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 Trackers [http://core.tcl.tk/tcllib/reportlist].
       Please also report any ideas for enhancements you may have for either package and/or documentation.

       When proposing code changes, please provide unified diffs, i.e the output of diff -u.

       Note further that attachments are strongly preferred over inlined patches. Attachments  can  be  made  by
       going  to the Edit form of the ticket immediately after its creation, and then using the left-most button
       in the secondary navigation bar.

KEYWORDS

       grammar, parsing, virtual machine

COPYRIGHT

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