'\"
'\" Generated from file 'me_vm\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2005 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
'\"
.TH "grammar::me_vm" n 0\&.2 tcllib "Grammar operations and usage"
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.BS
.SH NAME
grammar::me_vm \- Virtual machine for parsing token streams
.SH DESCRIPTION
Please go and read the document \fBgrammar::me_intro\fR first for
an overview of the various documents and their relations\&.
.PP
This document specifies a virtual machine for the controlled matching
and parsing of token streams, creating an
\fIabstract syntax tree\fR (short \fIAST\fR) reflecting the
structure of the input\&. Special machine features are the caching and
reuse of partial results, caching of the encountered input, and the
ability to backtrack in both input and AST creation\&.
.PP
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\&.
.PP
The following sections will discuss first the abstract state kept by
ME virtual machines, and then their instruction set\&.
.SH "MACHINE STATE"
A ME virtual machine manages the following state:
.TP
\fICurrent token\fR CT
The token from the input under consideration by the machine\&.
.sp
This information is used and modified by the instructions defined in
the section
\fBTERMINAL MATCHING\fR\&.
.TP
\fICurrent location\fR CL
The location of the \fIcurrent token\fR in the input stream, as
offset relative to the beginning of the stream\&. The first token is
considered to be at offset \fB0\fR\&.
.sp
This information is implicitly used and modified by the instructions
defined in the sections
\fBTERMINAL MATCHING\fR and
\fBNONTERMINAL MATCHING\fR,
and can be directly queried and modified by the instructions defined
in section
\fBINPUT LOCATION HANDLING\fR\&.
.TP
\fILocation stack\fR 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\&.
.sp
This information is implicitly used and modified by the instructions
defined in the sections
\fBTERMINAL MATCHING\fR and
\fBNONTERMINAL MATCHING\fR,
and can be directly queried and modified by the instructions defined
in section
\fBINPUT LOCATION HANDLING\fR\&.
.TP
\fIMatch status\fR OK
A boolean value, the result of the last attempt at matching input\&.
It is set to \fBtrue\fR if that attempt was successful, and
\fBfalse\fR otherwise\&.
.sp
This information is influenced by the instructions defined in the
sections
\fBTERMINAL MATCHING\fR,
\fBNONTERMINAL MATCHING\fR, and
\fBUNCONDITIONAL MATCHING\fR\&.
It is queried by the instructions defined in the section
\fBCONTROL FLOW\fR\&.
.TP
\fISemantic value\fR 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\&.
.sp
This information is influenced by the instructions defined in the
sections
\fBSEMANTIC VALUES\fR, and
\fBAST STACK HANDLING\fR\&.
.TP
\fIAST stack\fR AS
A stack of partial abstract syntax trees constructed by the machine
during matching\&.
.sp
This information is influenced by the instructions defined in the
sections
\fBSEMANTIC VALUES\fR, and
\fBAST STACK HANDLING\fR\&.
.TP
\fIAST Marker stack\fR 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\&.
.sp
This information is influenced by the instructions defined in the
sections
\fBSEMANTIC VALUES\fR, and
\fBAST STACK HANDLING\fR\&.
.TP
\fIError status\fR 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\&.
.sp
\fINote\fR 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\&.
.sp
This information is queried and influenced by the instructions defined
in the sections
\fBTERMINAL MATCHING\fR,
\fBNONTERMINAL MATCHING\fR, and
\fBERROR HANDLING\fR\&.
.TP
\fIError stack\fR ES
In addition to the above a stack of error information, to allow the
merging of current and older error information when performing
backtracking in choices after an unsucessful match\&.
.sp
This information is queried and influenced by the instructions defined
in the sections
\fBTERMINAL MATCHING\fR,
\fBNONTERMINAL MATCHING\fR, and
\fBERROR HANDLING\fR\&.
.TP
\fIReturn stack\fR RS
A stack of program counter values, i\&.e\&. locations in the code
controlling the virtual machine, for the management of subroutine
calls, i\&.e\&. the matching of nonterminal symbols\&.
.sp
This information is queried and influenced by the instructions defined
in the section
\fBNONTERMINAL MATCHING\fR\&.
.TP
\fINonterminal cache\fR NC
A cache of machine states (A 4-tuple containing a location in the
input, match status \fIOK\fR, semantic value \fISV\fR, and error
status \fIER\fR) keyed by name of nonterminal symbol and location in
the input stream\&.
