Karl-Friedrich Lenz reacts to my previous posting on how to distinguish programs from data, by insisting on the importance of having a simple definition of “program.” He is right about the value of a simple definition. And he is right to observe that my previous posting doesn’t argue against the existence of such a definition, although it does imply that the definition might be difficult to apply in practice. Lenz suggests a simple definition: “If the object instructs a computer to do something, it is a program. The remaining cases are data.”
Maximillian Dornseif weighs in with a thought-provoking example to show difficulty of applying this seemingly simple definition.
Another troublesome cases arises in logic programming, a style of programming that is implemented by programming languages like Prolog. In logic programming, you don’t tell the computer what to do or even how to do it. Instead, you specify the attributes of an object you want, and the computer figures out how to find or construct such an object. You state facts and relationships, and then you ask a question. At no point do you tell the computer what steps to execute or how to go about doing anything; that is all handled by a pre-packaged program called the Prolog Interpreter. Computer scientists talk about Prolog programs, but a Prolog program doesn’t seem to meet Lenz’s definition.
Now, we might try to stretch Lenz’s definition by saying that the Prolog program, even if it is only a listing of facts, does “instruct” the computer to do something, because the programmer wrote it knowing that it would cause the computer to behave in a certain way. But such a definition is too broad. A Word document is written with the purpose of causing the computer to do something, but that doesn’t make it a program. Besides, it seems unsatisfactory to call something a program or not based on the state of mind of its author.
Still, I’m not giving up on the quest for a simple definition.
