Ubuntu Manpage: grammar::me::tcl - Virtual machine implementation I for parsing token streams

NAME

       grammar::me::tcl - Virtual machine implementation I for parsing token streams

SYNOPSIS

       package require Tcl  8.4

       package require grammar::me::tcl  ?0.1?

       ::grammar::me::tcl cmd ...

       ::grammar::me::tcl init nextcmd ?tokmap?

       ::grammar::me::tcl lc location

       ::grammar::me::tcl tok from ?to?

       ::grammar::me::tcl tokens

       ::grammar::me::tcl sv

       ::grammar::me::tcl ast

       ::grammar::me::tcl astall

       ::grammar::me::tcl ctok

       ::grammar::me::tcl nc

       ::grammar::me::tcl next

       ::grammar::me::tcl ord

       ::grammar::me::tcl::ict_advance message

       ::grammar::me::tcl::ict_match_token tok message

       ::grammar::me::tcl::ict_match_tokrange tokbegin tokend message

       ::grammar::me::tcl::ict_match_tokclass code message

       ::grammar::me::tcl::inc_restore nt

       ::grammar::me::tcl::inc_save nt startlocation

       ::grammar::me::tcl::iok_ok

       ::grammar::me::tcl::iok_fail

       ::grammar::me::tcl::iok_negate

       ::grammar::me::tcl::icl_get

       ::grammar::me::tcl::icl_rewind oldlocation

       ::grammar::me::tcl::ier_get

       ::grammar::me::tcl::ier_clear

       ::grammar::me::tcl::ier_nonterminal message location

       ::grammar::me::tcl::ier_merge olderror

       ::grammar::me::tcl::isv_clear

       ::grammar::me::tcl::isv_terminal

       ::grammar::me::tcl::isv_nonterminal_leaf nt startlocation

       ::grammar::me::tcl::isv_nonterminal_range nt startlocation

       ::grammar::me::tcl::isv_nonterminal_reduce nt startlocation ?marker?

       ::grammar::me::tcl::ias_push

       ::grammar::me::tcl::ias_mark

       ::grammar::me::tcl::ias_pop2mark marker

_________________________________________________________________

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 is tied very strongly to Tcl. All the stacks in the machine state  are
       handled  through  the  Tcl  stack,  all  control  flow is handled by Tcl commands, and the
       remaining machine instructions  are  directly  mapped  to  Tcl  commands.  Especially  the
       matching  of  nonterminal  symbols  is  handled  by  Tcl  procedures  as well, essentially
       extending the machine implementation with custom instructions.

       Further on the implementation handles only a single machine which is uninteruptible during
       execution  and  hardwired  for pull operation. I.e.  it explicitly requests each new token
       through a callback, pulling them into its state.

       A related package is grammar::peg::interp which provides a generic  interpreter  /  parser
       for  parsing  expression grammars (PEGs), implemented on top of this implementation of the
       ME virtual machine.

API

The commands documented in this section do not implement any of the instructions of the ME
virtual machine. They provide the facilities for the initialization of the machine and the
retrieval of important information.

::grammar::me::tcl cmd ...
This is an ensemble command providing access to the commands listed in this
section. See the methods themselves for detailed specifications.

::grammar::me::tcl init nextcmd ?tokmap?
This command (re)initializes the machine. It returns the empty string. This command
has to be invoked before any other command of this package.

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 optional dictionary tokmap maps from tokens to integer numbers. If present the
numbers impose an order on the tokens, which is subsequently used by
ict_match_tokrange to determine if a token is in the specified range or not. If no
token map is specified the lexicographic order of th token names will be used
instead. This choice is especially asensible when using characters as tokens.

::grammar::me::tcl lc location
This command 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.

After a call of init the state used for the conversion is cleared, making further
conversions impossible until the machine has read tokens again.

::grammar::me::tcl tok from ?to?
This command 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.

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. In other words,
each element has the same structure as the result of the nextcmd callback given to
::grammar::me::tcl::init

This command places the same restrictions on its location arguments as
::grammar::me::tcl::lc.

::grammar::me::tcl tokens
This command returns the number of tokens currently known to the ME virtual
machine.

::grammar::me::tcl 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.

::grammar::me::tcl ast
This method returns the abstract syntax tree currently at the top of the AST stack
of the ME virtual machine. This is an abstract syntax tree as specified in the
document grammar::me_ast, section AST VALUES.

::grammar::me::tcl astall
This method returns the whole stack of abstract syntax trees currently known to the
ME virtual machine. Each element of the returned list is an abstract syntax tree as
specified in the document grammar::me_ast, section AST VALUES. The top of the
stack resides at the end of the list.

