grammar::me_vm(n) Grammar operations and usage grammar::me_vm(n)______________________________________________________________________________NAME
grammar::me_vm - Virtual machine for parsing token streams
DESCRIPTION
Please go and read the document grammar::me_intro first for an overview
of the various documents and their relations.
This document specifies a virtual machine for the controlled matching
and parsing of token streams, creating an abstract syntax tree (short
AST) reflecting the structure of the input. Special machine features
are the caching and reuse of partial results, caching of the encoun‐
tered input, and the ability to backtrack in both input and AST cre‐
ation.
These features make the specified virtual machine especially useful to
packrat parsers based on parsing expression grammars. It is however not
restricted to this type of parser. Normal LL and LR parsers can be
implemented with it as well.
The following sections will discuss first the abstract state kept by ME
virtual machines, and then their instruction set.
MACHINE STATE
A ME virtual machine manages the following state:
Current token CT
The token from the input under consideration by the machine.
This information is used and modified by the instructions
defined in the section TERMINAL MATCHING.
Current location CL
The location of the current token in the input stream, as offset
relative to the beginning of the stream. The first token is con‐
sidered to be at offset 0.
This information is implicitly used and modified by the instruc‐
tions defined in the sections TERMINAL MATCHING and NONTERMINAL
MATCHING, and can be directly queried and modified by the
instructions defined in section INPUT LOCATION HANDLING.
Location stack LS
In addition to the above a stack of locations, for backtracking.
Locations can put on the stack, removed from it, and removed
with setting the current location.
This information is implicitly used and modified by the instruc‐
tions defined in the sections TERMINAL MATCHING and NONTERMINAL
MATCHING, and can be directly queried and modified by the
instructions defined in section INPUT LOCATION HANDLING.
Match status OK
A boolean value, the result of the last attempt at matching
input. It is set to true if that attempt was successful, and
false otherwise.
This information is influenced by the instructions defined in
the sections TERMINAL MATCHING, NONTERMINAL MATCHING, and UNCON‐
DITIONAL MATCHING. It is queried by the instructions defined in
the section CONTROL FLOW.
Semantic value SV
The semantic value associated with (generated by) the last
attempt at matching input. Contains either the empty string or a
node for the abstract syntax tree constructed from the input.
This information is influenced by the instructions defined in
the sections SEMANTIC VALUES, and AST STACK HANDLING.
AST stack AS
A stack of partial abstract syntax trees constructed by the
machine during matching.
This information is influenced by the instructions defined in
the sections SEMANTIC VALUES, and AST STACK HANDLING.
AST Marker stack MS
In addition to the above a stack of stacks, for backtracking.
This is actually a stack of markers into the AST stack, thus
implicitly snapshooting the state of the AST stack at some point
in time. Markers can be put on the stack, dropped from it, and
used to roll back the AST stack to an earlier state.
This information is influenced by the instructions defined in
the sections SEMANTIC VALUES, and AST STACK HANDLING.
Error status ER
Error information associated with the last attempt at matching
input. Contains either the empty string or a list of 2 elements,
a location in the input and a list of error messages associated
with it, in this order.
Note that error information can be set even if the last attempt
at matching input was successful. For example the *-operator
(matching a sub-expression zero or more times) in a parsing
expression grammar is always successful, even if it encounters a
problem further in the input and has to backtrack. Such problems
must not be forgotten when continuing to match.
This information is queried and influenced by the instructions
defined in the sections TERMINAL MATCHING, NONTERMINAL MATCHING,
and ERROR HANDLING.
Error stack ES
In addition to the above a stack of error information, to allow
the merging of current and older error information when perform‐
ing backtracking in choices after an unsucessful match.
This information is queried and influenced by the instructions
defined in the sections TERMINAL MATCHING, NONTERMINAL MATCHING,
and ERROR HANDLING.
Return stack RS
A stack of program counter values, i.e. locations in the code
controlling the virtual machine, for the management of subrou‐
tine calls, i.e. the matching of nonterminal symbols.
This information is queried and influenced by the instructions
defined in the section NONTERMINAL MATCHING.
Nonterminal cache NC
A cache of machine states (A 4-tuple containing a location in
the input, match status OK, semantic value SV, and error status
ER) keyed by name of nonterminal symbol and location in the
input stream.
The key location is where machine started the attempt to match
the named nonterminal symbol, and the location in the value is
where machine ended up after the attempt completed, independent
of the success of the attempt.
This status is queried and influenced by the instructions
defined in the section NONTERMINAL MATCHING.
