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— 153 —
compare two strings using the ">" function.
CLIPS> (clear)
CLIPS> (> "duck2" "duck1")
[ARGACCES5] Function > expected argument #1 to be of type
integer or float
CLIPS>
It's not possible to do this comparison with the ">" function because it expects
NUMBER types as arguments.
However, it's easy to define a (defgeneric) which will overload the ">" to accept
STRING types as well as NUMBER types. For example, if the arguments of ">" are of
type STRING, the defgeneric will do a string comparison, letter by letter starting from
the left until the ASCII codes differ. In contrast, if the arguments of ">" are of type
NUMBER, the system compares the sign and magnitude of the numbers. The
user-defined ">" for STRING types is an explicit method, while a system-defined or
user-defined external function such as ">" for NUMBER type is an implicit method.
The technique of overloading a function name so that the method which implements it
is not known until run-time is another example of dynamic binding. Any object reference
of name or address may be bound at run-time in CLIPS to functions through dynamic
binding also.
Some languages such as Ada have a more restrictive type of overloading in which
the function name must be known at compile time rather than at run-time. The run-time
dynamic binding is the least restrictive since methods can be created during execution
by the (build) statement. However, you should be careful in using (build) since
dynamically creating constructs is often hard to debug. Also, the resulting code may be
difficult to verify and validate since you'll have to stop execution to examine the code.
Dynamic binding is a characteristics of a true object-oriented programming language.
Following is an example of a defgeneric, ">", for STRING types and its method.
CLIPS> (defgeneric >) ; Header declaraction. Actually
unnecessary
CLIPS> (defmethod > ((?a STRING) (?b STRING))
(> (str-compare ?a ?b) 0))
CLIPS> (> "duck2" "duck1") ; The overload ">" works correctly
TRUE
; in all three cases.
CLIPS> (> "duck1" "duck1")
FALSE
CLIPS> (> "duck1" "duck2")