Racc Grammar File Reference

Global Structure

Class Block and User Code Block

There's two block on toplevel. one is 'class' block, another is 'user code' block. 'user code' block MUST places after 'class' block.


You can insert comment about all places. Two style comment can be used, Ruby style (#.....) and C style (/*......*/) .

Class Block

The class block is formed like this:

  [precedance table]
  [token declearations]
  [expected number of S/R conflict]
  [semantic value convertion]
  [start rule]

CLASS_NAME is a name of parser class. This is the name of generating parser class.

If CLASS_NAME includes '::', Racc outputs module clause. For example, writing "class M::C" causes creating the code bellow:

module M
  class C

Grammar Block

The grammar block discripts grammar which is able to be understood by parser. Syntax is:

(token): (token) (token) (token).... (action)

(token): (token) (token) (token).... (action)
       | (token) (token) (token).... (action)
       | (token) (token) (token).... (action)

(action) is an action which is executed when its (token)s are found. (action) is a ruby code block, which is surrounded by braces:

{ print val[0]
  puts val[1] }

Note that you cannot use '%' string, here document, '%r' regexp in action.

Actions can be omitted. When it is omitted, '' (empty string) is used.

A return value of action is a value of left side value ($$). It is value of result, or returned value by "return" statement.

Here is an example of whole grammar block.

  goal: definition ruls source { result = val }

  definition: /* none */   { result = [] }
    | definition startdesig  { result[0] = val[1] }
    | definition
             precrule   # this line continue from upper line
        result[1] = val[1]

  startdesig: START TOKEN

You can use following special local variables in action.

The value of left-hand side (lhs). A default value is val[0].

An array of value of right-hand side (rhs).

A stack of values. DO NOT MODIFY this stack unless you know what you are doing.

Operator Precedance

This function is equal to '%prec' in yacc. To designate this block:

  nonassoc '++'
  left     '*' '/'
  left     '+' '-'
  right    '='

`right' is yacc's %right, `left' is yacc's %left.

`=' + (symbol) means yacc's %prec:

  nonassoc UMINUS
  left '*' '/'
  left '+' '-'

  exp: exp '*' exp
     | exp '-' exp
     | '-' exp       =UMINUS   # equals to "%prec UMINUS"


Racc has bison's "expect" directive.

# Example

class MyParser
  expect 3

This directive declears "expected" number of shift/reduce conflict. If "expected" number is equal to real number of conflicts, racc does not print confliction warning message.

Declaring Tokens

By declaring tokens, you can avoid many meanless bugs. If decleared token does not exist/existing token does not decleared, Racc output warnings. Declearation syntax is:



You can write options for racc command in your racc file.

options OPTION OPTION ...

Options are:

omit empty action call or not.

use/does not use local variable "result"

You can use 'no_' prefix to invert its meanings.

Converting Token Symbol

Token symbols are, as default,

You can change this default by "convert" block. Here is an example:

  PLUS 'PlusClass'      # We use PlusClass for symbol of `PLUS'
  MIN  'MinusClass'     # We use MinusClass for symbol of `MIN'

We can use almost all ruby value can be used by token symbol, except 'false' and 'nil'. These are causes unexpected parse error.

If you want to use String as token symbol, special care is required. For example:

  class '"cls"'            # in code, "cls"
  PLUS '"plus\n"'          # in code, "plus\n"
  MIN  "\"minus#{val}\""   # in code, \"minus#{val}\"

Start Rule

'%start' in yacc. This changes start rule.

start real_target

This statement will not be used forever, I think.

User Code Block

"User Code Block" is a Ruby source code which is copied to output. There are three user code block, "header" "inner" and "footer".

Format of user code is like this:

---- header
  ruby statement
  ruby statement
  ruby statement

---- inner
  ruby statement

If four '-' exist on line head, racc treat it as beginning of user code block. A name of user code must be one word.