::grammar::me::tcl ctok
This method returns the current token considered by the ME virtual machine.

::grammar::me::tcl nc
This method returns the contents of the nonterminal cache as a dictionary mapping
from "symbol,location" to match information.

::grammar::me::tcl next
This method returns the next token callback as specified during initialization of
the ME virtual machine.

::grammar::me::tcl ord
This method returns a dictionary containing the tokmap specified during
initialization of the ME virtual machine. ::grammar::me::tcl::ok This variable
contains the current match status OK. It is provided as variable instead of a
command because that makes access to this information faster, and the speed of
access is considered very important here as this information is used constantly to
determine the control flow.

MACHINE STATE

       Please go and read the document grammar::me_vm first for a specification of the  basic  ME
       virtual machine and its state.

       This  implementation  manages  the state described in that document, except for the stacks
       minus the AST stack. In other words, location stack, error stack, return  stack,  and  ast
       marker  stack  are  implicitly managed through standard Tcl scoping, i.e. Tcl variables in
       procedures, outside of this implementation.

MACHINE INSTRUCTIONS

Please go and read the document grammar::me_vm first for a specification of the basic ME
virtual machine and its instruction set.

This implementation maps all instructions to Tcl commands in the namespace
"::grammar::me::tcl", except for the stack related commands, nonterminal symbols and
control flow. Here we simply list the commands and explain the differences to the
specified instructions, if there are any. For their semantics see the aforementioned
specification. The machine commands are not reachable through the ensemble command
::grammar::me::tcl.

::grammar::me::tcl::ict_advance message
No changes.

::grammar::me::tcl::ict_match_token tok message
No changes.

::grammar::me::tcl::ict_match_tokrange tokbegin tokend message
If, and only if a token map was specified during initialization then the arguments
are the numeric representations of the smallest and largest tokens in the range.
Otherwise they are the relevant tokens themselves and lexicographic comparison is
used.

::grammar::me::tcl::ict_match_tokclass code message
No changes.

::grammar::me::tcl::inc_restore nt
Instead of taking a branchlabel the command returns a boolean value. The result
will be true if and only if cached information was found. The caller has to perform
the appropriate branching.

::grammar::me::tcl::inc_save nt startlocation
The command takes the start location as additional argument, as it is managed on
the Tcl stack, and not in the machine state.

icf_ntcall branchlabel

icf_ntreturn
These two instructions are not mapped to commands. They are control flow
instructions and handled in Tcl.

::grammar::me::tcl::iok_ok
No changes.

::grammar::me::tcl::iok_fail
No changes.

::grammar::me::tcl::iok_negate
No changes.

icf_jalways branchlabel

icf_jok branchlabel

icf_jfail branchlabel

icf_halt
These four instructions are not mapped to commands. They are control flow
instructions and handled in Tcl.

::grammar::me::tcl::icl_get
This command returns the current location CL in the input. It replaces icl_push.

::grammar::me::tcl::icl_rewind oldlocation
The command takes the location as argument as it comes from the Tcl stack, not the
machine state.

icl_pop
Not mapped, the stacks are not managed by the package.

::grammar::me::tcl::ier_get
This command returns the current error state ER. It replaces ier_push.

::grammar::me::tcl::ier_clear
No changes.

::grammar::me::tcl::ier_nonterminal message location
The command takes the location as argument as it comes from the Tcl stack, not the
machine state.

::grammar::me::tcl::ier_merge olderror
The command takes the second error state to merge as argument as it comes from the
Tcl stack, not the machine state.

::grammar::me::tcl::isv_clear
No changes.

::grammar::me::tcl::isv_terminal
No changes.

::grammar::me::tcl::isv_nonterminal_leaf nt startlocation
The command takes the start location as argument as it comes from the Tcl stack,
not the machine state.

::grammar::me::tcl::isv_nonterminal_range nt startlocation
The command takes the start location as argument as it comes from the Tcl stack,
not the machine state.

::grammar::me::tcl::isv_nonterminal_reduce nt startlocation ?marker?
The command takes start location and marker as argument as it comes from the Tcl
stack, not the machine state.

::grammar::me::tcl::ias_push
No changes.

::grammar::me::tcl::ias_mark
This command returns a marker for the current state of the AST stack AS. The marker
stack is not managed by the machine.

::grammar::me::tcl::ias_pop2mark marker
The command takes the marker as argument as it comes from the Tcl stack, not the
machine state. It replaces ias_mpop.

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 SF Trackers
       [http://sourceforge.net/tracker/?group_id=12883].   Please  also  report  any  ideas   for
       enhancements you may have for either package and/or documentation.

KEYWORDS

       grammar, parsing, virtual machine

COPYRIGHT

       Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>