.sp
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\&.
.sp
This status is queried and influenced by the instructions defined in
the section
\fBNONTERMINAL MATCHING\fR\&.
.PP
.SH "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\&.
.SS "TERMINAL MATCHING"
First the instructions to match tokens from the input stream, and
by extension all terminal symbols\&.
.PP
These instructions are the only ones which may retrieve a new token
from the input stream\&. This is a \fImay\fR and not a \fIwill\fR
because the instructions will a retrieve new token if, and only if the
current location \fICL\fR is at the head of the stream\&.
If the machine has backtracked (see \fBicl_rewind\fR) the instructions
will retrieve the token to compare against from the internal cache\&.
.PP
.TP
\fBict_advance\fR \fImessage\fR
This instruction tries to advance to the next token in the input
stream, i\&.e\&. the one after the current location \fICL\fR\&. The
instruction will fail if, and only if the end of the input stream is
reached, i\&.e\&. if there is no next token\&.
.sp
The sucess/failure of the instruction is remembered in the match
status \fIOK\fR\&. In the case of failure the error status \fIER\fR is
set to the current location and the message \fImessage\fR\&.
In the case of success the error status \fIER\fR is cleared, the new
token is made the current token \fICT\fR, and the new location is
made the current location \fICL\fR\&.
.sp
The argument \fImessage\fR is a reference to the string to put into
the error status \fIER\fR, if such is needed\&.
.TP
\fBict_match_token\fR \fItok\fR \fImessage\fR
This instruction tests the current token \fICT\fR for equality
with the argument \fItok\fR and records the result in the match
status \fIOK\fR\&. The instruction fails if the current token is
not equal to \fItok\fR\&.
.sp
In case of failure the error status \fIER\fR is set to the current
location \fICL\fR and the message \fImessage\fR, and the
current location \fICL\fR is moved one token backwards\&.
Otherwise, i\&.e\&. upon success, the error status \fIER\fR is cleared
and the current location \fICL\fR is not touched\&.
.TP
\fBict_match_tokrange\fR \fItokbegin\fR \fItokend\fR \fImessage\fR
This instruction tests the current token \fICT\fR for being in
the range of tokens from \fItokbegin\fR to \fItokend\fR
(inclusive) and records the result in the match status \fIOK\fR\&. The
instruction fails if the current token is not inside the range\&.
.sp
In case of failure the error status \fIER\fR is set to the current
location \fICL\fR and the message \fImessage\fR, and the current location
\fICL\fR is moved one token backwards\&.
Otherwise, i\&.e\&. upon success, the error status \fIER\fR is cleared
and the current location \fICL\fR is not touched\&.
.TP
\fBict_match_tokclass\fR \fIcode\fR \fImessage\fR
This instruction tests the current token \fICT\fR for being a member
of the token class \fIcode\fR and records the result in the match
status \fIOK\fR\&. The instruction fails if the current token is not a
member of the specified class\&.
.sp
In case of failure the error status \fIER\fR is set to the current
location \fICL\fR and the message \fImessage\fR, and the
current location \fICL\fR is moved one token backwards\&.
Otherwise, i\&.e\&. upon success, the error status \fIER\fR is cleared
and the current location \fICL\fR is not touched\&.
.sp
Currently the following classes are legal:
.RS
.TP
alnum
A token is accepted if it is a unicode alphabetical character, or a digit\&.
.TP
alpha
A token is accepted if it is a unicode alphabetical character\&.
.TP
digit
A token is accepted if it is a unicode digit character\&.
.TP
xdigit
A token is accepted if it is a hexadecimal digit character\&.
.TP
punct
A token is accepted if it is a unicode punctuation character\&.
.TP
space
A token is accepted if it is a unicode space character\&.
.RE
.PP
.PP
.SS "NONTERMINAL MATCHING"
The instructions in this section handle the matching of nonterminal
symbols\&. They query the nonterminal cache \fINC\fR for saved
information, and put such information into the cache\&.
.PP
The usage of the cache is a performance aid for backtracking parsers,
allowing them to avoid an expensive rematch of complex nonterminal
symbols if they have been encountered before\&.
