Adding declarations to JISON

Question

I have here an only slightly modified version of the JISON calculator example:

/* description: Parses end executes mathematical expressions. */

/* lexical grammar */
%lex
%%

\s+                   /* skip whitespace */
[0-9]+("."[0-9]+)?\b  return 'NUMBER'
"*"                   return '*'
"/"                   return '/'
"-"                   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 '^'
%right '!'
%right '%'
%left UMINUS

%start expressions

%% /* language grammar */

expressions
    : e EOF
        { typeof console !== 'undefined' ? console.log($1) : print($1);
          return $1; }
    ;

e
    : e '+' e
        {$$ = $1+$3;}
    | e '-' e
        {$$ = $1-$3;}
    | e '*' e
        {$$ = $1*$3;}
    | e '/' e
        {$$ = $1/$3;}
    | e '^' e
        {$$ = Math.pow($1, $3);}
    | e '!'
        {{
          $$ = fact($1);
        }}
    | e '%'
        {$$ = $1/100;}
    | '-' e %prec UMINUS
        {$$ = -$2;}
    | '(' e ')'
        {$$ = $2;}
    | NUMBER
        {$$ = Number(yytext);}
    | E
        {$$ = Math.E;}
    | PI
        {$$ = Math.PI;}
    ;

%%
/*why doesn't this work at runtime?
I see other examples defining declarations this way but I must be doing something wrong
I couldn't find a syntactically valid way of putting this declaration anywhere but here,
which is probably the issue*/
function fact(n) {
  var tot=1;
  for(var i=2;i<=n;++i) {
    tot*=i;
  }
  return tot;
}

Note the slight differences in the ! operator's definition. I'm trying to externally define the fact function rather than doing it inline.

As of now, it tells me at runtime fact is not defined. How can I fix this? Also, why does the calculator example use two braces around the factorial definition, {{ /*like so*/ }}?

Answer 1

To call a function defined under your productions (such as fact), you can use the mod-brace notation %{ and %} for multiline semantic actions:

e
    : e '+' e

    ...

    | e '!'
        %{
            // the %{ tells jison this is a multi-line js eval statement
            $$ = fact($1);
        %}
    ;

As a final solution, try this:

/* lexical grammar */
%lex
%%

\s+                   /* skip whitespace */
[0-9]+("."[0-9]+)?\b  return 'NUMBER'
"*"                   return '*'
"/"                   return '/'
"-"                   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 '^'
%right '!'
%right '%'
%left UMINUS

%start expressions

%% /* language grammar */

expressions
    : e EOF
        %{
            typeof console !== 'undefined' ? console.log($1) : print($1);
            return $1;
        %}
    ;

e
    : e '+' e
        {$$ = $1+$3;}
    | e '-' e
        {$$ = $1-$3;}
    | e '*' e
        {$$ = $1*$3;}
    | e '/' e
        {$$ = $1/$3;}
    | e '^' e
        {$$ = Math.pow($1, $3);}
    | e '!'
        %{
          $$ = fact($1);
        %}
    | e '%'
        {$$ = $1/100;}
    | '-' e %prec UMINUS
        {$$ = -$2;}
    | '(' e ')'
        {$$ = $2;}
    | NUMBER
        {$$ = Number(yytext);}
    | E
        {$$ = Math.E;}
    | PI
        {$$ = Math.PI;}
    ;

%%

function fact(n) {
  var tot=1;
  for(var i=2;i<=n;++i) {
    tot*=i;
  }
  return tot;
}

Answer 2

As noted by @Nucleon and @chaosbohne, the accepted answer is incorrect.

Directly from the Jison wiki (https://github.com/zaach/jison/wiki/Deviations-From-Flex-Bison)

Within lexer actions use %{ ... %} delimiters if you want to use block-style statements, e.g.:

.*  %{
  if (true) {
    console.log('test');
  }
  // ...
%}

Within parser actions use {{ .. }} delimiters for the same purpose.

So, if you are in the lexer, for multiline actions, use %{ action %}. In the parser, for multiline actions, use {{ action }}. If you are you are writing a single-line action, single braces {} work fine in the lexer and parser.