On Sat, Jun 11, 2022 at 09:53:39PM +0200, Ralf Hemmecke wrote:
> While implementing, I have a problem with
>
> x: Stream Integer := [k for k in 1..29]
> st := x(2..-2 by -1);
>
> The creation itself will succeed.
> Now what exactly should st be?
>
> Naively, that would be a stream consisting of 5 elements
>
> st := [x(2), x(1), x(0), x(-1), x(-2)] (*)
>
> Clearly the only sensible stream that I can return is
>
> st2 := [x(2), x(1)] (**)
If you want schematic representation it would be:
[x(2), x(1), bomb]
Like in:
f_elt(i : Integer) : Integer == (i < 2 => i + 1; error "invalid access")
st := stream(f_elt, 1)@Stream(Integer);
After that:
(6) -> st2 := rest(rest(st));
Type: Stream(Integer)
(7) -> empty?(st2)
Error signalled from user code in function f_elt:
invalid access
This may look perverse. However, it allows lazy evaluation,
and has nice property that if computation finishes without
error, then all values used in computation were in bound.
--
Waldek Hebisch
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