Reputation: 20808
I'm trying to modify this "Calculator" Jison example to be an expression parser rather than an expression interpreter. I want to output a JSON object that describes the expression instead of immediately evaluating it.
1 + 2
, I expect to get a JSON array that stringifies to ['+', 1, 2]
. Instead I get 1
. (I'm not concerned about whether I'm representing the expression in a useful AST model, I'm just trying to learn Jison)
/* description: Parses mathematical expressions. */
/* lexical grammar */
%lex
%%
\s+ /* skip whitespace */
[0-9]+("."[0-9]+)?\b return 'NUMBER'
"*" return '*'
"/" return '/'
"-" return '-'
"+" return '+'
"^" return '^'
"(" return '('
")" return ')'
"PI" return 'PI'
"E" return 'E'
<<EOF>> return 'EOF'
. return 'INVALID'
/lex
/* operator associations and precedence */
%left '+' '-'
%left '*' '/'
%left '^'
%left UMINUS
%start expressions
%% /* language grammar */
expressions
: e EOF
{ return $e; }
;
e
: e '+' e
{ return ['+', $e1, $e2]; }
| e '-' e
{ return ['-', $e1, $e2]; }
| e '*' e
{ return ['*', $e1, $e2]; }
| e '/' e
{ return ['/', $e1, $e2]; }
| e '^' e
{ return ['^', $e1, $e2]; }
| '-' e %prec UMINUS
{ return ['-', $e]; }
| '(' e ')'
{ return ['G', $e ]; }
| NUMBER
{ return Number($NUMBER); }
| E
{ return Math.E; }
| PI
{ return Math.PI; }
;
Upvotes: 0
Views: 344
Reputation: 56
In this line, instead of returning the sum of the operands in $$,
: e '+' e
{$$ = $1+$3;}
would it work to return the desired object?
: e '+' e
{$$ = ["+", $1, $3];}
Upvotes: 2