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wisent: Debugging actions

 
 3.5 Debugging semantic actions
 ==============================
 
 Each semantic action is represented by a symbol interned in an “obarray”
 that is part of the LALR(1) automaton (Compiling a grammar).
 ‘symbol-function’ on a semantic action symbol return the semantic action
 lambda expression.
 
    A semantic action symbol name has the form ‘NONTERMINAL:INDEX’, where
 NONTERMINAL is the name of the nonterminal symbol the action belongs to,
 and INDEX is an action sequence number within the scope of NONTERMINAL.
 For example, this nonterminal definition:
 
      input:
         line                     [input:0]
       | input line
         (format "%s %s" $1 $2)   [input:1]
       ;
 
    Will produce two semantic actions, and associated symbols:
 
 ‘input:0’
      A default action that returns ‘$1’.
 
 ‘input:1’
      That returns ‘(format "%s %s" $1 $2)’.
 
    Debugging uses the Lisp debugger to investigate what is happening
 during execution of semantic actions.  Three commands are available to
 debug semantic actions.  They receive two arguments:
 
    • The automaton that contains the semantic action.
 
    • The semantic action symbol.
 
  -- Command: wisent-debug-on-entry automaton function
      Request AUTOMATON’s FUNCTION to invoke debugger each time it is
      called.  FUNCTION must be a semantic action symbol that exists in
      AUTOMATON.
 
  -- Command: wisent-cancel-debug-on-entry automaton function
      Undo effect of ‘wisent-debug-on-entry’ on AUTOMATON’s FUNCTION.
      FUNCTION must be a semantic action symbol that exists in AUTOMATON.
 
  -- Command: wisent-debug-show-entry automaton function
      Show the source of AUTOMATON’s semantic action FUNCTION.  FUNCTION
      must be a semantic action symbol that exists in AUTOMATON.
 

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