[m-dev.] Ordered conjunction

[Erwan Jahier]

Ralph Becket <rbeck at microsoft.com> writes:

> I know this has come up before, but I just want to restate the case
> for an ordered conjunction (P && Q) which constrains the compiler to
> call P before Q (for && pick the symbol of your choice).

What about `then'? It seems to me that it a degenerated case of
if-then-else where there is no `else' branch (or where the `else'
branch it equivalent to `fail').  

> This allows one to write code like
> 
> :- pred num_in_rg(int::in, int::out, int::in) is nondet.
> 
> num_in_rg(Lo, N, Hi) :-
> 	Lo =< Hi &&
> 	( N = Lo ; num_in_rg(Lo + 1, N, Hi) ).
> 
> safely.  If the && is an ordinary, unordered conjunction then
> the subgoals might get rearranged and this predicate could
> fail to terminate.

I agree. Moreover, I think it is more in the mercury spirit to make
explicit if a conjunction is commutative or not, the same way that
you must be explicit about the mode you have in mind when you define
a predicate/function. 


> The current tension is that
> (1) writing code that really is order independent can get really
> obfuscated,

Well, when I say something like that, some people (actually, it
happened twice) argue that it is because I do not think (and program)
<<declaratively>>. What would you answer to that?

> (2) so one usually goes for `pretty' code and hopes for the best
> (accepting the appropriate guilt burden).
> 
> Would this be easy to add to the language?  If so, does anyone with
> the right skill set have the time?
> 
> It's probably not a high priority thing, but I reckon it's a wart
> that should be fixed for 1.0.
> 
> Ralph

-- 
R1.
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