[mercury-users] returning a term from a function? how to?

[David Overton]

On Wed, May 02, 2001 at 08:32:24PM -0700, Terrence Brannon wrote:
> the following program works! and I did it all by myself! however, 

Congratulations!

> 
> 1 - I used an in,in,out predicate, where I would have preferred a function:
> 
> :- pred add(rational,rational,rational).
> :- mode add(   in   ,   in   , out).
> add(rational(N1,D1),rational(N2,D2),rational(NA,DA)) :-
> 	DA = D1 * D2,
> 	NA = N1 * D2 + N2 * D1.
> 
> main -->
> 	  { add(rational(1,3),rational(2,5),rational(A,B)) },
> 
> 
> because I do not know
>  a - how to notate (declare) such a function
>  b - how to write such a function

A direct translation into a function would be:

	:- func add(rational,rational) = rational.
	:- mode add(   in   ,   in   ) =   out.
	add(rational(N1,D1),rational(N2,D2)) = rational(NA,DA) :-
		DA = D1 * D2,
		NA = N1 * D2 + N2 * D1.

which is a trivial transformation of the predicate version, but you
can write it more compactly by leaving out the mode declaration and
using a bit of functional notation:

	:- func add(rational, rational) = rational.

	add(rational(N1, D1), rational(N2, D2)) =
		rational(N1 * D2 + N2 * D1, D1 * D2).

>  c - how to write code to get back such a term from a function

Again, a simple transformation from the predicate version:

main -->
	  { rational(A,B) = add(rational(1,3),rational(2,5)) },
	  io__write_int(A),nl,
	  io__write_string("-"),nl,
	  io__write_int(B).



David
-- 
David Overton      Department of Computer Science & Software Engineering
PhD Student        The University of Melbourne, Victoria 3010, Australia
+61 3 8344 9159    http://www.cs.mu.oz.au/~dmo
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