Debugging semantic actions ​

Each semantic action is represented by a symbol interned in an obarray that is part of the LALR(1) automaton (see 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:

bash
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.