(* ML-Yacc Parser Generator (c) 1989 Andrew W. Appel, David R. Tarditi *)

(* parser.sml:  This is a parser driver for LR tables with an error-recovery
   routine added to it.  The routine used is described in detail in this
   article:

        'A Practical Method for LR and LL Syntactic Error Diagnosis and
         Recovery', by M. Burke and G. Fisher, ACM Transactions on
         Programming Langauges and Systems, Vol. 9, No. 2, April 1987,
         pp. 164-197.

    This program is an implementation is the partial, deferred method discussed
    in the article.  The algorithm and data structures used in the program
    are described below.

    This program assumes that all semantic actions are delayed.  A semantic
    action should produce a function from unit -> value instead of producing the
    normal value.  The parser returns the semantic value on the top of the
    stack when accept is encountered.  The user can deconstruct this value
    and apply the unit -> value function in it to get the answer.

    It also assumes that the lexer is a lazy stream.

    Data Structures:
    ----------------

        * The parser:

           The state stack has the type

                 (state * (semantic value * line # * line #)) list

           The parser keeps a queue of (state stack * lexer pair).  A lexer pair
         consists of a terminal * value pair and a lexer.  This allows the
         parser to reconstruct the states for terminals to the left of a
         syntax error, and attempt to make error corrections there.

           The queue consists of a pair of lists (x,y).  New additions to
         the queue are cons'ed onto y.  The first element of x is the top
         of the queue.  If x is nil, then y is reversed and used
         in place of x.

    Algorithm:
    ----------

        * The steady-state parser:

            This parser keeps the length of the queue of state stacks at
        a steady state by always removing an element from the front when
        another element is placed on the end.

            It has these arguments:

           stack: current stack
           queue: value of the queue
           lexPair ((terminal,value),lex stream)

        When SHIFT is encountered, the state to shift to and the value are
        are pushed onto the state stack.  The state stack and lexPair are
        placed on the queue.  The front element of the queue is removed.

        When REDUCTION is encountered, the rule is applied to the current
        stack to yield a triple (nonterm,value,new stack).  A new
        stack is formed by adding (goto(top state of stack,nonterm),value)
        to the stack.

        When ACCEPT is encountered, the top value from the stack and the
        lexer are returned.

        When an ERROR is encountered, fixError is called.  FixError
        takes the arguments to the parser, fixes the error if possible and
        returns a new set of arguments.

        * The distance-parser:

        This parser includes an additional argument distance.  It pushes
        elements on the queue until it has parsed distance tokens, or an
        ACCEPT or ERROR occurs.  It returns a stack, lexer, the number of
        tokens left unparsed, a queue, and an action option.
*)

signature FIFO =
  sig type 'a queue
      val empty : 'a queue
      exception Empty
      val get : 'a queue -> 'a * 'a queue
      val put : 'a * 'a queue -> 'a queue
  end

(* drt (12/15/89) -- the functor should be used in development work, but
   it wastes space in the release version.

functor ParserGen(structure LrTable : LR_TABLE
                  structure Stream : STREAM) : LR_PARSER =
*)

structure LrParser :> LR_PARSER =
   struct
      structure LrTable = LrTable
      structure Stream = Stream

      fun eqT (LrTable.T i, LrTable.T i') = i = i'

      structure Token : TOKEN =
        struct
            structure LrTable = LrTable
            datatype ('a,'b) token = TOKEN of LrTable.term * ('a * 'b * 'b)
            val sameToken = fn (TOKEN(t,_),TOKEN(t',_)) => eqT (t,t')
        end

      open LrTable
      open Token

      val DEBUG1 = false
      val DEBUG2 = false
      exception ParseError
      exception ParseImpossible of int

      structure Fifo :> FIFO =
        struct
          type 'a queue = ('a list * 'a list)
          val empty = (nil,nil)
          exception Empty
          fun get(a::x, y) = (a, (x,y))
            | get(nil, nil) = raise Empty
            | get(nil, y) = get(rev y, nil)
          fun put(a,(x,y)) = (x,a::y)
        end

