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Re: Question on pcase
From: |
Oleh Krehel |
Subject: |
Re: Question on pcase |
Date: |
Mon, 26 Oct 2015 16:55:15 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux) |
Hi Micheal,
Thanks for your work, it could be useful to many people.
Michael Heerdegen <address@hidden> writes:
> Someone who wants to try to learn pcase (Oleh? Oleh!) can help by
> reading it and telling me if it is understandable, and send corrections
> - or write a better introduction ;-) - and format it nicely for
> inclusion into Elpa or Emacs when it turns out to help people.
The reason I dislike `pcase' is not because I don't know how to use it
(the basic rules are actually pretty simple), it's because I think it
leads to code that's hard to understand, maintain and transform. I
dislike the trivial `if-let' and `when-let' for the same reasons.
I generally dislike any custom macro that includes `if' or binds
variables. This is because I can't reason about the code that uses these
macros unless I know exactly what they do in terms of binding variables
and selecting branches. These macros don't follow the substitution model
for procedure application (SMPA) [1], which is a valuable debugging
technique for me.
However, I'm willing to implement some tooling that will allow `pcase'
to follow SMPA.
Take this code for example, with "|" being the point:
(pcase which
(`all t)
|(`safe (member fun completion--capf-safe-funs))
(`optimist (not (member fun completion--capf-misbehave-funs))))
Here, the (`all t) branch is selected, and the `which' symbol
can be extracted from the context with `up-list'. I've implemented a
function that prints "pcase: nil" if the selected branch doesn't match,
and "pcase: t" when it matches.
This way, when I'm debugging a code with `pcase' I can see which branch
is the correct one without evaluating the code inside the branch. After
this, I can step into the correct branch and use SMPA.
Of course, this function would also need to bind the same variables that
a `pcase' branch would bind.
Taking your example:
(pcase x
('a 1)
("Hallo" 2)
|(thing (message "%s is neither equal to 'a nor to \"Hallo\"." thing)))
This is what I would like to have:
(equal (macroexpand '(eval-pcase-branch x
(thing (message "%s is neither equal to 'a nor to
\"Hallo\"." thing))))
'(progn
(setq thing x)
(message "pcase: t")))
After this, the inner body of the branch can be properly evaluated,
since `thing' is bound to `x' now.
So far, I've implemented some code that can check if each branch will be
followed, see
https://github.com/abo-abo/lispy/commit/d3ed4e4fee435a2a448ddc0722d07cd997ee59d3.
But the code that binds the variables, e.g. (setq thing x) will likely
be hard to implement. For instance, look at this example with macroexpand:
(setq test '(1 . 2))
(pcase test
(`(,foo . ,baz)
(cons baz foo)))
;; =>
;; (2 . 1)
(macroexpand '(pcase test
(`(,foo . ,baz)
(cons baz foo))))
;; =>
;; (if (consp test)
;; (let* ((x (car test))
;; (x (cdr test)))
;; (let ((baz x)
;; (foo x))
;; (cons baz foo)))
;; nil)
The macroexpanded code returns (2 . 2) when evaluated. This I don't
understand. Although, it still works fine with `eval':
(eval (macroexpand '(pcase test
(`(,foo . ,baz)
(cons baz foo)))))
;; =>
;; (2 . 1)
Maybe someone could explain the above, and also suggest the best way the
create variable bindings from a pcase branch.
thanks again,
Oleh
[1]: https://mitpress.mit.edu/sicp/full-text/sicp/book/node10.html
- Re: pcase docstring tweaks, (continued)
- Re: pcase docstring tweaks, Richard Stallman, 2015/10/27
- Re: pcase docstring tweaks, Michael Heerdegen, 2015/10/28
- RE: Question on pcase, Drew Adams, 2015/10/24
- Re: Question on pcase, Alan Mackenzie, 2015/10/24
- RE: Question on pcase, Drew Adams, 2015/10/24
- Re: Question on pcase, Johan Bockgård, 2015/10/24
- Re: Question on pcase, Michael Heerdegen, 2015/10/24
- Re: Question on pcase,
Oleh Krehel <=
- Re: Question on pcase, Michael Heerdegen, 2015/10/26
- Re: Question on pcase, Oleh Krehel, 2015/10/27
- Re: Question on pcase, Michael Heerdegen, 2015/10/26
- Re: Question on pcase, Oleh Krehel, 2015/10/27
- Re: Question on pcase, Michael Heerdegen, 2015/10/27
- Re: Question on pcase, Michael Heerdegen, 2015/10/27
- Re: Question on pcase, Michael Heerdegen, 2015/10/28
- Re: Question on pcase, Oleh Krehel, 2015/10/29
- Re: Question on pcase, Michael Heerdegen, 2015/10/29
- Re: Question on pcase, Andreas Schwab, 2015/10/26