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grammar::me::tcl(n) Grammar operations and usage grammar::me::tcl(n)

____________________________________________________________________________________________________________

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 ele-ments elements
ments 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 con-taining containing
taining 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 con-vert convert
vert 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 lex-eme, lexeme,
eme, 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 doc-ument document
ument 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 "sym-bol,location" "symbol,location"
bol,location" to match information.

::grammar::me::tcl next
This method returns the next token callback as specified during initialization of the ME vir-tual virtual
tual 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 informa-tion information
tion 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 implementa-tion. implementation.
tion.

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 branch-ing. branching.
ing.

::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 han-dled handled
dled 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://source -
forge.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

CATEGORY
Grammars and finite automata

grammar_me 0.1 grammar::me::tcl(n)

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