.PP
.TP
\fBinc_restore\fR \fIbranchlabel\fR \fInt\fR
This instruction checks if the nonterminal cache \fINC\fR contains
information about the nonterminal symbol \fInt\fR, at the current
location \fICL\fR\&. If that is the case the instruction will update
the machine state (current location \fICL\fR, match status \fIOK\fR,
semantic value \fISV\fR, and error status \fIER\fR) with the found
information and continue execution at the instruction refered to by
the \fIbranchlabel\fR\&. The new current location \fICL\fR will be the
last token matched by the nonterminal symbol, i\&.e\&. belonging to it\&.
.sp
If no information was found the instruction will continue execution at
the next instruction\&.
.sp
Together with \fBicf_ntcall\fR it is possible to generate code for
memoized and non-memoized matching of nonterminal symbols, either as
subroutine calls, or inlined in the caller\&.
.TP
\fBinc_save\fR \fInt\fR
This instruction saves the current state of the machine (current
location \fICL\fR, match status \fIOK\fR, semantic value \fISV\fR,
and error status \fIER\fR), to the nonterminal cache \fINC\fR\&. It
will also pop an entry from the location stack \fILS\fR and save it
as the start location of the match\&.
.sp
It is expected to be called at the end of matching a nonterminal
symbol, with \fInt\fR the name of the nonterminal symbol the code was
working on\&. This allows the instruction \fBinc_restore\fR to check for
and retrieve the data, should we have to match this nonterminal symbol
at the same location again, during backtracking\&.
.TP
\fBicf_ntcall\fR \fIbranchlabel\fR
This instruction invokes the code for matching the nonterminal symbol
\fInt\fR as a subroutine\&. To this end it stores the current program
counter \fIPC\fR on the return stack \fIRS\fR, the current location
\fICL\fR on the location stack \fILS\fR, and then continues
execution at the address \fIbranchlabel\fR\&.
.sp
The next matching \fBicf_ntreturn\fR will cause the execution to
continue at the instruction coming after the call\&.
.TP
\fBicf_ntreturn\fR
This instruction will pop an entry from the return stack \fIRS\fR,
assign it to the program counter \fIPC\fR, and then continue
execution at the new address\&.
.PP
.PP
.SS "UNCONDITIONAL MATCHING"
The instructions in this section are the remaining match
operators\&. They change the match status \fIOK\fR directly and
unconditionally\&.
.TP
\fBiok_ok\fR
This instruction sets the match status \fIOK\fR to \fBtrue\fR,
indicating a successful match\&.
.TP
\fBiok_fail\fR
This instruction sets the match status \fIOK\fR to \fBfalse\fR,
indicating a failed match\&.
.TP
\fBiok_negate\fR
This instruction negates the match status \fIOK\fR, turning a failure
into a success and vice versa\&.
.PP
.PP
.SS "CONTROL FLOW"
The instructions in this section implement both conditional and
unconditional control flow\&. The conditional jumps query the match
status \fIOK\fR\&.
.TP
\fBicf_jalways\fR \fIbranchlabel\fR
This instruction sets the program counter \fIPC\fR to the address
specified by \fIbranchlabel\fR and then continues execution from
there\&. This is an unconditional jump\&.
.TP
\fBicf_jok\fR \fIbranchlabel\fR
This instruction sets the program counter \fIPC\fR to the address
specified by \fIbranchlabel\fR\&. This happens if, and only if the match
status \fIOK\fR indicates a success\&. Otherwise it simply continues
execution at the next instruction\&. This is a conditional jump\&.
.TP
\fBicf_jfail\fR \fIbranchlabel\fR
This instruction sets the program counter \fIPC\fR to the address
specified by \fIbranchlabel\fR\&. This happens if, and only if the match
status \fIOK\fR indicates a failure\&. Otherwise it simply continues
execution at the next instruction\&. This is a conditional jump\&.
.TP
\fBicf_halt\fR
This instruction halts the machine and blocks any further execution\&.
.PP
.SS "INPUT LOCATION HANDLING"
The instructions in this section are for backtracking, they manipulate
the current location \fICL\fR 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 \fICL\fR to saved
locations, making them one of the components enabling the
implementation of backtracking parsers\&.
.TP
\fBicl_push\fR
This instruction pushes a copy of the current location \fICL\fR on
the location stack \fILS\fR\&.
.TP
\fBicl_rewind\fR
This instruction pops an entry from the location stack \fILS\fR and
then moves the current location \fICL\fR back to this point in the
input\&.
