Mon Nov 9 17:17:40 1981
>From RMS@MIT-AI Mon Nov 9 16:43:23 1981
Union types do not suffice for writing MEMBER.
They may be appear to be sufficient, if you look at a single program rather than at changing the program. But MEMBER is supposed to work on any type which does or EVER WILL exist. It should not be necessary to alter the definition of MEMBER and 69 other functions, or even to recompile them, every time a new type is added.
The only way to avoid this is if the system allows new alternatives to be added to a previously defined union type. And this must have the effect of causing all variables, functions and structures previously declared using the union type to accept the new alternatives, without the user's having to know what variables, functions or structures they are. Then you can define a union type called EVERYTHING and add all new types to that union. But then the system should save you trouble and automatically add all new types to the union EVERYTHING. This is tantamount to permitting you not to declare the type.
External variables are vital for extensible systems. A demonstration of this is in my EMACS paper, AI memo 519a.
Beliefs such as that type declarations are always desirable, and that external variables should be replaced by explicit parameters, and that it is good to control where a function can be called from, are based on a static way of looking at a program. There is only one program being considered, and the goal is to express that one program in the clearest possible manner, making all errors (in other words, all changes!) as difficult as possible. Naturally, mechanisms that incorporate redundancy are used for this.
The extensible way of thinking says that we want to make it as easy as possible to get to as many different useful variations as possible. It is not one program that we want, but options on many programs. Taking this as a goal, one reaches very different conclusions about what is good in a programming language. It looks less like ADA and more like Lisp.
It is no coincidence that "computer scientists" have been using the static view in designing their languages. The research has been sponsored for the sake of military projects for which that viewpoint may be appropriate. But it is not appropriate for other things. An editor designed to be reliable enough to be "man-rated" is a waste of effort; and if this unnecessary feature interferes with extensibility, it is actually harmful.
HT Usenet historian Chris.