grammar::me_vm -
Virtual machine for parsing token streams
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 encountered input, and the
ability to backtrack in both input and AST creation.
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.
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
considered to be at offset 0.
This information is implicitly used and modified by the instructions
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 instructions
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
UNCONDITIONAL 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 performing
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 subroutine
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.
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.
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 backtracked (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
instruction 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
status OK. The instruction fails if the current token is not a
member 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 character, or a digit.
- alpha
-
A token is accepted if it is a unicode alphabetical character.
- digit
-
A token is accepted if it is a unicode digit character.
- xdigit
-
A token is accepted if it is a hexadecimal digit character.
- punct
-
A token is accepted if it is a unicode punctuation character.
- space
-
A token is accepted if it is a unicode space character.
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
symbols 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 branchlabel. 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 execution 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 status 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
continue 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.
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.
The instructions in this section implement both conditional and
unconditional 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 execution.
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 components 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.
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 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).
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 nonterminal
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 nonterminal
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.
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.
This document, and the package it describes, will undoubtedly contain
bugs and other problems.
Please report such in the category
grammar_me of the
http://sourceforge.net/tracker/?group_id=12883.
Please also report any ideas for enhancements you may have for either
package and/or documentation.
virtual machine, parsing, grammar