[comment {-*- tcl -*- doctools manpage}] [manpage_begin grammar::me_vm n 0.1] [copyright {2005 Andreas Kupries }] [moddesc {Grammar operations and usage}] [titledesc {Virtual machine for parsing token streams}] [category {Grammars and finite automata}] [description] [keywords {virtual machine}] [keywords parsing grammar] Please go and read the document [syscmd grammar::me_intro] first for an overview of the various documents and their relations. [para] This document specifies a virtual machine for the controlled matching and parsing of token streams, creating an [term {abstract syntax tree}] (short [term 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. [para] 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. [para] The following sections will discuss first the abstract state kept by ME virtual machines, and then their instruction set. [section {MACHINE STATE}] A ME virtual machine manages the following state: [list_begin definitions] [def "[term {Current token}] CT"] The token from the input under consideration by the machine. [para] This information is used and modified by the instructions defined in the section [sectref {TERMINAL MATCHING}]. [def "[term {Current location}] CL"] The location of the [term {current token}] in the input stream, as offset relative to the beginning of the stream. The first token is considered to be at offset [const 0]. [para] This information is implicitly used and modified by the instructions defined in the sections [sectref {TERMINAL MATCHING}] and [sectref {NONTERMINAL MATCHING}], and can be directly queried and modified by the instructions defined in section [sectref {INPUT LOCATION HANDLING}]. [def "[term {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. [para] This information is implicitly used and modified by the instructions defined in the sections [sectref {TERMINAL MATCHING}] and [sectref {NONTERMINAL MATCHING}], and can be directly queried and modified by the instructions defined in section [sectref {INPUT LOCATION HANDLING}]. [def "[term {Match status}] OK"] A boolean value, the result of the last attempt at matching input. It is set to [const true] if that attempt was successful, and [const false] otherwise. [para] This information is influenced by the instructions defined in the sections [sectref {TERMINAL MATCHING}], [sectref {NONTERMINAL MATCHING}], and [sectref {UNCONDITIONAL MATCHING}]. It is queried by the instructions defined in the section [sectref {CONTROL FLOW}]. [def "[term {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. [para] This information is influenced by the instructions defined in the sections [sectref {SEMANTIC VALUES}], and [sectref {AST STACK HANDLING}]. [def "[term {AST stack}] AS"] A stack of partial abstract syntax trees constructed by the machine during matching. [para] This information is influenced by the instructions defined in the sections [sectref {SEMANTIC VALUES}], and [sectref {AST STACK HANDLING}]. [def "[term {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. [para] This information is influenced by the instructions defined in the sections [sectref {SEMANTIC VALUES}], and [sectref {AST STACK HANDLING}]. [def "[term {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. [para] [emph 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. [para] This information is queried and influenced by the instructions defined in the sections [sectref {TERMINAL MATCHING}], [sectref {NONTERMINAL MATCHING}], and [sectref {ERROR HANDLING}]. [def "[term {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. [para] This information is queried and influenced by the instructions defined in the sections [sectref {TERMINAL MATCHING}], [sectref {NONTERMINAL MATCHING}], and [sectref {ERROR HANDLING}]. [def "[term {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. [para] This information is queried and influenced by the instructions defined in the section [sectref {NONTERMINAL MATCHING}]. [def "[term {Nonterminal cache}] NC"] A cache of machine states (A 4-tuple containing a location in the input, match status [term OK], semantic value [term SV], and error status [term ER]) keyed by name of nonterminal symbol and location in the input stream. [para] 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. [para] This status is queried and influenced by the instructions defined in the section [sectref {NONTERMINAL MATCHING}]. [list_end] [section {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. [subsection {TERMINAL MATCHING}] First the instructions to match tokens from the input stream, and by extension all terminal symbols. [para] These instructions are the only ones which may retrieve a new token from the input stream. This is a [emph may] and not a [emph will] because the instructions will a retrieve new token if, and only if the current location [term CL] is at the head of the stream. If the machine has backtracked (see [cmd icl_rewind]) the instructions will retrieve the token to compare against from the internal cache. [para] [list_begin definitions] [def "[cmd ict_advance] [arg message]"] This instruction tries to advance to the next token in the input stream, i.e. the one after the current location [term 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. [para] The sucess/failure of the instruction is remembered in the match status [term OK]. In the case of failure the error status [term ER] is set to the current location and the message [arg message]. In the case of success the error status [term ER] is cleared, the new token is made the current token [term CT], and the new location is made the current location [term CL]. [para] The argument [arg message] is a reference to the string to put into