4.4 Conditionals

These conditional forms augment Emacs Lisp’s simple if, and, or, and cond forms.

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.