`Racc` Grammar File Reference

Global Structure

Class Block and User Code Block

There are two top-level blocks: the ‘class’ block, and the ‘user code’ block. The ‘user code’ block MUST be after the ‘class’ block.

Comment

Comments can be added about everywhere. Two comment styles are supported: Ruby style (‘# …`) and C style (`/* … */`).

Class Block

The class block is formed like this:

class CLASS_NAME
  [precedence table]
  [token declarations]
  [expected number of S/R conflict]
  [options]
  [semantic value conversion]
  [start rule]
rule
  GRAMMARS

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 below:

module M
  class C
    :
    :
  end
end

Grammar Block

The grammar block describes the grammar 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.

rule
  goal: definition rules 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.

* result ($$)

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

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

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

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

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

Operator Precedence

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

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

‘right’ is yacc’s %right, ‘left’ is yacc’s %left.

‘=’ + (symbol) means yacc’s %prec:

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

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

expect

Racc supports Bison’s “expect” directive to declare the expected number of shift/reduce conflicts.

class MyParser
  expect 3
rule
    :
    :

Then warnings are issued only when the effective number of conflicts differs.

Declaring Tokens

Declaring tokens avoids many bugs.

Racc outputs warnings for declared tokens that do not exist, or existing tokens not declared. The 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 ...

Options are:

* omit_action_call

omit empty action call or not.

* result_var

use/does not use local variable “result”

You can use ‘no_’ prefix to invert its meanings.

Converting Token Symbol

Token symbols are, as default,

* naked token strings in racc file (TOK, XFILE, this_is_token, ...)
  --&gt; symbol (:TOK, :XFILE, :this_is_token, ...)
* quoted strings (':', '.', '(', ...)
  --&gt; same string (':', '.', '(', ...)

You can change this default using a “convert” block. Here is an example:

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

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:

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 the start symbol.

start real_target

User Code Block

A “User Code Block” is a piece of Ruby source code copied in the output. There are three user code blocks, “header” “inner” and “footer”.

User code blocks are introduced by four ‘-’ at the beginning of a line, followed by a single-word name:

---- header
  ruby statement
  ruby statement
  ruby statement

---- inner
  ruby statement
     :
     :

Racc Grammar File Reference