b c)
  (`(a b ,x) x))

returns "c".

Phil