      type ('a,'b) elem = (state * ('a * 'b * 'b))
      type ('a,'b) stack = ('a,'b) elem list
      type ('a,'b) lexv = ('a,'b) token
      type ('a,'b) lexpair = ('a,'b) lexv * (('a,'b) lexv Stream.stream)
      type ('a,'b) distanceParse =
                 ('a,'b) lexpair *
                 ('a,'b) stack *
                 (('a,'b) stack * ('a,'b) lexpair) Fifo.queue *
                 int ->
                   ('a,'b) lexpair *
                   ('a,'b) stack *
                   (('a,'b) stack * ('a,'b) lexpair) Fifo.queue *
                   int *
                   action option

      type ('a,'b) ecRecord =
         {is_keyword : term -> bool,
          preferred_change : (term list * term list) list,
          error : string * 'b * 'b -> unit,
          errtermvalue : term -> 'a,
          terms : term list,
          showTerminal : term -> string,
          noShift : term -> bool}

      local
         val print = fn s => TextIO.output(TextIO.stdOut,s)
         val println = fn s => (print s; print "\n")
         val showState = fn (STATE s) => "STATE " ^ (Int.toString s)
      in
        fun printStack(stack: ('a,'b) stack, n: int) =
         case stack
           of (state,_) :: rest =>
                 (print("\t" ^ Int.toString n ^ ": ");
                  println(showState state);
                  printStack(rest, n+1))
            | nil => ()

        fun prAction showTerminal
                 (stack as (state,_) :: _, next as (TOKEN (term,_),_), action) =
             (println "Parse: state stack:";
              printStack(stack, 0);
              print("       state="
                         ^ showState state
                         ^ " next="
                         ^ showTerminal term
                         ^ " action="
                        );
              case action
                of SHIFT state => println ("SHIFT " ^ (showState state))
                 | REDUCE i => println ("REDUCE " ^ (Int.toString i))
                 | ERROR => println "ERROR"
                 | ACCEPT => println "ACCEPT")
        | prAction _ (_,_,action) = ()
     end

    (* ssParse: parser which maintains the queue of (state * lexvalues) in a
        steady-state.  It takes a table, showTerminal function, saction
        function, and fixError function.  It parses until an ACCEPT is
        encountered, or an exception is raised.  When an error is encountered,
        fixError is called with the arguments of parseStep (lexv,stack,and
        queue).  It returns the lexv, and a new stack and queue adjusted so
        that the lexv can be parsed *)

    val ssParse =
      fn (table,showTerminal,saction,fixError,arg) =>
        let val prAction = prAction showTerminal
            val action = LrTable.action table
            val goto = LrTable.goto table
            fun parseStep(args as
                         (lexPair as (TOKEN (terminal, value as (_,leftPos,_)),
                                      lexer
                                      ),
                          stack as (state,_) :: _,
                          queue)) =
              let val nextAction = action (state,terminal)
                  val _ = if DEBUG1 then prAction(stack,lexPair,nextAction)
                          else ()
              in case nextAction
                 of SHIFT s =>
                  let val newStack = (s,value) :: stack
                      val newLexPair = Stream.get lexer
                      val (_,newQueue) =Fifo.get(Fifo.put((newStack,newLexPair),
                                                            queue))
                  in parseStep(newLexPair,(s,value)::stack,newQueue)
                  end
                 | REDUCE i =>
                     (case saction(i,leftPos,stack,arg)
                      of (nonterm,value,stack as (state,_) :: _) =>
                          parseStep(lexPair,(goto(state,nonterm),value)::stack,
                                    queue)
                       | _ => raise (ParseImpossible 197))
                 | ERROR => parseStep(fixError args)
                 | ACCEPT =>
                        (case stack
                         of (_,(topvalue,_,_)) :: _ =>
                                let val (token,restLexer) = lexPair
                                in (topvalue,Stream.cons(token,restLexer))
                                end
                          | _ => raise (ParseImpossible 202))
              end
            | parseStep _ = raise (ParseImpossible 204)
        in parseStep
        end

