javascript - Debugging in Jison -

i'm using jison write parser. grammar:

{     "program": [         ["statements eof", "return $1;"]     ],     "statements": [         ["statement",            "$$ = $1;"],         ["statements statement", "$$ = $1 + '\\n' + $2;"]     ],     "statement": [         ["expression newline", "$$ = $1 + ';';"]     ],     "expression": [         ["number",                "$$ = yytext;"],         ["expression expression", "$$ = $1 + ', ' + $2;"]     ] } 

when run following error message:

conflict in grammar: multiple actions possible when lookahead token number in state 9 - reduce rule: expression -> expression expression - shift token (then go state 5)  states conflicts: state 9   expression -> expression expression . #lookaheads= newline number   expression -> expression .expression   expression -> .number   expression -> .expression expression 

what supposed make of debug message? how explain message in simple english? period in expression -> expression expression . mean? .expression , .number? how different expression , number respectively?

what supposed make of debug message?

a grammar conflict means parser can reach state follow multiple rules, doesn't have enough information determine 1 follow (or worse, grammar ambiguous). you'll have tweak grammar eliminate conflicts. often, takes practice right.

what period in expression -> expression expression . mean?

the period represents position of parser. so, in rule, parser have parsed 2 expressions, , in state 9. when period @ end of rule, means rule can "reduced", , grouped single expression non-terminal in case. however, can reduce if next token (the lookahead) newline or number.

in expression -> .number, parser has encountered number token, can "shift", move new state.

the conflict occurs because parser can reduce or shift when encounters number token.

edit: resolve conflict, need split expression rule distinct non-terminals. having same non-terminal in sequence bound produce conflicts.

e.g.

{     "program": [         ["statements eof", "return $1;"]     ],     "statements": [         ["statement",            "$$ = $1;"],         ["statements statement", "$$ = $1 + '\\n' + $2;"]     ],     "statement": [         ["expression newline", "$$ = $1 + ';';"]     ],     "expression": [         ["expression expression_base", "$$ = $1 + ', ' + $2;"],         ["expression_base", "$$ = $1;"]     ],     "expression_base": [         ["number",                "$$ = yytext;"]     ] } 

here's nice resource more background on these types of grammars.