MACHINE INSTRUCTIONS
With the machine state specified it is now possible to explain the
instruction set of ME virtual machines. They are grouped roughly by the
machine state they influence and/or query.
TERMINAL MATCHING
First the instructions to match tokens from the input stream, and by
extension all terminal symbols.
These instructions are the only ones which may retrieve a new token
from the input stream. This is a may and not a will because the
instructions will a retrieve new token if, and only if the current
location CL is at the head of the stream. If the machine has back‐
tracked (see icl_rewind) the instructions will retrieve the token to
compare against from the internal cache.
ict_advance message
This instruction tries to advance to the next token in the input
stream, i.e. the one after the current location CL. The instruc‐
tion will fail if, and only if the end of the input stream is
reached, i.e. if there is no next token.
The sucess/failure of the instruction is remembered in the match
status OK. In the case of failure the error status ER is set to
the current location and the message message. In the case of
success the error status ER is cleared, the new token is made
the current token CT, and the new location is made the current
location CL.
The argument message is a reference to the string to put into
the error status ER, if such is needed.
ict_match_token tok message
This instruction tests the current token CT for equality with
the argument tok and records the result in the match status OK.
The instruction fails if the current token is not equal to tok.
In case of failure the error status ER is set to the current
location CL and the message message, and the current location CL
is moved one token backwards. Otherwise, i.e. upon success, the
error status ER is cleared and the current location CL is not
touched.
ict_match_tokrange tokbegin tokend message
This instruction tests the current token CT for being in the
range of tokens from tokbegin to tokend (inclusive) and records
the result in the match status OK. The instruction fails if the
current token is not inside the range.
In case of failure the error status ER is set to the current
location CL and the message message, and the current location CL
is moved one token backwards. Otherwise, i.e. upon success, the
error status ER is cleared and the current location CL is not
touched.
ict_match_tokclass code message
This instruction tests the current token CT for being a member
of the token class code and records the result in the match sta‐
tus OK. The instruction fails if the current token is not a mem‐
ber of the specified class.
In case of failure the error status ER is set to the current
location CL and the message message, and the current location CL
is moved one token backwards. Otherwise, i.e. upon success, the
error status ER is cleared and the current location CL is not
touched.
Currently the following classes are legal:
alnum A token is accepted if it is a unicode alphabetical char‐
acter, or a digit.
alpha A token is accepted if it is a unicode alphabetical char‐
acter.
digit A token is accepted if it is a unicode digit character.
xdigit A token is accepted if it is a hexadecimal digit charac‐
ter.
punct A token is accepted if it is a unicode punctuation char‐
acter.
space A token is accepted if it is a unicode space character.
NONTERMINAL MATCHING
The instructions in this section handle the matching of nonterminal
symbols. They query the nonterminal cache NC for saved information, and
put such information into the cache.
The usage of the cache is a performance aid for backtracking parsers,
allowing them to avoid an expensive rematch of complex nonterminal sym‐
bols if they have been encountered before.
inc_restore branchlabel nt
This instruction checks if the nonterminal cache NC contains
information about the nonterminal symbol nt, at the current
location CL. If that is the case the instruction will update the
machine state (current location CL, match status OK, semantic
value SV, and error status ER) with the found information and
continue execution at the instruction refered to by the branch‐
label. The new current location CL will be the last token
matched by the nonterminal symbol, i.e. belonging to it.
If no information was found the instruction will continue execu‐
tion at the next instruction.
Together with icf_ntcall it is possible to generate code for
memoized and non-memoized matching of nonterminal symbols,
either as subroutine calls, or inlined in the caller.
inc_save nt
This instruction saves the current state of the machine (current
location CL, match status OK, semantic value SV, and error sta‐
tus ER), to the nonterminal cache NC. It will also pop an entry
from the location stack LS and save it as the start location of
the match.
It is expected to be called at the end of matching a nonterminal
symbol, with nt the name of the nonterminal symbol the code was
working on. This allows the instruction inc_restore to check for
and retrieve the data, should we have to match this nonterminal
symbol at the same location again, during backtracking.
icf_ntcall branchlabel
This instruction invokes the code for matching the nonterminal
symbol nt as a subroutine. To this end it stores the current
program counter PC on the return stack RS, the current location
CL on the location stack LS, and then continues execution at the
address branchlabel.
The next matching icf_ntreturn will cause the execution to con‐
tinue at the instruction coming after the call.
icf_ntreturn
This instruction will pop an entry from the return stack RS,
assign it to the program counter PC, and then continue execution
at the new address.
UNCONDITIONAL MATCHING
The instructions in this section are the remaining match operators.
