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bug#49809: [PATCH] Add macro 'pcase-setq'
From: |
Stefan Monnier |
Subject: |
bug#49809: [PATCH] Add macro 'pcase-setq' |
Date: |
Thu, 12 Aug 2021 12:13:58 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) |
> Undefined behavior has no use per se, unless it has a larger impact on
> efficiency.
There is indeed a significant difference in efficiency.
E.g. the code for (pcase-let ((`(,x ,y) (EXP))) (VAL x y)) looks like:
byte code:
args: nil
0 constant EXP
1 call 0
2 dup
3 car-safe
4 stack-ref 1
5 cdr-safe
6 dup
7 car-safe
8 stack-ref 1
9 cdr-safe
10 stack-ref 3
11 stack-ref 2
12 constant VAL
13 stack-ref 2
14 stack-ref 2
15 call 2
16 return
Instead of:
byte code:
args: nil
0 constant EXP
1 call 0
2 dup
3 consp
4 goto-if-nil-else-pop 3
7 dup
8 car-safe
9 stack-ref 1
10 cdr-safe
11 dup
12 consp
13 goto-if-nil-else-pop 2
16 dup
17 car-safe
18 stack-ref 1
19 cdr-safe
20 dup
21 not
22 goto-if-nil-else-pop 1
25 stack-ref 3
26 stack-ref 2
27 constant VAL
28 stack-ref 2
29 stack-ref 2
30 call 2
31 discardN-preserve-tos 2
33:1 discardN-preserve-tos 2
35:2 discardN-preserve-tos 2
37:3 return
For a semantic where the let is skipped when the pattern fails to match
and
byte code:
args: nil
0 constant EXP
1 call 0
2 dup
3 consp
4 goto-if-nil 3
7 dup
8 car-safe
9 stack-ref 1
10 cdr-safe
11 dup
12 consp
13 goto-if-nil 2
16 dup
17 car-safe
18 stack-ref 1
19 cdr-safe
20 dup
21 goto-if-nil 1
24 stack-ref 4
25 constant error
26 constant "No clause matching `%S'"
27 stack-ref 2
28 call 2
29 return
30:1 stack-ref 3
31 stack-ref 2
32 constant VAL
33 stack-ref 2
34 stack-ref 2
35 call 2
36 return
37:2 stack-ref 2
38 constant error
39 constant "No clause matching `%S'"
40 stack-ref 2
41 call 2
42 return
43:3 dup
44 constant error
45 constant "No clause matching `%S'"
46 stack-ref 2
47 call 2
48 return
For the semantics where we signal an error when the pattern fails to match.
> And why again do you think it is not a good idea?
Because if you want different behaviors when the pattern matches and
when it doesn't, you should use `pcase`.
Stefan
- bug#49809: [PATCH] Add macro 'pcase-setq', (continued)
- bug#49809: [PATCH] Add macro 'pcase-setq', Michael Heerdegen, 2021/08/12
- bug#49809: [PATCH] Add macro 'pcase-setq', Okam, 2021/08/12
- bug#49809: [PATCH] Add macro 'pcase-setq', Stefan Monnier, 2021/08/12
- bug#49809: [PATCH] Add macro 'pcase-setq', Michael Heerdegen, 2021/08/12
- bug#49809: [PATCH] Add macro 'pcase-setq', Stefan Monnier, 2021/08/13
- bug#49809: [PATCH] Add macro 'pcase-setq', Michael Heerdegen, 2021/08/13
- bug#49809: [PATCH] Add macro 'pcase-setq', Stefan Monnier, 2021/08/07
- bug#49809: [PATCH] Add macro 'pcase-setq', Michael Heerdegen, 2021/08/08
- bug#49809: [PATCH] Add macro 'pcase-setq', Stefan Monnier, 2021/08/09
- bug#49809: [PATCH] Add macro 'pcase-setq', Michael Heerdegen, 2021/08/09
- bug#49809: [PATCH] Add macro 'pcase-setq',
Stefan Monnier <=