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wisent: Wisent Parsing

 
 3 Wisent Parsing
 ****************
 
 The Wisent’s parser is what is called a “bottom-up” or “shift-reduce”
 parser which repeatedly:
 
 “shift”
      That is pushes the value of the last lexical token read (the
      look-ahead token) into a value stack, and reads a new one.
 
 “reduce”
      That is replaces a nonterminal by its semantic value.  The values
      of the components which form the right hand side of a rule are
      popped from the value stack and reduced by the semantic action of
      this rule.  The result is pushed back on top of value stack.
 
    The parser will stop on:
 
 “accept”
      When all input has been successfully parsed.  The semantic value of
      the start nonterminal is on top of the value stack.
 
 “error”
      When a syntax error (an unexpected token in input) has been
      detected.  At this point the parser issues an error message and
      either stops or calls a recovery routine to try to resume parsing.
 
    The above elementary actions are driven by the LALR(1) automaton
 built by ‘wisent-compile-grammar’ from a context-free grammar.
 
    The Wisent’s parser is entered by calling the function:
 
  -- Function: wisent-parse automaton lexer &optional error start
      Parse input using the automaton specified in AUTOMATON.
 
      AUTOMATON
           Is an LALR(1) automaton generated by ‘wisent-compile-grammar’
           (Wisent Grammar).
 
      LEXER
           Is a function with no argument called by the parser to obtain
           the next terminal (token) in input (Writing a lexer).
 
      ERROR
           Is an optional reporting function called when a parse error
           occurs.  It receives a message string to report.  It defaults
           to the function ‘wisent-message’ (Report errors).
 
      START
           Specify the start symbol (nonterminal) used by the parser as
           its goal.  It defaults to the start symbol defined in the
           grammar (Wisent Grammar).
 
    The following two normal hooks permit doing some useful processing
 respectively before starting parsing, and after the parser terminated.
 
  -- Variable: wisent-pre-parse-hook
      Normal hook run just before entering the LR parser engine.
 
  -- Variable: wisent-post-parse-hook
      Normal hook run just after the LR parser engine terminated.
 

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· Writing a lexer
· Actions goodies
· Report errors
· Error recovery
· Debugging actions
 

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