Re: Question on pcase

[Michael Heerdegen <michael_heerdegen@web.de>]

Oleh Krehel <ohwoeowho@gmail.com> writes:

> I debug with lispy [1], my package for Elisp and stuff. It's just a
> sophisticated "C-x C-e" - all the program stack is in the global
> variables.

Ah, in global variables.  Now I understand why you wanted to use `setq'.

Hmm, as I said before, it's impossible in the general case to separate a
`pcase' pattern into a side effect free matching part and a
binding/setting part.  That's because when variable binding is done,
bindings established while performing the matching need to be in effect;
look at the example at the end to see what I mean.

So, the best I can offer is a function that transforms a PATTERN into a
function F that takes one argument EXP, the expression to be matched.  F
returns non-nil when the PATTERN would match EXP, else nil.  When the
PATTERN matches, all bindings created as a side effect from matching
that could be referred to in the CODE part of a (PATTERN CODE) `pcase'
branch are set with `setq'.

--8<---------------cut here---------------start------------->8---
;; -*- lexical-binding: t -*-
(defun abo-transform-pcase-pattern (pattern)
  (let ((arg (make-symbol "expression")))
    `(lambda (,arg) ,(pcase--u
                 `((,(pcase--match arg (pcase--macroexpand pattern))
                    ,(lambda (vars)
                       `(progn ,@(mapcar (lambda (b) `(setq ,(car b) ,(cdr b)))
                                         vars)))))))))
--8<---------------cut here---------------end--------------->8---

Because this is a function, you need to quote the PATTERN.

Example: transform the pattern `(,a ,b)

(abo-transform-pcase-pattern '`(,a ,b))

==>

(lambda
  (#1=#:expression)
  (if
      (consp #1#)
      (let*
          ((#5=#:x
            (car #1#))
           (#2=#:x
            (cdr #1#)))
        (if
            (consp #2#)
            (let*
                ((#4=#:x
                  (car #2#))
                 (#3=#:x
                  (cdr #2#)))
              (if
                  (null #3#)
                  (progn
                    (setq b #4#)
                    (setq a #5#))
                nil))
          nil))
    nil))


Regards,

Michael.