[mercury-users] Mercury needs a Tutorial

[Thomas Charles Conway]

On Mon, Feb 15, 1999 at 06:44:51PM EST, Bas de Bakker wrote:
> >>>>> "David" == David Powers <powers at ist.flinders.edu.au> writes:
> 
> A few selected replies...
> 
>     David> Given that doing the factorial examples like this in Prolog
>     David> would return the sum of the appropriate number of 1's, do I
>     David> assume that = acts like Prolog's is and evaluates
>     David> expressions.
> 
> Not exactly, it's rather the other way around.  You can use 'is' in
> Mercury, but it's just another spelling for '='.  Expressions are
> always evaluated, e.g., you can write 'factorial(N1 - N2 + N3, F)'.

Well, not `always'. The operator `+' is declared as a function:

:- func (int + int) = int.

There is no reason at all why you couldn't build up an expression
(as you do in Prolog) and then have a function/predicate to evaluate
it:

:- import_module int.

:- type expr
	--->	num(int)
	;	(expr + expr)
	;	(expr - expr)
		...
	.

:- pred (int is expr).
:- mode (out is in) is det.

X is num(X).
(X + Y) is Ex + Ey :-
	X is Ex,
	Y is Ey.
(X - Y) is Ex - Ey :-
	X is Ex,
	Y is Ey.
...

Unfortunately, since Mercury doesn't have undiscriminated union
types, you need the num/1 wrapper around constants (besides,
with out the wrapper, code like the following would be ambiguous:
	F = 1 + 2 + 3
is F an expr or an int?).

Thomas
-- 
Thomas Conway <conway at cs.mu.oz.au> )O+
To a killer whale, otters are like hairy popcorn -- Paul Dayton