Path: utzoo!attcan!uunet!mcvax!unido!ecrcvax!micha
From: micha@ecrcvax.UUCP (Micha Meier)
Newsgroups: comp.lang.prolog
Subject: Re: Clause fusion (Disjunctions)
Message-ID: <539@ecrcvax.UUCP>
Date: 19 May 88 07:41:06 GMT
References: <983@cresswell.quintus.UUCP> <5501@megaron.arizona.edu>
Reply-To: micha@ecrcvax.UUCP (Micha Meier)
Organization: ECRC, Munich 81, West Germany
Lines: 47
Posted: Thu May 19 08:41:06 1988

Actually I don't want to contribute to this very topic, Saumya has analyzed
it in detail. What I'm concerned with is the indentation of disjunctions.
Two forms which were presented are
>
> 	association(Stack, Key, Val, Result) :-
>        pop_stack(Key1, Val1, Stack, Stack1) ->
>		 (Key1 \== Key ->
> 		     association(Stack1, Key, Val, Result) ;
>			(Val1 = Val,
>                Result = present
>			)
>		 ) ;
>           Result = conflict.
>--------------------
>
>
>> 	association(Stack, Key, Val, Result) :-
>> 		pop_stack(Key1, Val1, Stack, Stack1),
>> 		(   Key1 \== Key ->
>> 		    association(Stack1, Key, Val, Result)
>> 		;   Val1 = Val ->
>> 		    Result = present
>> 		;   Result = conflict
>> 		).

I find the two unreadable - what is the best way to indent the disjunctions
so that the resulting clause is easy to read and understand?

The general form of an if-then-else construct in a clause is

	p :-
		a, b, c,
		((q, r, s) -> (t, u, v);(w, x, y)),
		d, e, f.

Some of the parentheses can be omitted, but should they?
Richard proposes that nested if-then-else's are treated at the same level,
which leads to confusions since then the indentation is context dependent
(an if-then-else inside another one cannot be indented independently).
I have tried several variants, I have also seen various programs from
other people but I'm still searching. Isn't this problem a topic
for the standardization committee? The way if-then-else's and disjunctions
are written influences greatly the readability of the programs, if you
do it the wrong way round it's almost guaranteed that no-one will
notice any 'declarative meaning' of the program.

--Micha