the error status [term ER], if such is needed. [def "[cmd ict_match_token] [arg tok] [arg message]"] This instruction tests the current token [term CT] for equality with the argument [arg tok] and records the result in the match status [term OK]. The instruction fails if the current token is not equal to [arg tok]. [para] In case of failure the error status [term ER] is set to the current location [term CL] and the message [arg message], and the current location [term CL] is moved one token backwards. Otherwise, i.e. upon success, the error status [term ER] is cleared and the current location [term CL] is not touched. [def "[cmd ict_match_tokrange] [arg tokbegin] [arg tokend] [arg message]"] This instruction tests the current token [term CT] for being in the range of tokens from [arg tokbegin] to [arg tokend] (inclusive) and records the result in the match status [term OK]. The instruction fails if the current token is not inside the range. [para] In case of failure the error status [term ER] is set to the current location [term CL] and the message [arg message], and the current location [term CL] is moved one token backwards. Otherwise, i.e. upon success, the error status [term ER] is cleared and the current location [term CL] is not touched. [def "[cmd ict_match_tokclass] [arg code] [arg message]"] This instruction tests the current token [term CT] for being a member of the token class [arg code] and records the result in the match status [term OK]. The instruction fails if the current token is not a member of the specified class. [para] In case of failure the error status [term ER] is set to the current location [term CL] and the message [arg message], and the current location [term CL] is moved one token backwards. Otherwise, i.e. upon success, the error status [term ER] is cleared and the current location [term CL] is not touched. [para] Currently the following classes are legal: [list_begin definitions] [def alnum] A token is accepted if it is a unicode alphabetical character, or a digit. [def alpha] A token is accepted if it is a unicode alphabetical character. [def digit] A token is accepted if it is a unicode digit character. [def xdigit] A token is accepted if it is a hexadecimal digit character. [def punct] A token is accepted if it is a unicode punctuation character. [def space] A token is accepted if it is a unicode space character. [list_end] [list_end] [para] [subsection {NONTERMINAL MATCHING}] The instructions in this section handle the matching of nonterminal symbols. They query the nonterminal cache [term NC] for saved information, and put such information into the cache. [para] 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. [para] [list_begin definitions] [def "[cmd inc_restore] [arg branchlabel] [arg nt]"] This instruction checks if the nonterminal cache [term NC] contains information about the nonterminal symbol [arg nt], at the current location [term CL]. If that is the case the instruction will update the machine state (current location [term CL], match status [term OK], semantic value [term SV], and error status [term ER]) with the found information and continue execution at the instruction refered to by the [arg branchlabel]. The new current location [term CL] will be the last token matched by the nonterminal symbol, i.e. belonging to it. [para] If no information was found the instruction will continue execution at the next instruction. [para] Together with [cmd 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. [def "[cmd inc_save] [arg nt]"] This instruction saves the current state of the machine (current location [term CL], match status [term OK], semantic value [term SV], and error status [term ER]), to the nonterminal cache [term NC]. It will also pop an entry from the location stack [term LS] and save it as the start location of the match. [para] It is expected to be called at the end of matching a nonterminal symbol, with [arg nt] the name of the nonterminal symbol the code was working on. This allows the instruction [cmd inc_restore] to check for and retrieve the data, should we have to match this nonterminal symbol at the same location again, during backtracking. [def "[cmd icf_ntcall] [arg branchlabel]"] This instruction invokes the code for matching the nonterminal symbol [arg nt] as a subroutine. To this end it stores the current program counter [term PC] on the return stack [term RS], the current location [term CL] on the location stack [term LS], and then continues execution at the address [arg branchlabel]. [para] The next matching [cmd icf_ntreturn] will cause the execution to continue at the instruction coming after the call. [def [cmd icf_ntreturn]] This instruction will pop an entry from the return stack [term RS], assign it to the program counter [term PC], and then continue execution at the new address. [list_end] [para] [subsection {UNCONDITIONAL MATCHING}] The instructions in this section are the remaining match operators. They change the match status [term OK] directly and unconditionally. [list_begin definitions] [def [cmd iok_ok]] This instruction sets the match status [term OK] to [const true], indicating a successful match. [def [cmd iok_fail]] This instruction sets the match status [term OK] to [const false], indicating a failed match. [def [cmd iok_negate]] This instruction negates the match status [term OK], turning a failure into a success and vice versa. [list_end] [para] [subsection {CONTROL FLOW}] The instructions in this section implement both conditional and unconditional control flow. The conditional jumps query the match status [term OK]. [list_begin definitions] [def "[cmd icf_jalways] [arg branchlabel]"] This instruction sets