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 '())

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