CLHS: Macro CASE, CCASE, ECASE

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Macro CASE, CCASE, ECASE

Syntax:

case keyform {normal-clause}* [otherwise-clause] => result*

ccase keyplace {normal-clause}* => result*

ecase keyform {normal-clause}* => result*

    normal-clause::= (keys form*)

    otherwise-clause::= ({otherwise | t} form*)

    clause::= normal-clause | otherwise-clause

Arguments and Values:

keyform---a form; evaluated to produce a test-key.

keyplace---a form; evaluated initially to produce a test-key. Possibly also used later as a place if no keys match.

test-key---an object produced by evaluating keyform or keyplace.

keys---a designator for a list of objects. In the case of case, the symbols t and otherwise may not be used as the keys designator. To refer to these symbols by themselves as keys, the designators `(t)` and `(otherwise)`, respectively, must be used instead.

forms---an implicit progn.

results---the values returned by the forms in the matching clause.

Description:

These macros allow the conditional execution of a body of forms in a clause that is selected by matching the test-key on the basis of its identity.

The keyform or keyplace is evaluated to produce the test-key.

Each of the normal-clauses is then considered in turn. If the test-key is the same as any key for that clause, the forms in that clause are evaluated as an implicit progn, and the values it returns are returned as the value of the case, ccase, or ecase form.

These macros differ only in their behavior when no normal-clause matches; specifically:

case

  If no normal-clause matches, and there is an otherwise-clause, then that otherwise-clause automatically matches; the forms in that clause are evaluated as an implicit progn, and the values it returns are returned as the value of the case.

  If there is no otherwise-clause, case returns nil.

ccase

  If no normal-clause matches, a correctable error of type type-error is signaled. The offending datum is the test-key and the expected type is type equivalent to `(member` key1`` key2`...)`. The store-value restart can be used to correct the error.

  If the store-value restart is invoked, its argument becomes the new test-key, and is stored in keyplace as if by `(setf` keyplace`` test-key`)`. Then ccase starts over, considering each clause anew.

  The subforms of keyplace might be evaluated again if none of the cases holds.

ecase

  If no normal-clause matches, a non-correctable error of type type-error is signaled. The offending datum is the test-key and the expected type is type equivalent to `(member` key1`` key2`...)`.

  Note that in contrast with ccase, the caller of ecase may rely on the fact that ecase does not return if a normal-clause does not match.

Examples:

     (dolist (k '(1 2 3 :four #\v () t 'other))
        (format t "~S "
           (case k ((1 2) 'clause1)
                   (3 'clause2)
                   (nil 'no-keys-so-never-seen)
                   ((nil) 'nilslot)
                   ((:four #\v) 'clause4)
                   ((t) 'tslot)
                   (otherwise 'others))))
    >>  CLAUSE1 CLAUSE1 CLAUSE2 CLAUSE4 CLAUSE4 NILSLOT TSLOT OTHERS
    =>  NIL
     (defun add-em (x) (apply #'+ (mapcar #'decode x)))
    =>  ADD-EM
     (defun decode (x)
       (ccase x
         ((i uno) 1)
         ((ii dos) 2)
         ((iii tres) 3)
         ((iv cuatro) 4)))
    =>  DECODE
     (add-em '(uno iii)) =>  4
     (add-em '(uno iiii))
    >>  Error: The value of X, IIII, is not I, UNO, II, DOS, III,
    >>         TRES, IV, or CUATRO.
    >>   1: Supply a value to use instead.
    >>   2: Return to Lisp Toplevel.
    >>  Debug> :CONTINUE 1
    >>  Value to evaluate and use for X: 'IV
    =>  5

Side Effects:

The debugger might be entered. If the store-value restart is invoked, the value of keyplace might be changed.

Affected By:

ccase and ecase, since they might signal an error, are potentially affected by existing handlers and *debug-io*.

Exceptional Situations:

ccase and ecase signal an error of type type-error if no normal-clause matches.

See Also:

cond, typecase, setf, Section 5.1 (Generalized Reference)

Notes:

    (case test-key
      {((key*) form*)}*)
    ==
    (let ((#1=#:g0001 test-key))
      (cond {((member #1# '(key*)) form*)}*))

The specific error message used by ecase and ccase can vary between implementations. In situations where control of the specific wording of the error message is important, it is better to use case with an otherwise-clause that explicitly signals an error with an appropriate message.

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