— Special Form: when test forms...

This is a variant of if where there are no “else” forms, and possibly several “then” forms. In particular,

          (when test a b c)

is entirely equivalent to

          (if test (progn a b c) nil)

— Special Form: unless test forms...

This is a variant of if where there are no “then” forms, and possibly several “else” forms:

          (unless test a b c)

is entirely equivalent to

          (when (not test) a b c)

— Special Form: case keyform clause...

This macro evaluates keyform, then compares it with the key values listed in the various clauses. Whichever clause matches the key is executed; comparison is done by eql. If no clause matches, the case form returns nil. The clauses are of the form

          (keylist body-forms...)

where keylist is a list of key values. If there is exactly one value, and it is not a cons cell or the symbol nil or t, then it can be used by itself as a keylist without being enclosed in a list. All key values in the case form must be distinct. The final clauses may use t in place of a keylist to indicate a default clause that should be taken if none of the other clauses match. (The symbol otherwise is also recognized in place of t. To make a clause that matches the actual symbol t, nil, or otherwise, enclose the symbol in a list.)

For example, this expression reads a keystroke, then does one of four things depending on whether it is an ‘a’, a ‘b’, a <RET> or <LFD>, or anything else.

          (case (read-char)
            (?a (do-a-thing))
            (?b (do-b-thing))
            ((?\r ?\n) (do-ret-thing))
            (t (do-other-thing)))

— Special Form: ecase keyform clause...

This macro is just like case, except that if the key does not match any of the clauses, an error is signalled rather than simply returning nil.

— Special Form: typecase keyform clause...

This macro is a version of case that checks for types rather than values. Each clause is of the form ‘(type body...)’. See Type Predicates, for a description of type specifiers. For example,

          (typecase x
            (integer (munch-integer x))
            (float (munch-float x))
            (string (munch-integer (string-to-int x)))
            (t (munch-anything x)))

The type specifier t matches any type of object; the word otherwise is also allowed. To make one clause match any of several types, use an (or ...) type specifier.

— Special Form: etypecase keyform clause...

This macro is just like typecase, except that if the key does not match any of the clauses, an error is signalled rather than simply returning nil.