24.3. Special Forms for Exhaustive Case Analysis

The syntax for etypecase and ctypecase is the same as for typecase, except that no otherwise clause is permitted. Similarly, the syntax for ecase and ccase is the same as for case except for the otherwise clause.

etypecase and ecase are similar to typecase and case, respectively, but signal a non-continuable error rather than returning nil if no clause is selected.

ctypecase and ccase are also similar to typecase and case, but signal a continuable error if no clause is selected.

[Macro]
etypecasekeyform{(type{form}*)}*

old_change_begin
This control construct is similar to typecase, but no explicit otherwise or t clause is permitted. If no clause is satisfied, etypecase signals an error with a message constructed from the clauses. It is not permissible to continue from this error. To supply an application-specific error message, the user should use typecase with an otherwise clause containing a call to error. The name of this function stands for ``exhaustive type case’’ or ``error-checking type case.’’ For example:

(setq x 1/3) 
(etypecase x 
  (integer x) 
  (symbol (symbol-value x))) 
Error: The value of X, 1/3, is neither 
       an integer nor a symbol. 
>

change_begin
X3J13 voted in June 1988 (CONDITION-SYSTEM) to adopt a proposal for a Common Lisp Condition System. This proposal modifies the definition of etypecase to specify its interaction with the condition system. See section 29.4.3.
change_end

[Macro]
ctypecasekeyplace{(type{form}*)}*

old_change_begin
This control construct is similar to typecase, but no explicit otherwise or t clause is permitted. The keyplace must be a generalized variable reference acceptable to setf. If no clause is satisfied, ctypecase signals an error with a message constructed from the clauses. Continuing from this error causes ctypecase to accept a new value from the user, store it into keyplace, and start over, making the type tests again. Subforms of keyplace may be evaluated multiple times. The name of this function stands for ``continuable exhaustive type case.’’
old_change_end

change_begin
X3J13 voted in June 1988 (CONDITION-SYSTEM) to adopt a proposal for a Common Lisp Condition System. This proposal modifies the definition of ctypecase to specify its interaction with the condition system. See section 29.4.3.

X3J13 voted in March 1988 (PUSH-EVALUATION-ORDER) to clarify order of evaluation (see section 7.2).
change_end

[Macro]
ecasekeyform{({({key}*) |key} {form}*)}*

old_change_begin
This control construct is similar to case, but no explicit otherwise or t clause is permitted. If no clause is satisfied, ecase signals an error with a message constructed from the clauses. It is not permissible to continue from this error. To supply an error message, the user should use case with an otherwise clause containing a call to error. The name of this function stands for ``exhaustive case’’ or ``error-checking case.’’ For example:

(setq x 1/3)     
(ecase x 
  (alpha (foo)) 
  (omega (bar)) 
  ((zeta phi) (baz))) 
Error: The value of X, 1/3, is not 
       ALPHA, OMEGA, ZETA, or PHI.

change_begin
X3J13 voted in June 1988 (CONDITION-SYSTEM) to adopt a proposal for a Common Lisp Condition System. This proposal modifies the definition of ecase to specify its interaction with the condition system. See section 29.4.3.
change_end

[Macro]
ccasekeyplace{({({key}*) |key} {form}*)}*

old_change_begin
This control construct is similar to case, but no explicit otherwise or t clause is permitted. The keyplace must be a generalized variable reference acceptable to setf. If no clause is satisfied, ccase signals an error with a message constructed from the clauses. Continuing from this error causes ccase to accept a new value from the user, store it into keyplace, and start over, making the clause tests again. Subforms of keyplace may be evaluated multiple times. The name of this function stands for ``continuable exhaustive case.’’
old_change_end

change_begin
X3J13 voted in June 1988 (CONDITION-SYSTEM) to adopt a proposal for a Common Lisp Condition System. This proposal modifies the definition of ccase to specify its interaction with the condition system. See section 29.4.3.

X3J13 voted in March 1988 (PUSH-EVALUATION-ORDER) to clarify order of evaluation (see section 7.2).
change_end

Rationale: The special forms etypecase, ctypecase, ecase, and ccase are included in Common Lisp, even though a user could write them himself using the other standard facilities provided, because it is likely that many users will want these. Common Lisp therefore provides a standard consistent set rather than allowing a variety of incompatible dialects to develop.

In addition, experience has shown that some Lisp programmers are too lazy to put an appropriate otherwise clause into every case statement to check for cases they didn’t anticipate, even if they would agree that it will probably hurt them later. If an otherwise clause can be included very easily by adding one character to the name of the construct, it is perhaps more likely that programmers will take the trouble to do it.

The e versions do nothing more than supply automatically generated otherwise clauses, but correct implementation of the c versions requires some care. It is therefore especially important that the c versions be provided by the system so users don’t have to puzzle them out on their own. Individual implementations may be able to do a better job of supporting these special forms, using their own idiosyncratic facilities, than can be done using the error-signaling facilities defined by Common Lisp.

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