On 01/10/2008 09:42 AM, Martin Rubey wrote:
> Dear Gaby,
>
> I saw you implemented map!$Stack as follows:
>
> map!(f: S -> S, s: %) == -- from HOAGG
> map!(f, deref s)$List(S)
> s
>
> I have two questions:
>
> 1) why didn't you say
>
> map!(f: S -> S, s: %) == ref map!(f, deref s)$List(S)
>
> 2) isn't
>
> map!(f: S -> S, s: %) == -- from HOAGG
> map!(f, deref s)$List(S)
> s
>
> assuming that map! stores its result in s? I'm not sure whether this is
> ok:
Unfortunately 1) and 2) are not equivalent with the current
implementation of the Reference domain. In boolean.spad.pamphlet one finds
Reference(S:Type): Type with
ref : S -> %
++ ref(n) creates a pointer (reference) to the object n.
...
== add
Rep := Record(value: S)
p = q == EQ(p, q)$Lisp
ref v == [v]
...
That looks to me like that for
a: Reference Integer := ref 1;
b: Reference Integer := ref deref a;
it does not hold
a = b
so 1) would create a new memory location whereas 2) would not.
I hope my SPAD understanding is correct.
> (14) -> l := [1,16,81,1]
>
> (14) [1,16,81,1]
> Type: List PositiveInteger
> (15) -> sort! l
>
> (15) [1,1,16,81]
> Type: List PositiveInteger
> (16) -> l
>
> (16) [1,16,81]
> Type: List PositiveInteger
>
>
> comments?
Yes. Whenever you call a destructive function on something, you should
be *very* careful. Where is it documented that after calling sort!(l)
the variable l still has a proper value. It could well be that sort!
leaves l in a state like, for example,
[1, <pointer outside the data segment>, ...
It could happen that after a destructive function there is no NIL at the
last entry of the list, but rather a pointer to a wild place in memory
that still holds something nice but will be reclaimed the next time the
garbage collector runs.
That said ...
*Don't use l after you have called sort!(l) unless you say l:=sort!(l).*
Ralf
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