9.1.2 Creating Conditions

The function make-condition can be used to construct a condition object explicitly. Functions such as error, cerror, signal, and warn operate on conditions and might create condition objects implicitly. Macros such as ccase, ctypecase, ecase, etypecase, check-type, and assert might also implicitly create (and signal) conditions.

9.1.2.1 Condition Designators

A number of the functions in the condition system take arguments which are identified as condition designators. By convention, those arguments are notated as

 datum &rest arguments

Taken together, the datum and the arguments are “designators for a condition of default type default-type.” How the denoted condition is computed depends on the type of the datum:

• If the datum is a symbol naming a condition type ...

  The denoted condition is the result of

        (apply #'make-condition datum arguments)
  

• If the datum is a format control ...

  The denoted condition is the result of

        (make-condition defaulted-type
                        :format-control datum
                        :format-arguments arguments)
  

  where the defaulted-type is a subtype of default-type.

• If the datum is a condition ...

  The denoted condition is the datum itself. In this case, unless otherwise specified by the description of the operator in question, the arguments must be null; that is, the consequences are undefined if any arguments were supplied.

Note that the default-type gets used only in the case where the datum string is supplied. In the other situations, the resulting condition is not necessarily of type default-type.

Here are some illustrations of how different condition designators can denote equivalent condition objects:

(let ((c (make-condition 'arithmetic-error :operator '/ :operands '(7 0))))
  (error c))
≡ (error 'arithmetic-error :operator '/ :operands '(7 0))

(error "Bad luck.")
≡ (error 'simple-error :format-control "Bad luck." :format-arguments '())