Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!linus!philabs!cmcl2!seismo!mcvax!boring!lambert
From: lambert@boring.uucp (Lambert Meertens)
Newsgroups: net.lang
Subject: Re: and if you put this in your language ...
Message-ID: <6843@boring.UUCP>
Date: Mon, 24-Mar-86 10:52:15 EST
Article-I.D.: boring.6843
Posted: Mon Mar 24 10:52:15 1986
Date-Received: Fri, 11-Apr-86 06:49:14 EST
References: <1187@mmintl.UUCP> <800009@ccvaxa>
Reply-To: lambert@boring.UUCP (Lambert Meertens)
Organization: CWI, Amsterdam
Lines: 65
Keywords: Refinement
Apparently-To: rnews@mcvax

In article <800009@ccvaxa> aglew@ccvaxa.UUCP writes:

>                                     Finally, something I'm not sure whether
> it's my idea, or Knuth's, or somebody else's, there is the concept of an
> ASIDE. An ASIDE is a labelled block of code that can only be entered via
> a goto to the label at the top of the aside (and can only be exited in one of
> the normal ways to exit a block). Code that would seemm to fall through to
> an aside branches around it. So above you really have
> 	goto END
> 	AB: begin Z; goto END end
> 	END: ...
> The aside has saved you a goto and a label - it makes it seem just a bit less
> like spaghetti.
> [...]
> but if Z is not simply a function call then you have to make sure that two
> copies of code get updated. That's what macros are for [...]
> A big #define in line looks ugly, and is really just like an aside, except
> that it hides a bit of the concept of doing work to get simpler cases.

Some languages have "refinements", which are somewhat like macros but they
really are part of the language and do not get expanded by a preprocessor.
Mainly useful for programming in a relaxed top-down style, but also good to
prevent code duplication.  The languages I know of that support refinements
are (in the chronological order in which the refinements entered the design)
ELAN, ABC (the upcoming revision of our B) and SETL.  In ABC, a possible way
of expressing the program is

       SELECT:
          a:
             X
             SELECT:
                b: Y
                ELSE: Z
          ELSE: Z
       ...
    X: <definition of X>
    Y: <definition of Y>
    Z: <definition of Z>

X and Y may be expanded in place, but for Z this gives code duplication.
Tests may also be refined, so if the evaluation of a and b is fast enough,
you can also write

       SELECT:
          a AND b:
             X
             Y
          a AND NOT b:
             X
             Z
          NOT a:
             Z

I wrote an article to make propaganda for refinements: Program Text and
Program Structure, in: Constructing Quality Software (P.G. Hibbard and
S.A. Schuman, eds), 271-281, North-Holland, 1978.

If any other languages than the three mentioned above sport refinements, I
would like to hear about them.

-- 

     Lambert Meertens
     ...!{seismo,okstate,garfield,decvax,philabs}!lambert@mcvax.UUCP
     CWI (Centre for Mathematics and Computer Science), Amsterdam