When XLISP encounters an error while evaluating an expression, it attempts to handle the error in the following way:
If the symbol *breakenable* is true, the message corresponding to the error is printed. If the error is correctable, the correction message is printed.
If the symbol *tracenable* is true, a trace back is printed. The number of entries printed depends on the value of the symbol *tracelimit*. If this symbol is set to something other than a number, the entire trace back stack is printed.
XLISP then enters a 'read-eval-print' loop to allow the user to examine the state of the interpreter in the context of the error. This loop differs from the normal top-level 'read-eval-print' loop in that if the user invokes the continue function:
1> (continue)
XLISP will continue from a correctable error. If the user invokes the clean-up function:
1> (clean-up)
XLISP will abort the break loop and return to the top level or the next lower numbered break loop. When in a break loop, XLISP prefixes the break level to the normal prompt with a number.
If the symbol *breakenable* is NIL, XLISP looks for a surrounding errset function. If one is found, XLISP examines the value of the print flag. If this flag is true, the error message is printed. In any case, XLISP causes the errset function call to return NIL.
If there is no surrounding errset function, XLISP prints the error message and returns to the top level.