A function in CLIPS is a piece of executable code identified by a specific name which returns a useful value or performs a useful side effect (such as displaying information). Throughout the CLIPS documentation, the word function is generally used to refer only to functions which return a value (whereas commands and actions are used to refer to functions which have a side effect but generally do not return a value).
There are several types of functions. User defined functions and system defined functions are pieces of code that have been written in an external language (such as C, FORTRAN, or Ada) and linked with the CLIPS environment. System defined functions are those functions that have been defined internally by the CLIPS environment. User defined functions are functions that have been defined externally of the CLIPS environment. A complete list of system defined functions can be found in appendix I.
The deffunction construct allows users to define new functions directly in the CLIPS environment using CLIPS syntax. Functions defined in this manner appear and act like other functions, however, instead of being directly executed (as code written in an external language would be) they are interpreted by the CLIPS environment. Deffunctions are also discussed in section 2.5.2.1 in the context of procedural knowledge representation.
Generic functions can be defined using the defgeneric and defmethod constructs. Generic functions allow different pieces of code to be executed depending upon the arguments passed to the generic function. Thus, a single function name can be overloaded with more than one piece of code. Generic functions are also discussed in section 2.5.2.2 in the context of procedural knowledge representation.
F calls in CLIPS use a prefix notation ñ the arguments to a function always appear after the function name. Function calls begin with a left parenthesis, followed by the name of the function, then the arguments to the function follow (each argument separated by one or more spaces). Arguments to a function can be primitive data types, variables, or another function call. The function call is then closed with a right parenthesis. Some examples of function calls using the addition (+) and multiplication (*) functions are shown following.
(+ 3 4 5) (* 5 6.0 2) (+ 3 (* 8 9) 4) (* 8 (+ 3 (* 2 3 4) 9) (* 3 4))
While a function refers to a piece of executable code identified
by a specific name, an expression refers
to a function which has its arguments specified (which may or
may not be functions calls as well). Thus the previous examples
are expressions which make calls to the * and + functions.