Writing GNU Emacs Extensions

Control Structures

Variables

To reference a variable, simply use its name (a symbol). To assign a variable, use setq.

(setq x 17)        ; assign 17 to variable x
x ⇒ 17             ; value of variable x

To make temporary variables that are in effect only in a certain region of code, use let.

(let ((var1 value1)
      (var2 value2)
       ...)
   body1 body2 ...)

In a let, all the values are computed in an unspecified order before any of the vars are assigned. The variant let* (whose syntax is identical to let) evaluates value_i and assigns it to var_i before evaluating value_i+1.

Sequencing

To evaluate a sequence of expressions where only a single expression is allowed, use progn.

(progn expr1 expr2 ...)

Evaluates each expr in turn. Returns the value of the last expr.

To evaluate a sequence of expressions and return the value of the first subexpression instead of the last, use prog1.

Conditionals

Emacs Lisp has two kinds of conditional expression: if and cond.

(if test
    then
else1 else2 ...)

Evaluates test. If the result is non-nil, evaluates then. Otherwise, evaluates each else expression in turn. Returns the value of the last expression it evaluates.

(cond ((test1 body11 body12 ...)
       (test2 body21 body22 ...)
       ...))

Evaluates test₁. If the result is non-nil, evaluates each body1 in turn. Otherwise evaluates test2. If the result is non-nil, evaluates each body2, and so on with each "cond clause." Returns the value of the last expression it evaluates.

A common practice is to place a catch-all clause at the end like this:

(cond ((test ...