Racc grammar reference

This is temporary specification.

global structure

blocks in file

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

comment

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

class block

class block is like this:

    class class_name
      [precedance table]
      [token declearations]
      [expected number of S/R conflict]
      [options]
      [semantic value convertion]
      [start rule]
    rule
      RULES
    end

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

module M
  class C
    :
  end
end

rule block

'rule block' discripts grammar which is able to be understood by parser. For example:

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

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

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

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

Then, here's a sample of whole 'rule block'.

rule
  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

end

result ($$)

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

val ($1,$2,$3...)

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

_values (...$-2,$-1,$0)

a stack of values. DO NOT CHANGE this stack unless you know what you are doing.

Operator precedance

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

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

'%prec' can be used. format is like this:

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

  rule
    exp: exp '*' exp
       | exp '-' exp
       | '-' exp       = UMINUS   # this!!!
           :

expect

Racc has bison's "expect" directive.

# Example

class MyParser
rule
  expect 3
    :

Token Declearation

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

  token TOKEN_NAME AND_IS_THIS
        ALSO_THIS_IS AGAIN_AND_AGAIN THIS_IS_LAST

Options

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

  options OPTION OPTION ...

omit_action_call

if omit empty action call

result_var

if use variable "result"

Convert Token Symbol

Token symbols are, as default,

naked token string in racc file (TOK, XFILE, this_is_token, ...)

symbol of it (:TOK, :XFILE, :this_is_token, ...)

quoted string (':', '.', '(', ...)

same string (':', '.', '(', ...)

    convert
      PLUS 'PlusClass'      # not use :PLUS but PlusClass
      MIN  'MinusClass'     # not use :MIN but MinusClass
    end

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

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

Start Rule

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

      start real_target

this statement won't be used forever, I think.

User Code

'user block' is Ruby source code which is copied to output. In racc command, Three spetial user code 'header' 'inner' 'footer' are used.
Until version 0.10.2, they are called prepare/inner/driver. It is also OK, but they will be eliminated in future.

format of user code is like this:

---- header
  ruby statement
  ruby statement
  ruby statement

---- inner
  ruby statement
     :

You can include other file as user code like this:

---- footer = init.rb run.rb

print 'this code is executed too.'

Copyright (c) 1999-2002 Minero Aoki <aamine@loveruby.net>