.TP
\fBicl_pop\fR
This instruction pops an entry from the location stack \fILS\fR and
discards it\&.
.PP
.PP
.SS "ERROR HANDLING"
The instructions in this section provide read and write access to the
error status \fIER\fR of the machine\&.
.TP
\fBier_push\fR
This instruction pushes a copy of the current error status \fIER\fR
on the error stack \fIES\fR\&.
.TP
\fBier_clear\fR
This instruction clears the error status \fIER\fR\&.
.TP
\fBier_nonterminal\fR \fImessage\fR
This instruction checks if the error status \fIER\fR contains an
error whose location is just past the location found in the top entry
of the location stack \fILS\fR\&.
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 \fImessage\fR\&.
.TP
\fBier_merge\fR
This instruction pops an entry from the error stack \fIES\fR, merges
it with the current error status \fIER\fR and stores the result of
the merge as the new error status \fIER\fR\&.
.sp
The merge is performed as described below:
.sp
If one of the two error states is empty the other is chosen\&. If
neither error state is empty, and refering to different locations,
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)\&.
.PP
.SS "SEMANTIC VALUES"
The instructions in this section manipulate the semantic value
\fISV\fR\&.
.TP
\fBisv_clear\fR
This instruction clears the semantic value \fISV\fR\&.
.TP
\fBisv_terminal\fR
This instruction creates a terminal AST node for the current token
\fICT\fR, makes it the semantic value \fISV\fR, and also pushes the
node on the AST stack \fIAS\fR\&.
.TP
\fBisv_nonterminal_leaf\fR \fInt\fR
This instruction creates a nonterminal AST node without any children
for the nonterminal \fInt\fR, and makes it the semantic value
\fISV\fR\&.
.sp
This instruction should be executed if, and only if the match status
\fIOK\fR indicates a success\&.
In the case of a failure \fBisv_clear\fR should be called\&.
.TP
\fBisv_nonterminal_range\fR \fInt\fR
This instruction creates a nonterminal AST node for the nonterminal
\fInt\fR, with a single terminal node as its child, and makes this AST
the semantic value \fISV\fR\&. The terminal node refers to the input
string from the location found on top of the location stack \fILS\fR
to the current location \fICL\fR (both inclusive)\&.
.sp
This instruction should be executed if, and only if the match status
\fIOK\fR indicates a success\&.
In the case of a failure \fBisv_clear\fR should be called\&.
.TP
\fBisv_nonterminal_reduce\fR \fInt\fR
This instruction creates a nonterminal AST node for the nonterminal
\fInt\fR and makes it the semantic value \fISV\fR\&.
.sp
All entries on the AST stack \fIAS\fR above the marker found in the
top entry of the AST Marker stack \fIMS\fR become children of the new
node, with the entry at the stack top becoming the rightmost child\&. If
the AST Marker stack \fIMS\fR is empty the whole stack is used\&. The
AST marker stack \fIMS\fR is left unchanged\&.
.sp
This instruction should be executed if, and only if the match status
\fIOK\fR indicates a success\&.
In the case of a failure \fBisv_clear\fR should be called\&.
.PP
.PP
.SS "AST STACK HANDLING"
The instructions in this section manipulate the AST stack \fIAS\fR,
and the AST Marker stack \fIMS\fR\&.
.TP
\fBias_push\fR
This instruction pushes the semantic value \fISV\fR on the AST stack
\fIAS\fR\&.
.TP
\fBias_mark\fR
This instruction pushes a marker for the current state of the AST
stack \fIAS\fR on the AST Marker stack \fIMS\fR\&.
.TP
\fBias_mrewind\fR
This instruction pops an entry from the AST Marker stack \fIMS\fR and
then proceeds to pop entries from the AST stack \fIAS\fR until the
state represented by the popped marker has been reached again\&.
Nothing is done if the AST stack \fIAS\fR is already smaller than
indicated by the popped marker\&.
.TP
\fBias_mpop\fR
This instruction pops an entry from the AST Marker stack \fIMS\fR and
discards it\&.
.PP
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIgrammar_me\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH KEYWORDS
grammar, parsing, virtual machine
.SH CATEGORY
Grammars and finite automata
.SH COPYRIGHT
.nf
Copyright (c) 2005 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>

.fi
