Provide functionalities for Bring Your Own Stack

[yui-knk]

Ruby’s Ripper library requires their own semantic value stack to manage Ruby Objects returned by user defined callback method. Currently Ripper uses semantic value stack (yyvsa) which is used by parser to manage Node. This hack introduces some limitation on Ripper.
For example, Ripper can not execute semantic analysis depending on Node structure.

Lrama introduces two features to support another semantic value stack by parser generator users.

1. Callback entry points

User can emulate semantic value stack by these callbacks. Lrama provides these five callbacks. Registered functions are called when each event happen. For example %after-shift function is called when shift happens on original semantic value stack.

• %after-shift function_name
• %before-reduce function_name
• %after-reduce function_name
• %after-shift-error-token function_name
• %after-pop-stack function_name

2. $:n variable to access index of each grammar symbols

User also needs to access semantic value of their stack in grammar action. $:n provides the way to access to it. $:n is translated to the minus index from the top of the stack. For example

primary: k_if expr_value then compstmt if_tail k_end
          {
          /*% ripper: if!($:2, $:4, $:5) %*/
          /* $:2 = -5, $:4 = -3, $:5 = -2. */
          }