[mercury-users] Re: (the lack of) unique modes

[Robert Ernst Johann JESCHOFNIK]

On Fri, 5 Nov 1999, Fergus Henderson wrote:

> Or you could try using `array' and using
> `unsafe_promise_unique' (in the `builtin' standard library module)
> to work around the cases where the compiler's uniqueness analysis is
> overly conservative.

As I understand it, unsafe_promise_unique/2 gives back an object with
instantiatedness `unique', and this will cause mode errors with things
like array__set, that expect something else, for example, `uniq_array'.

I had this problem with the graphics assignment I recently did, and I
believe Ralph recently banged his head against the same problem as well.
Luckily, I happened to be in the right place at the right time (4pm the
day the assignment was due) and caught Tom Conway in a hallway, and he
provided a workaround. I believe I sent this to Ralph in an email a few
weeks ago, but it is short, so here it is again.

	:- func u(array(T)) = array(T).
	:- mode (u(in) = array_uo) is det.
	:- pragma c_code(u(A::in) = (B::array_uo),
			[will_not_call_mercury, thread_safe],
			"B = A;"
			).

So I guess you could use (something very similar to) this to solve your 
map woes, Tomas. Just make some custom :- inst's for your types, and then
modes (a_uo, a_di, etc) that use them, and inside a call, use the above
function to ensure uniqueness.
eg,
(assuming `C' is a 2d array or higher, and do_something/2 uses destructive
update on the array it is given)

	array__lookup(C, Pos, Element),
	do_something(u(Element), NewElement),
	...


heh. I think I have finally got my head around uniqueness now. :)


Rob
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