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# grammar::me::cpu(n) 0.2 tcllib "Grammar operations and usage"

## Name

grammar::me::cpu - Virtual machine implementation II for parsing token streams

## Synopsis

- package require Tcl 8.4
- package require grammar::me::cpu ?0.2?

- ::grammar::me::cpu meName matchcode
- meName option ?arg arg ...?
- meName lc location
- meName tok ?from ?to??
- meName pc state
- meName iseof state
- meName at state
- meName cc state
- meName sv
- meName ok
- meName error
- meName lstk state
- meName astk state
- meName mstk state
- meName estk state
- meName rstk state
- meName nc state
- meName ast
- meName halted
- meName code
- meName eof
- meName put tok lex line col
- meName putstring string lvar cvar
- meName run ?n?
- meName pull nextcmd
- meName reset
- meName destroy

## Description

This package provides an implementation of the ME virtual machine. Please go and read the document grammar::me_intro first if you do not know what a ME virtual machine is.

This implementation provides an object-based API and the machines are not truly tied to Tcl. A C implementation of the same API is quite possible.

Internally the package actually uses the value-based machine manipulation commands as provided by the package grammar::me::cpu::core to perform its duties.

## API

### CLASS API

The package directly provides only a single command for the construction of ME virtual machines.

::grammar::me::cpu meName matchcode

  The command creates a new ME machine object with an associated global Tcl command whose name is meName. This command may be used to invoke various operations on the machine. It has the following general form:

  meName option ?arg arg ...?

    Option and the args determine the exact behavior of the command.

  The argument matchcode contains the match instructions the machine has to execute while parsing the input stream. Please read section MATCH CODE REPRESENTATION of the documentation for the package grammar::me::cpu::core for the specification of the structure of this value.

  The tokmap argument taken by the implementation provided by the package grammar::me::tcl is here hidden inside of the match instructions and therefore not needed.

### OBJECT API

All ME virtual machine objects created by the class command specified in section CLASS API support the methods listed below.

The machines provided by this package provide methods for operation in both push- and pull-styles. Push-style means that tokens are pushed into the machine state when they arrive, triggering further execution until they are consumed. In other words, this allows the machine to be suspended and resumed at will and an arbitrary number of times, the quasi-parallel operation of several machines, and the operation as part of the event loop.

meName lc location

  This method converts the location of a token given as offset in the input stream into the associated line number and column index. The result of the command is a 2-element list containing the two values, in the order mentioned in the previous sentence. This allows higher levels to convert the location information found in the error status and the generated AST into more human readable data.

  Note that the command is not able to convert locations which have not been reached by the machine yet. In other words, if the machine has read 7 tokens the command is able to convert the offsets 0 to 6, but nothing beyond that. This also shows that it is not possible to convert offsets which refer to locations before the beginning of the stream.

meName tok ?from ?to??

  This method returns a Tcl list containing the part of the input stream between the locations from and to (both inclusive). If to is not specified it will default to the value of from. If from is not specified either the whole input stream is returned.

  Each element of the returned list is a list of four elements, the token, its associated lexeme, line number, and column index, in this order. This command places the same restrictions on its location arguments as the method lc.

meName pc state

  This method takes the state value of a ME virtual machine and returns the current value of the stored program counter.

meName iseof state

  This method takes the state value of a ME virtual machine and returns the current value of the stored eof flag.

meName at state

  This method takes the state value of a ME virtual machine and returns the current location in the input stream.

meName cc state

  This method takes the state value of a ME virtual machine and returns the current token.

meName sv

  This command returns the current semantic value SV stored in the machine. This is an abstract syntax tree as specified in the document grammar::me_ast, section AST VALUES.

meName ok

  This method returns the current match status OK.

meName error

  This method returns the current error status ER.

meName lstk state

  This method takes the state value of a ME virtual machine and returns the location stack.

meName astk state

  This method takes the state value of a ME virtual machine and returns the AST stack.

meName mstk state

  This method takes the state value of a ME virtual machine and returns the AST marker stack.

meName estk state

  This method takes the state value of a ME virtual machine and returns the error stack.

meName rstk state

  This method takes the state value of a ME virtual machine and returns the subroutine return stack.

meName nc state

  This method takes the state value of a ME virtual machine and returns the nonterminal match cache as a dictionary.

meName ast

  This method returns the current top entry of the AST stack AS. This is an abstract syntax tree as specified in the document grammar::me_ast, section AST VALUES.

meName halted

  This method returns a boolean value telling the caller whether the engine has halted execution or not. Halt means that no further matching is possible, and the information retrieved via the other method is final. Attempts to run the engine will be ignored, until a reset is made.

meName code

  This method returns the code information used to construct the object. In other words, the match program executed by the machine.

meName eof

  This method adds an end of file marker to the end of the input stream. This signals the machine that the current contents of the input queue are the final parts of the input and nothing will come after. Attempts to put more characters into the queue will fail.

meName put tok lex line col

  This method adds the token tok to the end of the input stream, with associated lexeme data lex and line/column information.

meName putstring string lvar cvar

  This method adds each individual character in the string as a token to the end of the input stream, from first to last. The lexemes will be empty and the line/col information is computed based on the characters encountered and the data in the variables lvar and cvar.

meName run ?n?

  This methods causes the engine to execute match instructions until either

  - n instructions have been executed, or

  - a halt instruction was executed, or

  - the input queue is empty and the code is asking for more tokens to process.

  If no limit n was set only the last two conditions are checked for.

meName pull nextcmd

  This method implements pull-style operation of the machine. It causes it to execute match instructions until either a halt instruction is reached, or the command prefix nextcmd ceases to deliver more tokens.

  The command prefix nextcmd represents the input stream of characters and is invoked by the machine whenever the a new character from the stream is required. The instruction for handling this is ict_advance. The callback has to return either the empty list, or a list of 4 elements containing the token, its lexeme attribute, and its location as line number and column index, in this order. The empty list is the signal that the end of the input stream has been reached. The lexeme attribute is stored in the terminal cache, but otherwise not used by the machine.

  The end of the input stream for this method does not imply that method eof is called for the machine as a whole. By avoiding this and still asking for an explicit call of the method it is possible to mix push- and pull-style operation during the lifetime of the machine.

meName reset

  This method resets the machine to its initial state, discarding any state it may have.

meName destroy

  This method deletes the object and releases all resurces it claimed.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category grammar_me of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

## Keywords

grammar, parsing, virtual machine

## Category

Grammars and finite automata

## Copyright

Copyright © 2005-2006 Andreas Kupries <andreas_kupries@users.sourceforge.net>