Editing Lisp (programming language) (section)

===Operators===
Arithmetic operators are treated similarly. The expression
<syntaxhighlight lang="Lisp">
 (+ 1 2 3 4)
</syntaxhighlight>
evaluates to 10. The equivalent under [[infix notation]] would be "{{Lisp2|1 + 2 + 3 + 4}}".

Lisp has no notion of operators as implemented in [[ALGOL]]-derived languages. Arithmetic operators in Lisp are [[variadic function]]s (or ''n-ary''), able to take any number of arguments. A C-style '++' increment operator is sometimes implemented under the name <code>incf</code> giving syntax
<syntaxhighlight lang="Lisp">
 (incf x)
</syntaxhighlight>
equivalent to <code>(setq x (+ x 1))</code>, returning the new value of <code>x</code>.

"Special operators" (sometimes called "special forms") provide Lisp's control structure. For example, the special operator {{Lisp2|if}} takes three arguments. If the first argument is non-nil, it evaluates to the second argument; otherwise, it evaluates to the third argument. Thus, the expression
<syntaxhighlight lang="Lisp">
 (if nil
     (list 1 2 "foo")
     (list 3 4 "bar"))
</syntaxhighlight>
evaluates to {{Lisp2|(3 4 "bar")}}. Of course, this would be more useful if a non-trivial expression had been substituted in place of {{Lisp2|nil}}.

Lisp also provides logical operators '''and''', '''or''' and '''not'''. The '''and''' and '''or''' operators do [[short-circuit evaluation]] and will return their first nil and non-nil argument respectively.
<syntaxhighlight lang="Lisp">
 (or (and "zero" nil "never") "James" 'task 'time)
</syntaxhighlight>
will evaluate to "James".