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 represents each ME virtual machine as a Tcl value and provides commands to manipulate and query such values to show the effects of executing instructions, adding tokens, retrieving state, etc.

The values fully follow the paradigm of Tcl that every value is a string and while also allowing C implementations for a proper Tcl_ObjType to keep all the important data in native data structures. Because of the latter it is recommended to access the state values only through the commands of this package to ensure that internal representation is not shimmered away.

The actual structure used by all state values is described in section CPU STATE.

API

The package directly provides only a single command, and all the functionality is made available through its methods.

::grammar::me::cpu::core disasm asm

This method returns a list containing a disassembly of the match instructions in asm. The format of asm is specified in the section MATCH PROGRAM REPRESENTATION.

Each element of the result contains instruction label, instruction name, and the instruction arguments, in this order. The label can be the empty string. Jump destinations are shown as labels, strings and tokens unencoded. Token names are prefixed with their numeric id, if, and only if a tokmap is defined. The two components are separated by a colon.

::grammar::me::cpu::core asm asm

This method returns code in the format as specified in section MATCH PROGRAM REPRESENTATION generated from ME assembly code asm, which is in the format as returned by the method disasm.

::grammar::me::cpu::core new asm

This method creates state value for a ME virtual machine in its initial state and returns it as its result.

The argument matchcode contains a Tcl representation of the match instructions the machine has to execute while parsing the input stream. Its format is specified in the section MATCH PROGRAM REPRESENTATION.

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.

::grammar::me::cpu::core lc state location

This method takes the state value of a ME virtual machine and uses it to convert a location in the input stream (as offset) into a line number and column index. The result of the method is a 2-element list containing the two pieces in the order mentioned in the previous sentence.

Note that the method cannot convert locations which the machine has not yet read from the input stream. In other words, if the machine has read 7 characters so far it is possible 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.

This utility allows higher levels to convert the location offsets found in the error status and the AST into more human readable data.

::grammar::me::cpu::core tok state ?from ?to??

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

This method places the same restrictions on its location arguments as the method lc.

::grammar::me::cpu::core pc state

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

::grammar::me::cpu::core iseof state

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

::grammar::me::cpu::core at state

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

::grammar::me::cpu::core cc state

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

::grammar::me::cpu::core sv state

This method takes the state value of a ME virtual machine and returns the current semantic value stored in it. This is an abstract syntax tree as specified in the document grammar::me_ast , section AST VALUES.

::grammar::me::cpu::core ok state

This method takes the state value of a ME virtual machine and returns the match status stored in it.

::grammar::me::cpu::core error state

This method takes the state value of a ME virtual machine and returns the current error status stored in it.

::grammar::me::cpu::core lstk state

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

::grammar::me::cpu::core astk state

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

::grammar::me::cpu::core mstk state

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

::grammar::me::cpu::core estk state

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

::grammar::me::cpu::core rstk state

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

::grammar::me::cpu::core nc state

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

::grammar::me::cpu::core ast state

This method takes the state value of a ME virtual machine and returns the abstract syntax tree currently at the top of the AST stack stored in it. This is an abstract syntax tree as specified in the document grammar::me_ast , section AST VALUES.

::grammar::me::cpu::core halted state

This method takes the state value of a ME virtual machine and returns the current halt status stored in it, i.e. if the machine has stopped or not.

::grammar::me::cpu::core code state

This method takes the state value of a ME virtual machine and returns the code stored in it, i.e. the instructions executed by the machine.

::grammar::me::cpu::core eof statevar

This method takes the state value of a ME virtual machine as stored in the variable named by statevar and modifies it so that the eof flag inside is set. This signals to the machine that whatever token are in the input queue are the last to be processed. There will be no more.

::grammar::me::cpu::core put statevar tok lex line col

This method takes the state value of a ME virtual machine as stored in the variable named by statevar and modifies it so that the token tok is added to the end of the input queue, with associated lexeme data lex and line/column information.

The operation will fail with an error if the eof flag of the machine has been set through the method eof.

::grammar::me::cpu::core run statevar ?n?

This method takes the state value of a ME virtual machine as stored in the variable named by statevar, executes a number of instructions and stores the state resulting from their modifications back into the variable.

The execution loop will run 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.

MATCH PROGRAM REPRESENTATION

A match program is represented by nested Tcl list. The first element, asm, is a list of integer numbers, the instructions to execute, and their arguments. The second element, pool , is a list of strings, referenced by the instructions, for error messages, token names, etc. The third element, tokmap, provides ordering information for the tokens, mapping their names to their numerical rank. This element can be empty, forcing lexicographic comparison when matching ranges.

All ME instructions are encoded as integer numbers, with the mapping given below. A number of the instructions, those which handle error messages, have been given an additional argument to supply that message explicitly instead of having it constructed from token names, etc. This allows the machine state to store only the message ids instead of the full strings.

Jump destination arguments are absolute indices into the asm element, refering to the instruction to jump to. Any string arguments are absolute indices into the pool element. Tokens, characters, messages, and token (actually character) classes to match are coded as references into the pool as well.

"ict_advance message"
"ict_match_token tok message"
"ict_match_tokrange tokbegin tokend message"
"ict_match_tokclass code message"
"inc_restore branchlabel nt"
"inc_save nt"
"icf_ntcall branchlabel"
"icf_ntreturn"
"iok_ok"
"iok_fail"
"iok_negate"
"icf_jalways branchlabel"
"icf_jok branchlabel"
"icf_jfail branchlabel"
"icf_halt"
"icl_push"
"icl_rewind"
"icl_pop"
"ier_push"
"ier_clear"
"ier_nonterminal message"
"ier_merge"
"isv_clear"
"isv_terminal"
"isv_nonterminal_leaf nt"
"isv_nonterminal_range nt"
"isv_nonterminal_reduce nt"
"ias_push"
"ias_mark"
"ias_mrewind"
"ias_mpop"

CPU STATE

A state value is a list containing the following elements, in the order listed below:

code: Match instructions, see MATCH PROGRAM REPRESENTATION.
pc: Program counter, int.
halt: Halt flag, boolean.
eof: Eof flag, boolean
tc: Terminal cache, and input queue. Structure see below.
cl: Current location, int.
ct: Current token, string .
ok: Match status, boolean.
sv: Semantic value, list .
er: Error status, list .
ls: Location stack, list .
as: AST stack, list .
ms: AST marker stack, list .
es: Error stack, list .
rs: Return stack, list .
nc: Nonterminal cache, dictionary.

tc, the input queue of tokens waiting for processing and the terminal cache containing the tokens already processing are one unified data structure simply holding all tokens and their information, with the current location separating that which has been processed from that which is waiting. Each element of the queue/cache is a list containing the token, its lexeme information, line number, and column index, in this order.

All stacks have their top element aat the end, i.e. pushing an item is equivalent to appending to the list representing the stack, and popping it removes the last element.

er, the error status is either empty or a list of two elements, a location in the input, and a list of messages, encoded as references into the pool element of the code.

nc, the nonterminal cache is keyed by nonterminal name and location, each value a four-element list containing current location, match status, semantic value, and error status, in this order.

KEYWORDS

grammar , parsing , virtual machine

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

Copyright © 2006 for compilation: ActiveState