    (*  distanceParse: parse until n tokens are shifted, or accept or
        error are encountered.  Takes a table, showTerminal function, and
        semantic action function.  Returns a parser which takes a lexPair
        (lex result * lexer), a state stack, a queue, and a distance
        (must be > 0) to parse.  The parser returns a new lex-value, a stack
        with the nth token shifted on top, a queue, a distance, and action
        option. *)

    val distanceParse =
      fn (table,showTerminal,saction,arg) =>
        let val prAction = prAction showTerminal
            val action = LrTable.action table
            val goto = LrTable.goto table
            fun parseStep(lexPair,stack,queue,0) = (lexPair,stack,queue,0,NONE)
              | parseStep(lexPair as (TOKEN (terminal, value as (_,leftPos,_)),
                                      lexer
                                     ),
                          stack as (state,_) :: _,
                          queue,distance) =
              let val nextAction = action(state,terminal)
                  val _ = if DEBUG1 then prAction(stack,lexPair,nextAction)
                          else ()
              in case nextAction
                 of SHIFT s =>
                  let val newStack = (s,value) :: stack
                      val newLexPair = Stream.get lexer
                  in parseStep(newLexPair,(s,value)::stack,
                               Fifo.put((newStack,newLexPair),queue),distance-1)
                  end
                 | REDUCE i =>
                    (case saction(i,leftPos,stack,arg)
                      of (nonterm,value,stack as (state,_) :: _) =>
                         parseStep(lexPair,(goto(state,nonterm),value)::stack,
                                 queue,distance)
                      | _ => raise (ParseImpossible 240))
                 | ERROR => (lexPair,stack,queue,distance,SOME nextAction)
                 | ACCEPT => (lexPair,stack,queue,distance,SOME nextAction)
              end
           | parseStep _ = raise (ParseImpossible 242)
        in parseStep : ('_a,'_b) distanceParse
        end

(* mkFixError: function to create fixError function which adjusts parser state
   so that parse may continue in the presence of an error *)

fun mkFixError({is_keyword,terms,errtermvalue,
              preferred_change,noShift,
              showTerminal,error,...} : ('_a,'_b) ecRecord,
             distanceParse : ('_a,'_b) distanceParse,
             minAdvance,maxAdvance)

            (lexv as (TOKEN (term,value as (_,leftPos,_)),_),stack,queue) =
    let val _ = if DEBUG2 then
                        error("syntax error found at " ^ (showTerminal term),
                              leftPos,leftPos)
                else ()

        fun tokAt(t,p) = TOKEN(t,(errtermvalue t,p,p))

        val minDelta = 3

        (* pull all the state * lexv elements from the queue *)

        val stateList =
           let fun f q = let val (elem,newQueue) = Fifo.get q
                         in elem :: (f newQueue)
                         end handle Fifo.Empty => nil
           in f queue
           end

        (* now number elements of stateList, giving distance from
           error token *)

        val (_, numStateList) =
              List.foldr (fn (a,(num,r)) => (num+1,(a,num)::r)) (0, []) stateList

        (* Represent the set of potential changes as a linked list.

           Values of datatype Change hold information about a potential change.

           oper = oper to be applied
           pos = the # of the element in stateList that would be altered.
           distance = the number of tokens beyond the error token which the
             change allows us to parse.
           new = new terminal * value pair at that point
           orig = original terminal * value pair at the point being changed.
         *)

        datatype ('a,'b) change = CHANGE of
           {pos : int, distance : int, leftPos: 'b, rightPos: 'b,
            new : ('a,'b) lexv list, orig : ('a,'b) lexv list}


         val showTerms = concat o map (fn TOKEN(t,_) => " " ^ showTerminal t)

         val printChange = fn c =>
          let val CHANGE {distance,new,orig,pos,...} = c
          in (print ("{distance= " ^ (Int.toString distance));
              print (",orig ="); print(showTerms orig);
              print (",new ="); print(showTerms new);
              print (",pos= " ^ (Int.toString pos));
              print "}\n")
          end

        val printChangeList = app printChange

(* parse: given a lexPair, a stack, and the distance from the error
   token, return the distance past the error token that we are able to parse.*)

        fun parse (lexPair,stack,queuePos : int) =
            case distanceParse(lexPair,stack,Fifo.empty,queuePos+maxAdvance+1)
             of (_,_,_,distance,SOME ACCEPT) =>
                        if maxAdvance-distance-1 >= 0
                            then maxAdvance
                            else maxAdvance-distance-1
              | (_,_,_,distance,_) => maxAdvance - distance - 1