They change the match status OK directly and unconditionally.
iok_ok This instruction sets the match status OK to true, indicating a
successful match.
iok_fail
This instruction sets the match status OK to false, indicating a
failed match.
iok_negate
This instruction negates the match status OK, turning a failure
into a success and vice versa.
CONTROL FLOW
The instructions in this section implement both conditional and uncon‐
ditional control flow. The conditional jumps query the match status OK.
icf_jalways branchlabel
This instruction sets the program counter PC to the address
specified by branchlabel and then continues execution from
there. This is an unconditional jump.
icf_jok branchlabel
This instruction sets the program counter PC to the address
specified by branchlabel. This happens if, and only if the match
status OK indicates a success. Otherwise it simply continues
execution at the next instruction. This is a conditional jump.
icf_jfail branchlabel
This instruction sets the program counter PC to the address
specified by branchlabel. This happens if, and only if the match
status OK indicates a failure. Otherwise it simply continues
execution at the next instruction. This is a conditional jump.
icf_halt
This instruction halts the machine and blocks any further execu‐
tion.
INPUT LOCATION HANDLING
The instructions in this section are for backtracking, they manipulate
the current location CL of the machine state. They allow a user of the
machine to query and save locations in the input, and to rewind the
current location CL to saved locations, making them one of the compo‐
nents enabling the implementation of backtracking parsers.
icl_push
This instruction pushes a copy of the current location CL on the
location stack LS.
icl_rewind
This instruction pops an entry from the location stack LS and
then moves the current location CL back to this point in the
input.
icl_pop
This instruction pops an entry from the location stack LS and
discards it.
ERROR HANDLING
The instructions in this section provide read and write access to the
error status ER of the machine.
ier_push
This instruction pushes a copy of the current error status ER on
the error stack ES.
ier_clear
This instruction clears the error status ER.
ier_nonterminal message
This instruction checks if the error status ER contains an error
whose location is just past the location found in the top entry
of the location stack LS. Nothing happens if no such error is
found. Otherwise the found error is replaced by an error at the
location found on the stack, having the message message.
ier_merge
This instruction pops an entry from the error stack ES, merges
it with the current error status ER and stores the result of the
merge as the new error status ER.
The merge is performed as described below:
If one of the two error states is empty the other is chosen. If
neither error state is empty, and refering to different loca‐
tions, then the error state with the location further in the
input is chosen. If both error states refer to the same location
their messages are merged (with removing duplicates).
SEMANTIC VALUES
The instructions in this section manipulate the semantic value SV.
isv_clear
This instruction clears the semantic value SV.
isv_terminal
This instruction creates a terminal AST node for the current
token CT, makes it the semantic value SV, and also pushes the
node on the AST stack AS.
isv_nonterminal_leaf nt
This instruction creates a nonterminal AST node without any
children for the nonterminal nt, and makes it the semantic value
SV.
This instruction should be executed if, and only if the match
status OK indicates a success. In the case of a failure
isv_clear should be called.
isv_nonterminal_range nt
This instruction creates a nonterminal AST node for the nonter‐
minal nt, with a single terminal node as its child, and makes
this AST the semantic value SV. The terminal node refers to the
input string from the location found on top of the location
stack LS to the current location CL (both inclusive).
This instruction should be executed if, and only if the match
status OK indicates a success. In the case of a failure
isv_clear should be called.
isv_nonterminal_reduce nt
This instruction creates a nonterminal AST node for the nonter‐
minal nt and makes it the semantic value SV.
All entries on the AST stack AS above the marker found in the
top entry of the AST Marker stack MS become children of the new
node, with the entry at the stack top becoming the rightmost
child. If the AST Marker stack MS is empty the whole stack is
used. The AST marker stack MS is left unchanged.
This instruction should be executed if, and only if the match
status OK indicates a success. In the case of a failure
isv_clear should be called.
AST STACK HANDLING
The instructions in this section manipulate the AST stack AS, and the
AST Marker stack MS.
ias_push
This instruction pushes the semantic value SV on the AST stack
AS.
ias_mark
This instruction pushes a marker for the current state of the
AST stack AS on the AST Marker stack MS.
ias_mrewind
This instruction pops an entry from the AST Marker stack MS and
then proceeds to pop entries from the AST stack AS until the
state represented by the popped marker has been reached again.
Nothing is done if the AST stack AS is already smaller than
indicated by the popped marker.
ias_mpop
This instruction pops an entry from the AST Marker stack MS and
discards it.
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
COPYRIGHT
Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>
grammar_me 0.1 grammar::me_vm(n)
