grammar2.en - racc: Ruby Standard Library Documentation

# `Racc` Grammar File Reference¶ ↑

## Global Structure¶ ↑

## Class Block and User Code Block¶ ↑

There are two blocks on the toplevel. One is the ‘class’ block, the other is the ‘user code’ block. The ‘user code’ block MUST be placed after the ‘class’ block.

## Comments¶ ↑

You can insert comments about all places. Two styles of comments can be used, 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 conflicts]
      [options]
      [semantic value conversion]
      [start rule]
    rule
      GRAMMARS

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

If CLASS_NAME includes ‘::’, `Racc` outputs the module clause. For example, writing “class M::C” causes the code below to be created:

    module M
      class C
        :
        :
      end
    end

## Grammar Block¶ ↑

The grammar block describes grammar which is able to be understood by the 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 the left side value ($$). It is the value of the result, or the returned value by ‘return` statement.

Here is an example of the whole grammar block.

    rule
      goal: definition rules source { result = val }

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

      startdesig: START TOKEN

You can use the following special local variables in action:

- result ($$)

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

- val ($1,$2,$3…)

An array of value of the 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` has bison’s “expect” directive.

    # Example

    class MyParser
      expect 3
    rule
        :
        :

This directive declares “expected” number of shift/reduce conflicts. If “expected” number is equal to real number of conflicts, `Racc` does not print conflict warning message.

## Declaring Tokens¶ ↑

By declaring tokens, you can avoid many meaningless bugs. If declared token does not exist or existing token does not decleared, `Racc` output warnings. Declaration 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

omits empty action call or not.

- result_var

uses local variable “result” or not.

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

## Converting Token Symbol¶ ↑

Token symbols are, as default,

    * naked token string in Racc file (TOK, XFILE, this_is_token, ...)
      --> symbol (:TOK, :XFILE, :this_is_token, ...)
    * quoted string (':', '.', '(', ...)
      --> same string (':', '.', '(', ...)

You can change this default by “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 cause 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 start rule.

    start real_target

## User Code Block¶ ↑

“User Code Block” is a Ruby source code which is copied to output. There are three user code blocks, “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 the line head, `Racc` treats it as the beginning of the user code block. The name of the user code block must be one word.