4.3.3 Redefining Procedures during Program Execution

You can redefine procedures during the execution of the program, which can be very useful while debugging. The normal way to do this is to use the ‘break’ option of the debugger to enter a break state (see break/0, ref-ere-int), and then load an altered version of some procedures. If you do this, then it is advisable, after redefining the procedures and exiting from the break state, to wind the computation back to the first call to any of the procedures you are changing: you can do this by using the ‘retry’ option with an argument that is the invocation number of that call. If you do not wind the computation back like this, then:

• if you are in the middle of executing a procedure that you redefine, then you will find that the old definition of the procedure continues to be used until it exits or fails;
• if you should backtrack into a procedure you have just redefined, then alternative clauses in the old definition will still be used.

See mpg-ref-break.