Condition Designators (original) (raw)

ANSI Common Lisp 9 Conditions 9.1 Condition System Concepts 9.1.2 Creating 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 '())