LISP: Errors In Lisp

1. Errors in Lisp

2. overview General error-signaling functions Specialized error signaling Forms and Macros Special Forms for exhaustive Case Analysis

3. List functions may signal an error when given incorrect arguments. Each implementation of common lisp will provide an interactive debugger that prints the error message along with suitable contextual information such as which function detected the error. Conditions subsume and generalize the notation of errors. The conditions system also provides means for handling conditions and for restarting a computation after a condition has been signaled.

4. General error-signaling functions These functions provide mechanisms for warnings, breaks, continuable errors, and fatal errors. The caller specifies an error message( a string) that may be processed by the error handling mechanism. Error message should not contain a new-line character either at the beginning or at the end.

6.  cerror continue-format-string error-format-string &rest args Cerror is used to signal continuable errors, it allows the program to be continued from debugger after resolving the error. break &optional format-string &rest args Break prints the message and goes directly into the debugger, without allowing any possibility of interception by programmed error-handling facilities.

7. Specialized error signaling Forms and Macros These facilitate the user to insert error checks into the code. Check-type place typespec [string] Check-type signals an error if the contents of place are not of the desired type. The user will be asked for a new value, the check-type will store the new value in place and start over. The place must be a generalized variable reference acceptable to setf. Typespec must be a type specifier. (it is not evaluated.) String must be an English description of the type, starting with an indefinite article ( “a” or “an”). (it is evaluated.)

8. assert test-form [({place}* ) [string {arg}*]] assert signals an error if the value of the test-form is nil. Continuing from this error allows the user to alter the values of some variables , and assert will start over, evaluating the test form again. assert returns nil.

10. To supply an application-specific error, the user must use typecase with an otherwise clause containing a call to error.  Ctypecase keyplace { (type {form}* )}* The keyplace must be a generalized variable reference acceptable to setf. If n o clause is satisfied, ctypecase signals an error with a message constructed from the clauses.

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