Re: choice of character for relational division

On Mar 31, 11:04 pm, Gene Wirchenko <g..._at_ocis.net> wrote:
> "Marshall" <marshall.spi..._at_gmail.com> wrote:
>
> [snip]
>
> >So, if you had to choose an ascii character for
> >relational division, which one would you use
> >and why?
>
> I would not do so unless the meaning was fairly obvious, and it
> did not horribly overload the character.

It's hard to come up with something that's obvious for relational division, since it's such an infrequently used operation. However there is certainly the issue of *not* picking something that is familiarly associated with some other operation, such as the sad case of using << to mean "write to a stream." The overloading issue is certainly important, though.

> Instead, I would pick a
> three-or-so-letter name. ("mod" is used by some. ALGOL used "DIV"
> and "REM".)

Yeah, I'm starting to see the value. One issue is that I think it makes sense to use the same charset for both join, union, and their inverses. (I.e., you wouldn't want a language to use "+" and "minus" for addition and subtraction.) Since I'm having relation join, union be the same thing as boolean and, or, so it's not clear which one to pick. If I used join/union, then the inverses would be divide/minus I suppose, but then that would look weird with booleans. If I use and/or, then what are the inverses?

> "|" is used a lot in one of my computing science classes. "|x|"
> can mean the absolute value of x, or the size of x. "x|y" means that
> x divides evenly into y. "|" is also used something like "such that",
> as in {x | x>0}. I do not like this overloading.

It's a design issue. There is a balance to be struck. On the one hand, attempting to completely avoid any overloading of any character is going to lead to something that is wildly verbose, and probably not very readable. On the other hand too much overloading and programs start to look like line noise, and again readability suffers.

Using |x| doesn't seem like a great choice to me because we already have abs(x) which is hardly any longer and much more obvious. (And to many programmers, familiar.)

Using | as the "divides" operator is probably quite familiar to math students, and if you're studying prime numbers for example, divisibility is likely to be something you're talking about a lot. It also figures in to the Fundamental Theorem of Arithmetic. But it's not much use in programming.

On the other hand, | to mean "such that" is set theory and set builder notation is awesome.

  /: (a, b | b != 0)

"division is a function that has parameters a and b where b is not zero." Can't beat that.

For a long time I was playing with a variation of set builder notation for a programming notation. (You can see similar things in list comprehensions in languages like Clean.) Ultimately I dropped it because I found better ways to accomplish the same thing, but it was pretty compact and readable.

Marshall Received on Mon Apr 02 2007 - 01:29:19 CEST