问题
I would like to parse sentences in propositional logic using BNFC. I wrote the following BNF grammar to facilitate this:
Negation. N ::= "(" "-" L")";
Conjuction. C ::= "(" L "&" L ")";
Disjuction. D ::= "(" L "|" L ")";
Implication. I ::= "(" L "=>" L ")";
Equivalence. E ::= "(" L "<=>" L ")";
Atom. L ::= Ident | N | C | D | I | E ;
However, with this construction I get the following error:
syntax error at line 6, column 27 before `|'
What is syntactically incorrect about the specification I provided?
Edit 1
Ok, so it looks like bnfc really does not like the idea of using the symbol | for union. How do I then assign multiple productions to a single rule, if not via a union? I do not want to have to define Atom1. L ::= Ident ;, Atom2. L ::= N ; and so forth, but is this necessary if I want this to work?
Edit 2
Ok, so giving different labels to each L-production, as in
Negation. N ::= "(" "-" L")";
Conjuction. C ::= "(" L "&" L ")";
Disjuction. D ::= "(" L "|" L ")";
Implication. I ::= "(" L "=>" L ")";
Equivalence. E ::= "(" L "<=>" L ")";
Atom1. L ::= Ident ;
Atom2. L ::= N ;
Atom3. L ::= C ;
Atom4. L ::= D ;
Atom5. L ::= I ;
Atom6. L ::= E ;
allowed the file logic.cf to pass through bnfc without any errors. However, when the file is compiled using the command
bnfc -m -c file.cf
and I then try running make, I get the following error when Make tries to run gcc on the bnfc-generated file Printer.c:
gcc -g -W -Wall -c Absyn.c
flex -Plogic -oLexer.c logic.l
gcc -g -W -Wall -c Lexer.c
Lexer.c:1477:16: warning: ‘input’ defined but not used [-Wunused-function]
static int input (void)
^~~~~
Lexer.c:1434:17: warning: ‘yyunput’ defined but not used [-Wunused-function]
static void yyunput (int c, char * yy_bp )
^~~~~~~
bison -t -plogic logic.y -o Parser.c
gcc -g -W -Wall -c Parser.c
gcc -g -W -Wall -c Printer.c
Printer.c: In function ‘ppL’:
Printer.c:289:20: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
ppIdent(_p_->u.atom_.ident_, 0);
^~~~~
atom1_
Printer.c:296:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
ppN(_p_->u.atom_.n_, 0);
^~~~~
atom1_
Printer.c:303:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
ppC(_p_->u.atom_.c_, 0);
^~~~~
atom1_
Printer.c:310:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
ppD(_p_->u.atom_.d_, 0);
^~~~~
atom1_
Printer.c:317:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
ppI(_p_->u.atom_.i_, 0);
^~~~~
atom1_
Printer.c:324:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
ppE(_p_->u.atom_.e_, 0);
^~~~~
atom1_
Printer.c: In function ‘ppInteger’:
Printer.c:336:31: warning: unused parameter ‘i’ [-Wunused-parameter]
void ppInteger(Integer n, int i)
^
Printer.c: In function ‘ppDouble’:
Printer.c:342:29: warning: unused parameter ‘i’ [-Wunused-parameter]
void ppDouble(Double d, int i)
^
Printer.c: In function ‘ppChar’:
Printer.c:348:25: warning: unused parameter ‘i’ [-Wunused-parameter]
void ppChar(Char c, int i)
^
Printer.c: In function ‘ppString’:
Printer.c:354:29: warning: unused parameter ‘i’ [-Wunused-parameter]
void ppString(String s, int i)
^
Printer.c: In function ‘ppIdent’:
Printer.c:360:28: warning: unused parameter ‘i’ [-Wunused-parameter]
void ppIdent(String s, int i)
^
Printer.c: In function ‘shL’:
Printer.c:507:20: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
shIdent(_p_->u.atom_.ident_);
^~~~~
atom1_
Printer.c:522:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
shN(_p_->u.atom_.n_);
^~~~~
atom1_
Printer.c:537:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
shC(_p_->u.atom_.c_);
^~~~~
atom1_
Printer.c:552:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
shD(_p_->u.atom_.d_);
^~~~~
atom1_
Printer.c:567:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
shI(_p_->u.atom_.i_);
^~~~~
atom1_
Printer.c:582:16: error: ‘union <anonymous>’ has no member named ‘atom_’; did you mean ‘atom1_’?
shE(_p_->u.atom_.e_);
^~~~~
atom1_
Makefile:42: recipe for target 'Printer.o' failed
make: *** [Printer.o] Error 1
I have no idea what this means. Why is it trying to find atom_, when I've not specified such a thing in logic.cf If there are any people more experienced with the internals of bnfc, I wouldn't mind hearing from you.
Edit 3
Ok, so writing the labels as
Negation. N ::= "(" "-" L ")";
Conjuction. C ::= "(" L "&" L ")";
Disjuction. D ::= "(" L "|" L ")";
Implication. I ::= "(" L "=>" L ")";
Equivalence. E ::= "(" L "<=>" L ")";
Atom. L ::= Ident;
AtomN. L ::= N ;
AtomC. L ::= C ;
AtomD. L ::= D ;
AtomI. L ::= I ;
AtomE. L ::= E ;
somehow magically allowed make to pass. However, my parser isn't exactly working, as something as simple as
echo "p" | ./Testlogic
returns with
error: line 1: syntax error at p
Isn't p a valid identifier, and so the production Atom. L ::= Ident; should allow it to pass? Why is this not the case?
回答1:
It turns out BNFC is a bit picky, when it comes to the order of the productions. I had to write
Atom. L ::= Ident;
AtomN. L ::= N ;
AtomC. L ::= C ;
AtomD. L ::= D ;
AtomI. L ::= I ;
AtomE. L ::= E ;
Negation. N ::= "(" "-" L ")";
Conjuction. C ::= "(" L "&" L ")";
Disjuction. D ::= "(" L "|" L ")";
Implication. I ::= "(" L "=>" L ")";
Equivalence. E ::= "(" L "<=>" L ")";
in addition to getting rid of the numbers at the ends of the labels to make it work. I am now able to parse simple sentences in predicate logic:
$ echo "(a&(a<=>b))" | ./Testlogic
Parse Succesful!
[Abstract Syntax]
(AtomC [(Conjuction (Atom ["a"]) (AtomE [(Equivalence (Atom ["a"]) (Atom ["b"]))]))])
[Linearized Tree]
(a & (a <=> b))
来源：https://stackoverflow.com/questions/61154549/using-bnfc-to-determine-a-basic-language-for-propositional-logic-syntax-error