[m-dev.] for review: direct reuse

[Ralph Becket]

>From Peter Ross on 21/09/2000 10:40:55
> By any chance did you come to a meeting where we discussed this?

:)

> case b)
> 
> :- pred split(tree23(T)::in, tree23(T)::out) is det.
> 
> split(X, Y) :-
>     X => three(L, LK, M, RK, R),
>     T <= two(L, LK, M), % reuse the three cell, but waste the two extra
>                         % words at the end of the cell.
>     Y <= two(T, RK, R).

At first sight, this might seem fairly extravagant (40% wastage), until
you consider that applying reuse in an imperative language would probably 
require something of the following order:

struct node {
	enum {TWO, THREE, FOUR} tag;
	union {
		struct two_node two;
		struct three_node three;
		struct four_node four;
	} body;
}
...
struct four_node {
	Word key1; Word value1; ...; Word key3; Word value3;
	struct node *tree0; struct node *tree1; ...; struct node *tree4;
}

which is certainly worse, except for the fact that, under the Mercury
scheme, having overwritten a larger cell with a smaller one, we no longer
have the information that there is some extra space available if we wanted
to go in the other direction.

Ralph

--
Ralph Becket      |      MSR Cambridge      |      rbeck at microsoft.com 
--------------------------------------------------------------------------
mercury-developers mailing list
Post messages to:       mercury-developers at cs.mu.oz.au
Administrative Queries: owner-mercury-developers at cs.mu.oz.au
Subscriptions:          mercury-developers-request at cs.mu.oz.au
--------------------------------------------------------------------------