(* catList: concatenate results of scanning list *)

        fun catList l f = List.foldr (fn(a,r)=> f a @ r) [] l

        fun keywordsDelta new = if List.exists (fn(TOKEN(t,_))=>is_keyword t) new
                       then minDelta else 0

        fun tryChange{lex,stack,pos,leftPos,rightPos,orig,new} =
             let val lex' = List.foldr (fn (t',p)=>(t',Stream.cons p)) lex new
                 val distance = parse(lex',stack,pos+length new-length orig)
              in if distance >= minAdvance + keywordsDelta new
                   then [CHANGE{pos=pos,leftPos=leftPos,rightPos=rightPos,
                                distance=distance,orig=orig,new=new}]
                   else []
             end


(* tryDelete: Try to delete n terminals.
              Return single-element [success] or nil.
              Do not delete unshiftable terminals. *)


    fun tryDelete n ((stack,lexPair as (TOKEN(term,(_,l,r)),_)),qPos) =
        let fun del(0,accum,left,right,lexPair) =
                  tryChange{lex=lexPair,stack=stack,
                            pos=qPos,leftPos=left,rightPos=right,
                            orig=rev accum, new=[]}
              | del(n,accum,left,right,(tok as TOKEN(term,(_,_,r)),lexer)) =
                   if noShift term then []
                   else del(n-1,tok::accum,left,r,Stream.get lexer)
         in del(n,[],l,r,lexPair)
        end

(* tryInsert: try to insert tokens before the current terminal;
       return a list of the successes  *)

        fun tryInsert((stack,lexPair as (TOKEN(_,(_,l,_)),_)),queuePos) =
               catList terms (fn t =>
                 tryChange{lex=lexPair,stack=stack,
                           pos=queuePos,orig=[],new=[tokAt(t,l)],
                           leftPos=l,rightPos=l})

(* trySubst: try to substitute tokens for the current terminal;
       return a list of the successes  *)

        fun trySubst ((stack,lexPair as (orig as TOKEN (term,(_,l,r)),lexer)),
                      queuePos) =
              if noShift term then []
              else
                  catList terms (fn t =>
                      tryChange{lex=Stream.get lexer,stack=stack,
                                pos=queuePos,
                                leftPos=l,rightPos=r,orig=[orig],
                                new=[tokAt(t,r)]})

     (* do_delete(toks,lexPair) tries to delete tokens "toks" from "lexPair".
         If it succeeds, returns SOME(toks',l,r,lp), where
             toks' is the actual tokens (with positions and values) deleted,
             (l,r) are the (leftmost,rightmost) position of toks',
             lp is what remains of the stream after deletion
     *)
        fun do_delete(nil,lp as (TOKEN(_,(_,l,_)),_)) = SOME(nil,l,l,lp)
          | do_delete([t],(tok as TOKEN(t',(_,l,r)),lp')) =
               if eqT (t, t')
                   then SOME([tok],l,r,Stream.get lp')
                   else NONE
          | do_delete(t::rest,(tok as TOKEN(t',(_,l,r)),lp')) =
               if eqT (t,t')
                   then case do_delete(rest,Stream.get lp')
                         of SOME(deleted,l',r',lp'') =>
                               SOME(tok::deleted,l,r',lp'')
                          | NONE => NONE
                   else NONE

        fun tryPreferred((stack,lexPair),queuePos) =
            catList preferred_change (fn (delete,insert) =>
               if List.exists noShift delete then [] (* should give warning at
                                                 parser-generation time *)
               else case do_delete(delete,lexPair)
                     of SOME(deleted,l,r,lp) =>
                            tryChange{lex=lp,stack=stack,pos=queuePos,
                                      leftPos=l,rightPos=r,orig=deleted,
                                      new=map (fn t=>(tokAt(t,r))) insert}
                      | NONE => [])

        val changes = catList numStateList tryPreferred @
                        catList numStateList tryInsert @
                          catList numStateList trySubst @
                            catList numStateList (tryDelete 1) @
                              catList numStateList (tryDelete 2) @
                                catList numStateList (tryDelete 3)

        val findMaxDist = fn l =>
          foldr (fn (CHANGE {distance,...},high) => Int.max(distance,high)) 0 l

