


grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


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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.





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       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



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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


              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



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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


              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.




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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


              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



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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


              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.



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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


   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).





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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


   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.



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grammar::me_vm(n)  Grammar operations and usage grammar::me_vm(n)


       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>


























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