These comparison functions are part of a suite of object comparison functions that ScriptX provides. Because they are based on generic functions, you can override the generics in the classes and objects you define to extend the object Comparison protocol.

Chapter 6, "Defining Classes and Objects," describes how to specialize your classes to allow for comparison of objects.

For more information on the Comparison protocol, see the "Object System Kernel" chapter of the ScriptX Components Guide. For complete definitions of ScriptX comparison functions, see the "Global Functions" chapter of the ScriptX Class Reference.

Each class can override isComparable to indicate which classes it can be compared with. The default implementation of isComparable simply states that two objects are comparable if they are of the same class.

isComparable 1 2

true

isComparable 1 "banana"

false

The cmp global function is a general purpose comparison function. When it is called with two arguments (which must be comparable), it uses the equal ( = ) and lt ( < ) generic functions to return one of three values:

• @same if the arguments have the same values ( equal evaluates to true )

• @before if the first argument is less than the second ( lt returns true )

• @after if the first argument is greater than the second ( lt returns false )

cmp 1 1

@same

cmp 1 4

@before

cmp 4 1

@after

Universal comparison functions can compare two objects of different classes. If two objects are of the same class, and that class does not implement a method for localLT, then ult can still report an exception. Generally, all objects for which comparison is meaningful, such as numbers and strings, implement a method for localLT. To insure that the universal comparison functions work for a given object, use canObjectDo to query the object to see whether it implements localLT. For information on canObjectDo, see page 72.

The seven universal comparison functions are ueq, une, ult, ugt, uge, ule, and ucmp.

The functions ueq (universal equal) and une (universal not equal) test for object equality. The ueq function returns true only if:

• The classes of its arguments are the same, and

• the values of its arguments are equal

arg1 := 1.9 -- an instance of ImmediateFloat 

arg2 := 1.9 as Float

arg1 = arg2 -- equivalent to the equal global function

true

ueq arg1 arg2 -- test for universal equality

false

In this example, arg1 is assigned the number 1.9 (an instance of class ImmediateFloat), and arg2 is assigned the same value, coerced to the Float class. Because the values of these numbers are the same, the equality test returns true. However, because the classes of these objects are different, ueq returns false.

The global functions ugt, ult, uge, and ule are used for universal comparisons of magnitude. (They are the universal counterparts of the > (gt), < (lt), >= (ge), and <= (le) operations, respectively). Each of these functions returns true if one of the following rules is true:

• If the classes of the arguments are the same, then the methods return the same result that the non-universal comparison would have returned.

For example, for the expression ult 1 2, the classes are the same (ImmediateInteger), and the value 1 is indeed less than 2, so the function returns true. Since lt 1 2 returns true, ult 1 2 returns true.

• If the classes are different, then the names of the classes are compared alphabetically.

For example, for the expression ult 1 "this", the two classes (ImmediateInteger and String) are different, so the names of the classes are compared. Because ImmediateInteger is alphabetically "less than" String (that is, "I" comes before "S"), the expression returns true.

Finally, the ucmp function is the universal equivalent of the cmp function. ucmp returns one of the following values, based on the following rules:

• @same if the ueq function returns true for the arguments.

• @before if the ult function returns true for the arguments.

• @after if the ueq and ult functions both return false for the arguments.

This document is part of the ScriptX Language Guide, one of the volumes of the ScriptX Technical Reference Series. ScriptX is developed by the ScriptX Engineering Team at Apple Computer, successor to the Kaleida Engineering Team at Kaleida Labs, Inc.

Copyright 1996 Apple Computer, Inc. All Rights Reserved.