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Re: pcase ` meaning [Was: Re: Replace trivial pcase occurrences in the E

From: Michael Heerdegen
Subject: Re: pcase ` meaning [Was: Re: Replace trivial pcase occurrences in the Emacs sources]
Date: Mon, 29 Oct 2018 22:33:10 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

"Garreau, Alexandre" <address@hidden> writes:

> Even in pcase I don’t see how to do that except manually using (and var
> (guard (equal var val))),

You mean like (pred (equal val))?

> #+begin_src emacs-lisp
> (pcase-defmacro list (&rest args)
>   `(,'\`  ,(mapcar (lambda (thing) `(,'\, ,thing)) args)))
> #+end_src
> I tried to adapt for arrays:
> #+begin_src emacs-lisp
> (pcase-defmacro array (&rest args)
>   `[,'\` ,(mapcar (lambda (thing) `(,'\, ,thing)) args)])
> #+end_src

> But it doesn’t work x), that, plus my initial non-working
> “(pcase-defmacro list (&rest args) ``(,@args))”, I must find “`”, is,
> indeed, quite confusing (how is “,'” needed? how isn’t “,'\`” equivalent
> to “\`”?),

You want , to survive til after the macro expansion.  Backquote can't
know that it should ignore a , because you want it to be literally in
the expansion.  ,'\` let's backquote insert a literal , into the
expansion, which is what you want.  The underlying problem is that you
want to use (of course you don't have to!) backquote to construct a
backquote expression.  That happens here and there when writing Lisp,
it's not something special to pcase.

For understanding the definition above note that ``' (synonymous for
`backquote') is a (very normal) macro accepting one argument STRUCTURE,
whereby we also have an abbreviating reader syntax
`STRUCTURE == (` STRUCTURE).  We also have the reader syntax
,THING == (, THING).

Your pcase macro `array' could be defined like

#+begin_src emacs-lisp
(pcase-defmacro array (&rest args)
  `(,'\` [,@(mapcar (lambda (thing) `(,'\, ,thing)) args)]))

You could also write that without backquote (and also without the []
reader syntax).


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