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7 Deffunction Construct

With the deffunction construct, new functions may be defined directly in CLIPS. Deffunctions are equivalent in use to other functions; see section 2.3.2 for more detail on calling functions in CLIPS. The only differences between user-defined external functions and deffunctions are that deffunctions are written in CLIPS and executed by CLIPS interpretively and userdefined external functions are written in an external language, such as C, and executed by CLIPS directly. Also, deffunctions allow the addition of new functions without having to recompile and relink CLIPS.

A deffunction is comprised of five elements: 1) a name, 2) an optional comment, 3) a list of zero or more required parameters, 4) an optional wildcard parameter to handle a variable number of arguments and 5) a sequence of actions, or expressions, which will be executed in order when the deffunction is called.


(deffunction <name> [<comment>]
    (<regular-parameter>* [<wildcard-parameter>])
<regular-parameter>   ::= <single-field-variable>
<wildcard-parameter>  ::= <multifield-variable>

A deffunction must have a unique name different from all other system functions and generic functions. In particular, a deffunction cannot be overloaded like a system function (see section 8 for more detail). A deffunction must be declared prior to being called from another deffunction, generic function method, object message-handler, rule, or the top level. The only exception is a self recursive deffunction.

A deffunction may accept exactly or at least a specified number of arguments, depending on whether a is used or not. The regular parameters specify the minimum number of arguments that must be passed to the deffunction. Each of these parameters may be referenced like a normal single-field variable within the actions of the deffunction. If a wildcard parameter is present, the deffunction may be passed any number of arguments greater than or equal to the minimum. If no wildcard parameter is present, then the deffunction must be passed exactly the number of arguments specified by the s. All arguments to a deffunction that do not correspond to a regular parameter are grouped into a multifield value that can be referenced by the wildcard parameter. The standard CLIPS multifield functions, such as length and nth, can be applied to the wildcard parameter.


CLIPS> (clear)
(deffunction print-args (?a ?b $?c)
    (printout t ?a " " ?b " and " (length ?c) " extras: " ?c
CLIPS> (print-args 1 2)
1 2 and 0 extras: ()
CLIPS> (print-args a b c d)
a b and 2 extras: (c d)

When a deffunction is called, its s are executed in order. The of a deffunction is the evaluation of the last action. If a deffunction has no actions, its return value is the symbol FALSE. If an occurs while the deffunction is executing, any actions not yet executed will be aborted, and the deffunction will return the symbol FALSE.

Deffunctions may be self and mutually recursive. Self is accomplished simply by calling the deffunction from within its own actions.


(deffunction factorial (?a)
    (if (or (not (integerp ?a)) (< ?a 0)) then
        (printout t "Factorial Error!" crlf)
        (if (= ?a 0) then
            (* ?a (factorial (- ?a 1))))))

Mutual recursion between two deffunctions requires a forward declaration of one of the deffunctions. A forward declaration is simply a declaration of the deffunction without any actions. In the following example, the deffunction foo is forward declared so that it may be called by the deffunction bar. Then the deffunction foo is redefined with actions that call the deffunction bar.


(deffunction foo ())
(deffunction bar ()
(deffunction foo ()

Care should be taken with recursive deffunctions; too many levels of recursion can lead to an overflow of stack memory (especially on PC-type machines).

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