the program counter [term PC] to the address specified by [arg branchlabel] and then continues execution from there. This is an unconditional jump. [def "[cmd icf_jok] [arg branchlabel]"] This instruction sets the program counter [term PC] to the address specified by [arg branchlabel]. This happens if, and only if the match status [term OK] indicates a success. Otherwise it simply continues execution at the next instruction. This is a conditional jump. [def "[cmd icf_jfail] [arg branchlabel]"] This instruction sets the program counter [term PC] to the address specified by [arg branchlabel]. This happens if, and only if the match status [term OK] indicates a failure. Otherwise it simply continues execution at the next instruction. This is a conditional jump. [def [cmd icf_halt]] This instruction halts the machine and blocks any further execution. [list_end] [subsection {INPUT LOCATION HANDLING}] The instructions in this section are for backtracking, they manipulate the current location [term 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 [term CL] to saved locations, making them one of the components enabling the implementation of backtracking parsers. [list_begin definitions] [def [cmd icl_push]] This instruction pushes a copy of the current location [term CL] on the location stack [term LS]. [def [cmd icl_rewind]] This instruction pops an entry from the location stack [term LS] and then moves the current location [term CL] back to this point in the input. [def [cmd icl_pop]] This instruction pops an entry from the location stack [term LS] and discards it. [list_end] [para] [subsection {ERROR HANDLING}] The instructions in this section provide read and write access to the error status [term ER] of the machine. [list_begin definitions] [def [cmd ier_push]] This instruction pushes a copy of the current error status [term ER] on the error stack [term ES]. [def [cmd ier_clear]] This instruction clears the error status [term ER]. [def "[cmd ier_nonterminal] [arg message]"] This instruction checks if the error status [term ER] contains an error whose location is just past the location found in the top entry of the location stack [term 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 [arg message]. [def [cmd ier_merge]] This instruction pops an entry from the error stack [term ES], merges it with the current error status [term ER] and stores the result of the merge as the new error status [term ER]. [para] The merge is performed as described below: [para] 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). [list_end] [subsection {SEMANTIC VALUES}] The instructions in this section manipulate the semantic value [term SV]. [list_begin definitions] [def [cmd isv_clear]] This instruction clears the semantic value [term SV]. [def [cmd isv_terminal]] This instruction creates a terminal AST node for the current token [term CT], makes it the semantic value [term SV], and also pushes the node on the AST stack [term AS]. [def "[cmd isv_nonterminal_leaf] [arg nt]"] This instruction creates a nonterminal AST node without any children for the nonterminal [arg nt], and makes it the semantic value [term SV]. [para] This instruction should be executed if, and only if the match status [term OK] indicates a success. In the case of a failure [cmd isv_clear] should be called. [def "[cmd isv_nonterminal_range] [arg nt]"] This instruction creates a nonterminal AST node for the nonterminal [arg nt], with a single terminal node as its child, and makes this AST the semantic value [term SV]. The terminal node refers to the input string from the location found on top of the location stack [term LS] to the current location [term CL] (both inclusive). [para] This instruction should be executed if, and only if the match status [term OK] indicates a success. In the case of a failure [cmd isv_clear] should be called. [def "[cmd isv_nonterminal_reduce] [arg nt]"] This instruction creates a nonterminal AST node for the nonterminal [arg nt] and makes it the semantic value [term SV]. [para] All entries on the AST stack [term AS] above the marker found in the top entry of the AST Marker stack [term MS] become children of the new node, with the entry at the stack top becoming the rightmost child. If the AST Marker stack [term MS] is empty the whole stack is used. The AST marker stack [term MS] is left unchanged. [para] This instruction should be executed if, and only if the match status [term OK] indicates a success. In the case of a failure [cmd isv_clear] should be called. [list_end] [para] [subsection {AST STACK HANDLING}] The instructions in this section manipulate the AST stack [term AS], and the AST Marker stack [term MS]. [list_begin definitions] [def [cmd ias_push]] This instruction pushes the semantic value [term SV] on the AST stack [term AS]. [def [cmd ias_mark]] This instruction pushes a marker for the current state of the AST stack [term AS] on the AST Marker stack [term MS]. [def [cmd ias_mrewind]] This instruction pops an entry from the AST Marker stack [term MS] and then proceeds to pop entries from the AST stack [term AS] until the state represented by the popped marker has been reached again. Nothing is done if the AST stack [term AS] is already smaller than indicated by the popped marker. [def [cmd ias_mpop]] This instruction pops an entry from the AST Marker stack [term MS] and discards it. [list_end] [section {BUGS, IDEAS, FEEDBACK}] This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category [emph grammar_me] of the [uri {http://sourceforge.net/tracker/?group_id=12883} {Tcllib SF Trackers}]. Please also report any ideas for enhancements you may have for either package and/or documentation. [manpage_end]