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(bison.info)Rpcalc Expr

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Explanation of `expr'
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   The `exp' grouping has several rules, one for each kind of
expression.  The first rule handles the simplest expressions: those
that are just numbers.  The second handles an addition-expression,
which looks like two expressions followed by a plus-sign.  The third
handles subtraction, and so on.

     exp:      NUM
             | exp exp '+'     { $$ = $1 + $2;    }
             | exp exp '-'     { $$ = $1 - $2;    }
             ...
             ;

   We have used `|' to join all the rules for `exp', but we could
equally well have written them separately:

     exp:      NUM ;
     exp:      exp exp '+'     { $$ = $1 + $2;    } ;
     exp:      exp exp '-'     { $$ = $1 - $2;    } ;
             ...

   Most of the rules have actions that compute the value of the
expression in terms of the value of its parts.  For example, in the
rule for addition, `$1' refers to the first component `exp' and `$2'
refers to the second one.  The third component, `'+'', has no meaningful
associated semantic value, but if it had one you could refer to it as
`$3'.  When `yyparse' recognizes a sum expression using this rule, the
sum of the two subexpressions' values is produced as the value of the
entire expression.  Note: Actions.

   You don't have to give an action for every rule.  When a rule has no
action, Bison by default copies the value of `$1' into `$$'.  This is
what happens in the first rule (the one that uses `NUM').

   The formatting shown here is the recommended convention, but Bison
does not require it.  You can add or change whitespace as much as you
wish.  For example, this:

     exp   : NUM | exp exp '+' {$$ = $1 + $2; } | ...

means the same thing as this:

     exp:      NUM
             | exp exp '+'    { $$ = $1 + $2; }
             | ...

The latter, however, is much more readable.