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

[Richard A. O'Keefe]

	1 - I used an in,in,out predicate, where I would have preferred a function:
	
	 a - how to notate (declare) such a function
	 b - how to write such a function
	 c - how to write code to get back such a term from a function
	
:- func add(rational, rational) = rational.

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

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

Since Mercury is often compiled via some flavour of C, it is important
to ensure that text output ends with a newline.

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