Emacs 30 新加的 peg.el 的一个例子

Emacs-general
1

看 NEWS 的时候看到一个叫 peg.el 的包直接在 Emacs 30 里面内置了。正好最近在玩 LTL ，就写了个 LTL2BA 生成的自动机输出的 parser 玩玩。

Emacs 29 里面可以通过 package-install 来安装 peg 这个包，目前似乎没有太多文档可以参考，peg.el 里面的注释和 peg-test.el 的例子是不错的学习材料。我不打算详细介绍 peg.el 怎么用，这个帖子算是给出一个简单用例。

在 LTL 2 BA (lsv.fr) 输入 F(a1) && F(a2) && F(a3) 可以得到如下输出和图：

never { /* F(a1) && F(a2) && F(a3) */
T0_init :    /* init */
	if
	:: (1) -> goto T0_init
	:: (a3) -> goto T0_S2
	:: (a2) -> goto T0_S3
	:: (a2 && a3) -> goto T0_S4
	:: (a1) -> goto T1_S5
	:: (a1 && a3) -> goto T1_S6
	:: (a1 && a2) -> goto T2_S7
	:: (a1 && a2 && a3) -> goto accept_all
	fi;
T0_S2 :    /* 1 */
	if
	:: (1) -> goto T0_S2
	:: (a2) -> goto T0_S4
	:: (a1) -> goto T1_S6
	:: (a1 && a2) -> goto accept_all
	fi;
T0_S3 :    /* 2 */
	if
	:: (1) -> goto T0_S3
	:: (a3) -> goto T0_S4
	:: (a1) -> goto T2_S7
	:: (a1 && a3) -> goto accept_all
	fi;
T0_S4 :    /* 3 */
	if
	:: (1) -> goto T0_S4
	:: (a1) -> goto accept_all
	fi;
T1_S5 :    /* 4 */
	if
	:: (1) -> goto T1_S5
	:: (a3) -> goto T1_S6
	:: (a2) -> goto T2_S7
	:: (a2 && a3) -> goto accept_all
	fi;
T1_S6 :    /* 5 */
	if
	:: (1) -> goto T1_S6
	:: (a2) -> goto accept_all
	fi;
T2_S7 :    /* 6 */
	if
	:: (1) -> goto T2_S7
	:: (a3) -> goto accept_all
	fi;
accept_all :    /* 7 */
	skip
}

27005-B
27005-B883×453 28.4 KB

上面的输出格式似乎和 Spin - Formal Verification (spinroot.com) 有点关系。不过由于 ltl2ba 的输出并不复杂，这里就写出了如下凑合用的解析代码：

(defun ltl-buchi2list ()
  (peg-parse
   (translation_unit "never" _  statements)
   (statements "{" (+ _ statement _)"}")
   (statement state _ ":" _
	      (or (and "if" (list (+ transform)) "fi;")
		  (list "skip"))
	      `(state arrows -- (list state arrows)))
   (state (substring [a-z A-Z] (* [a-z A-Z 0-9 "_"])))
   (transform (list _ "::" _ condition _ "->" _ "goto" _ state _))
   (condition (or (and "(1)" `(-- (list "1")))
		  (list (and "("_ id _ id-list _")"))))
   (id-list (* _ "&&" _ id))
   (id (substring (opt "!") identifier))
   (identifier [a-z A-Z] (* [a-z A-Z 0-9 "_"]))
   (_ (* (or [" \n\t"] comment)))
   (comment "/*" (* (not (or "\n" (eob))) (any)))))

对以上文本可以得到如下解析结果：

(("accept_all" nil)
 ("T2_S7"
  ((("1") "T2_S7")
   (("a3") "accept_all")))
 ("T1_S6"
  ((("1") "T1_S6")
   (("a2") "accept_all")))
 ("T1_S5"
  ((("1") "T1_S5")
   (("a3") "T1_S6")
   (("a2") "T2_S7")
   (("a2" "a3") "accept_all")))
 ("T0_S4"
  ((("1") "T0_S4")
   (("a1") "accept_all")))
 ("T0_S3"
  ((("1") "T0_S3")
   (("a3") "T0_S4")
   (("a1") "T2_S7")
   (("a1" "a3") "accept_all")))
 ("T0_S2"
  ((("1") "T0_S2")
   (("a2") "T0_S4")
   (("a1") "T1_S6")
   (("a1" "a2") "accept_all")))
 ("T0_init"
  ((("1") "T0_init")
   (("a3") "T0_S2")
   (("a2") "T0_S3")
   (("a2" "a3") "T0_S4")
   (("a1") "T1_S5")
   (("a1" "a3") "T1_S6")
   (("a1" "a2") "T2_S7")
   (("a1" "a2" "a3") "accept_all"))))
3 个赞
2

简单看了一下，应该是原版 PEG，语法换成了 S-exp，加入了 substring、-- 引导的模式匹配之类的新语法。可能有误

原 PEG 论文，elpa 链接

另附：这个 stack operation 怎么让我回想起了 forth

2 个赞
3

感谢补充。

原本我准备手搓递归下降或者用 tree-sitter 的，偶然看到了 peg.el 就顺便试试，它这个 stack operation 用起来挺爽的我感觉。

4

另一种实现方式，通过函数调用实现的

1 个赞
5

草，再补充一个 OBJ 波形文件的 Parser，不过解析的格式不全：

(defun obj-parser ()
  (peg-parse
   (source (+ _ statement _))
   (statement (or mtllib usemtl g v vt vn s g f))
   (mtllib "mtllib" " " file)
   (usemtl "usemtl" " " name `(n -- (list 'usemtl n)))
   (g "g" " " name " " name
      `(n1 n2 -- (list 'g n1 n2)))
   (v "v" " " float " " float " " float
      `(f1 f2 f3 -- (list 'v f1 f2 f3)))
   (vt "vt" " " float " " float
       `(f1 f2 -- (list 'vt f1 f2)))
   (vn "vn" " " float " " float " " float
       `(f1 f2 f3 -- (list 'vn f1 f2 f3)))
   (s "s" " " (or int (substring "off"))
      `(id -- (list 's id)))
   (f "f" " " tri " " tri " " tri
      `(t1 t2 t3 -- (list 'f t1 t2 t3)))
   (file (+ [a-z A-Z 0-9 "_"]) "." (+ [a-z A-Z 0-9 "_"]))
   (name (substring (+ [a-z A-Z 0-9 "_"])))
   (float (substring (opt ["+-"]) (+ [0-9]) "." (+ [0-9])))
   (int (substring (+ [0-9])))
   (tri int "/" int "/" int
	`(i1 i2 i3 -- (list i1 i2 i3)))
   (_ (* (or [" \n\t"] comment)))
   (comment "#" (* (not (or "\n" (eob))) (any)))))
1 个赞
6

刚刚发现 python3 语法描述用的是 PEG

1 个赞