> But using / in Spad this would become a little awkward. Eg.
>
> y:NonZero(Integer):=retractIfCan(z)
> y case "failed" => error ...
> x/y
Well... we should have exceptions anyway...
And since NonZero(X) would be a subdomain of X, there is actually no
need to do any conversion at all I rather think of "telling the compiler
about extra properties of an element".
So maybe it would look better if in the end we could say something like
x: % := ...
y: % := ...
...
augment(y, NonZero %){
z := x/y
}
and the "augment" might throw a runtime exception if y=0.
This is similar to
if D has SomeCat then {
-- do something with the knowledge that D exports SomeCat
}
only that it is working on (element, Domain) instead of
has: (Domain, Category) -> Boolean
One might argue that the "augment" above is the same as
if not zero? y then z := x/y
Yes, nearly. The difference is that in the "augment" would transport the
NonZero knowledge to / while in the "if" construction / would not see
that y has already been checked for being non-zero.
Of course the "if" construction looks more natural. So I'd rather like a
way to transport such information to argument properties of /. These are
questions that should be tackled in the future. I don't see some
immediate remedy for that right now. Looks like a need to invent a more
powerful language. ;-)
Ralf
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