grammar::me::tcl(3tcl) Grammar operations and usage grammar::me::tcl(3tcl)
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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
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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 di-
rectly 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 in-
terpreter / 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 in-
structions of the ME virtual machine. They provide the facilities for
the initialization of the machine and the retrieval of important infor-
mation.
::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 char-
acters and is invoked by the machine whenever the a new charac-
ter 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 in-
dex, 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 ma-
chine.
The optional dictionary tokmap maps from tokens to integer num-
bers. 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 charac-
ters 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 in-
dex. 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 be-
fore 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 struc-
ture as the result of the nextcmd callback given to ::gram-
mar::me::tcl::init
This command places the same restrictions on its location argu-
ments 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 ab-
stract 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 vir-
tual 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. ::gram-
mar::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 specifica-
tion of the basic ME virtual machine and its state.
This implementation manages the state described in that document, ex-
cept 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 specifica-
tion of the basic ME virtual machine and its instruction set.
This implementation maps all instructions to Tcl commands in the name-
space "::grammar::me::tcl", except for the stack related commands, non-
terminal 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 ma-
chine commands are not reachable through the ensemble command ::gram-
mar::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 small-
est and largest tokens in the range. Otherwise they are the rel-
evant 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 informa-
tion 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 con-
trol 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 out-
put 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
CATEGORY
Grammars and finite automata
COPYRIGHT
Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>
tcllib 0.1 grammar::me::tcl(3tcl)