6.1 Methods for the + generic function
We need to make it possible to add one time to another. We could define a method with a name such as add or plus. However, the concept of adding times is the same as the concept of adding numbers. Dylan already provides the + generic function for adding numbers. Instead of inventing a new name for the addition operation, we define new methods on the built-in generic function +. We can extend + by defining new methods for it. In certain languages, this technique is called operator overloading.
Comparison with C++ and Java: In C++, operator overloading means customizing the action of any built-in operator for classes that you define. In Dylan, operators are just generic functions, and you can add methods to those generic functions for your classes. In C++, the meaning of an overloaded operator is resolved at compile time — the types of the operands must be known at compile time. Because Dylan operators are generic functions, the method is chosen dynamically according to the argument types — at run time, if the types may vary at run time.
Java does not allow operator overloading. The Java designers believe that overloading of operators results in inscrutable code (because the meaning of the operator can vary). Dylan and C++ designers believe that, judiciously used, operator overloading permits clearer, more concise code.
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N Feinberg/S E Keene/R Mathews/P Tucker Withington, DYLAN PROGRAMMING, (c) 1997 Harlequin Inc. Reproduced by permission of Addison-Wesley Longman Publishing Company, Inc. All rights reserved. No further copying, downloading or transmitting of this material is allowed without the prior written permission of the publisher.
