CLHS: Macro TYPECASE, CTYPECASE, ETYPECASE

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Macro TYPECASE, CTYPECASE, ETYPECASE

Syntax:

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

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

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

    normal-clause::= (type 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 types match.

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

type---a type specifier.

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 type.

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 of the type given by the clauses's type, the forms in that clause are evaluated as an implicit progn, and the values it returns are returned as the value of the typecase, ctypecase, or etypecase form.

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

typecase

  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 typecase.

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

ctypecase

  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 `(or` type1`` type2`...)`. 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 ctypecase starts over, considering each clause anew.

  If the store-value restart is invoked interactively, the user is prompted for a new test-key to use.

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

etypecase

  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 `(or` type1`` type2`...)`.

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

In all three cases, is permissible for more than one clause to specify a matching type, particularly if one is a subtype of another; the earliest applicable clause is chosen.

Examples:

    ;;; (Note that the parts of this example which use TYPE-OF
    ;;;  are implementation-dependent.)
     (defun what-is-it (x)
       (format t "~&~S is ~A.~%"
               x (typecase x
                   (float "a float")
                   (null "a symbol, boolean false, or the empty list")
                   (list "a list")
                   (t (format nil "a(n) ~(~A~)" (type-of x))))))
    =>  WHAT-IS-IT
     (map 'nil #'what-is-it '(nil (a b) 7.0 7 box))
    >>  NIL is a symbol, boolean false, or the empty list.
    >>  (A B) is a list.
    >>  7.0 is a float.
    >>  7 is a(n) integer.
    >>  BOX is a(n) symbol.
    =>  NIL
     (setq x 1/3)
    =>  1/3
     (ctypecase x
         (integer (* x 4))
         (symbol  (symbol-value x)))
    >>  Error: The value of X, 1/3, is neither an integer nor a symbol.
    >>  To continue, type :CONTINUE followed by an option number:
    >>   1: Specify a value to use instead.
    >>   2: Return to Lisp Toplevel.
    >>  Debug> :CONTINUE 1
    >>  Use value: 3.7
    >>  Error: The value of X, 3.7, is neither an integer nor a symbol.
    >>  To continue, type :CONTINUE followed by an option number:
    >>   1: Specify a value to use instead.
    >>   2: Return to Lisp Toplevel.
    >>  Debug> :CONTINUE 1
    >>  Use value: 12
    =>  48
     x =>  12

Affected By:

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

Exceptional Situations:

ctypecase and etypecase signal an error of type type-error if no normal-clause matches.

The compiler may choose to issue a warning of type style-warning if a clause will never be selected because it is completely shadowed by earlier clauses.

See Also:

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

Notes:

    (typecase test-key
      {(type form*)}*)
    ==
    (let ((#1=#:g0001 test-key))
      (cond {((typep #1# 'type) form*)}*))

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

Copyright 1996-2005, LispWorks Ltd. All rights reserved.