CODEWITHSUNDEEP
bisonyacc
GalaxyVintage
GalaxyVintage

Reputation: 666

Resolving shift/reduce conflicts in Bison

The grammar that has the conflict is as follow

method_call: T_ID T_LPAREN method_arg_list T_RPAREN T_SEMICOLON

value: T_ID T_LSB expr T_RSB
     | T_ID;

method_arg_list: /* Empty */
               | method_arg method_arg_list;

method_arg: string_constant
          | expr;

expr: value
    | '(' expr ')';

The problem is that when it reads a T_ID, it doesn't know whether it should shift and read the following '(' or reduce to a value. But shouldn't it check if the next token is a left parenthesis?

I am quite new to bison and parser in general and I am wondering what are the ways that I can rewrite the grammar and solve the conflict?

Edit

State 60

37 method_call: T_ID . T_LPAREN method_arg_list T_RPAREN T_SEMICOLON
67 value: T_ID . T_LSB expr T_RSB
68      | T_ID .

T_LPAREN  shift, and go to state 71
T_LSB     shift, and go to state 72

T_LPAREN  [reduce using rule 68 (value)]
$default  reduce using rule 68 (value)

Upvotes: 0

Views: 481

Answers (1)

Bruce David Wilner
Bruce David Wilner

Reputation: 467

Leffler is correct: we need to know about some higher-level sentential structures that may rely upon method calls and values and such.

I see what's going on from a CONCEPTUAL standpoint: we needn't get mired in any "y.output" nitty-gritty. Everything stems from your failure to separate arguments to the method call with commas. Also, I'd love to know why you're collecting your argument list right-recursively rather than left-recursively.

What is the difference between '(' and T_LPAREN, and between ')' and T_RPAREN? Does lex return one in one case, the other in another case? It seems to me that the simple pattern \( can't be mapped to both of these, but only to one; the same goes for the pattern \). Also, for readability, don't bother defining, e.g., T_LSB where '[' will do just fine.

Upvotes: 0

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