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pcase-if-let?
From: |
Michael Heerdegen |
Subject: |
pcase-if-let? |
Date: |
Thu, 29 Mar 2018 01:20:50 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) |
Hello,
should we add a `pcase-if-let' to pcase.el? While the name might sound
a bit obscure, in my experience I quite often wanted something like
this, instead of having to write
#+begin_src emacs-lisp
(pcase value
(pattern code
more code
...)
(_ other code
more other code
...))
#+end_src
for example.
Here is the result of a first naive implementation trial (which works
for me):
#+begin_src emacs-lisp
(defmacro pcase-if-let (bindings then &rest elses)
"Bind variables according to BINDINGS and eval THEN or ELSE.
This is like `if-let' but BINDINGS is a list of elements of the
form \(PATTERN VALUE-FORM) and it's tested whether all the
PATTERNs match instead of whether VALUE-FORMS are all non-nil."
(declare (indent 2)
(debug ((&rest (pcase-PAT &optional form))
form body)))
(if (null bindings)
'(let* () then)
(let ((sym (make-symbol "sym"))
(sucess-syms '())
(last-sucess-sym nil))
(dotimes (i (length bindings))
(push (make-symbol (format "binding-%d-sucess" i)) sucess-syms))
(cl-callf nreverse sucess-syms)
`(progn
(mapc (lambda (,sym) (setq ,sym nil)) ',sucess-syms)
(pcase nil
((and ,@(mapcar (pcase-lambda (`(,pattern ,value))
`(let (and ,@(if last-sucess-sym `((guard
,last-sucess-sym)) '())
,pattern
(let ,(setq last-sucess-sym (pop
sucess-syms)) t))
,value))
bindings))
(if ,last-sucess-sym ,then ,@elses)))))))
#+end_src
Note that there is no need for a `pcase-if' (just don't bind variables
in the patterns), so we could also use this name I think. Also note
that we can't unite this macro with `if-let' (because it is about
non-nil-ness of values, while `pcase-if-let' is about pattern matching,
so a binding like (SYMBOL VALUE) has different semantics).
Thanks,
Michael.
- pcase-if-let?,
Michael Heerdegen <=
- Re: pcase-if-let?, Davis Herring, 2018/03/28
- Re: pcase-if-let?, Michael Heerdegen, 2018/03/28
- Re: pcase-if-let?, Michael Heerdegen, 2018/03/28
- RE: pcase-if-let?, Drew Adams, 2018/03/29
- Re: pcase-if-let?, Michael Heerdegen, 2018/03/29
- RE: pcase-if-let?, Drew Adams, 2018/03/29
- Re: pcase-if-let?, Stefan Monnier, 2018/03/29
- Re: pcase-if-let?, Michael Heerdegen, 2018/03/29
- Re: pcase-if-let?, Stefan Monnier, 2018/03/29
- Re: pcase-if-let?, Michael Heerdegen, 2018/03/29