(* maxDist: max distance past error taken that we could parse *)

        val maxDist = findMaxDist changes

(* remove changes which did not parse maxDist tokens past the error token *)

        val changes = catList changes
              (fn(c as CHANGE{distance,...}) =>
                  if distance=maxDist then [c] else [])

      in case changes
          of (l as change :: _) =>
              let fun print_msg (CHANGE {new,orig,leftPos,rightPos,...}) =
                  let val s =
                      case (orig,new)
                          of (_::_,[]) => "deleting " ^ (showTerms orig)
                           | ([],_::_) => "inserting " ^ (showTerms new)
                           | _ => "replacing " ^ (showTerms orig) ^
                                 " with " ^ (showTerms new)
                  in error ("syntax error: " ^ s,leftPos,rightPos)
                  end

                  val _ =
                      (if length l > 1 andalso DEBUG2 then
                           (print "multiple fixes possible; could fix it by:\n";
                            app print_msg l;
                            print "chosen correction:\n")
                       else ();
                       print_msg change)

                  (* findNth: find nth queue entry from the error
                   entry.  Returns the Nth queue entry and the  portion of
                   the queue from the beginning to the nth-1 entry.  The
                   error entry is at the end of the queue.

                   Examples:

                   queue = a b c d e
                   findNth 0 = (e,a b c d)
                   findNth 1 =  (d,a b c)
                   *)

                  val findNth = fn n =>
                      let fun f (h::t,0) = (h,rev t)
                            | f (h::t,n) = f(t,n-1)
                            | f (nil,_) = let exception FindNth
                                          in raise FindNth
                                          end
                      in f (rev stateList,n)
                      end

                  val CHANGE {pos,orig,new,...} = change
                  val (last,queueFront) = findNth pos
                  val (stack,lexPair) = last

                  val lp1 = foldl(fn (_,(_,r)) => Stream.get r) lexPair orig
                  val lp2 = foldr(fn(t,r)=>(t,Stream.cons r)) lp1 new

                  val restQueue =
                      Fifo.put((stack,lp2),
                               foldl Fifo.put Fifo.empty queueFront)

                  val (lexPair,stack,queue,_,_) =
                      distanceParse(lp2,stack,restQueue,pos)

              in (lexPair,stack,queue)
              end
        | nil => (error("syntax error found at " ^ (showTerminal term),
                        leftPos,leftPos); raise ParseError)
    end

   val parse = fn {arg,table,lexer,saction,void,lookahead,
                   ec=ec as {showTerminal,...} : ('_a,'_b) ecRecord} =>
        let val distance = 15   (* defer distance tokens *)
            val minAdvance = 1  (* must parse at least 1 token past error *)
            val maxAdvance = Int.max(lookahead,0)(* max distance for parse check *)
            val lexPair = Stream.get lexer
            val (TOKEN (_,(_,leftPos,_)),_) = lexPair
            val startStack = [(initialState table,(void,leftPos,leftPos))]
            val startQueue = Fifo.put((startStack,lexPair),Fifo.empty)
            val distanceParse = distanceParse(table,showTerminal,saction,arg)
            val fixError = mkFixError(ec,distanceParse,minAdvance,maxAdvance)
            val ssParse = ssParse(table,showTerminal,saction,fixError,arg)
            fun loop (lexPair,stack,queue,_,SOME ACCEPT) =
                   ssParse(lexPair,stack,queue)
              | loop (lexPair,stack,queue,0,_) = ssParse(lexPair,stack,queue)
              | loop (lexPair,stack,queue,distance,SOME ERROR) =
                 let val (lexPair,stack,queue) = fixError(lexPair,stack,queue)
                 in loop (distanceParse(lexPair,stack,queue,distance))
                 end
              | loop _ = let exception ParseInternal
                         in raise ParseInternal
                         end
        in loop (distanceParse(lexPair,startStack,startQueue,distance))
        end
 end;
