On Wed, May 14, 2008 at 10:53 PM, Gabriel Dos Reis wrote:
> Ralf Hemmecke writes:
> ...
> | The code can also be made legal by changing the definition of
> | halfExtendedSubResultantGcd1 and halfExtendedSubResultantGcd2
> | to return something of type
> |
> | Record (gcd: P, coef: P)
> |
> | No "1" or "2" in the tag of the second field. *That* is an artificial
> | complication that is completely unnecessary.
>
> Yes, yes, yes. That would solve this *particular issue*.
> I'm concerned with the _pattern_.
> ...
So to summarize, the pattern is something like this:
if cond then
x := 1
else
x := 2.3
...
If this is allowed by the compiler, the question is what is the type
of 'x' outside the scope of the 'then' and 'else' clauses?
One answer is to introduce only local scope so that the variable x
does not exist outside the scope.
If the scope of 'x' is intended to be global, then the type of 'x'
must be a Union.
x:Union(Integer,Float)
if cond then
x := 1
else
x := 2.3
I do not see anything wrong with this proposal. It is compatible with
only a minor change to the existing library code - just like the other
scope change in OpenAxiom recently discussed by Gaby. For example one
can write:
hesrg:Union(c1:Record (gcd : P, coef1 : P), c2:Record (gcd :
P, coef2 : P))
if mdeg(p) < mdeg(ts_v)
then
hesrg := halfExtendedSubResultantGcd2(ts_v,p)$P
d: P := hesrg.c2.gcd; n: P := hesrg.c2.coef2
else
hesrg := halfExtendedSubResultantGcd1(p,ts_v)$P
d: P := hesrg.c1.gcd; n: P := hesrg.c1.coef1
g := gcd(n,d)
But of course this makes it even more clear that this segment of code
has a rather poor style and introduces this Union for on particularly
good purpose. As others have pointed out, it should be re-factored.
Regards,
Bill Page.
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