On Feb 10, 12:17 pm, Charles Bouillaguet <charles.bouillag...@gmail.com> wrote: > Why doesn't the first call to R in IntegralDomains() decide wether this is > true or not ?
Since in this case it just boils down to exactly the same check that is necessary for `R in Fields()`, that's definitely an option. However, in general deciding whether a ring is an integral domain might be rather hard, or more generally, I bet that there are categories with subcategories for which membership is undecidable. In that case, the only way to read a question like `R in IntegralDomains()` would be "Do you happen to know for sure that R is an integral domain" or "Did this ring get constructed in a way that guarantees it's an integral domain"? In that light, the first answer may not be so disturbing. However, the fact that an answer changes due to a seemingly unrelated query (and which could happen in completely unrelated code, just because both happen to use the same ring constructor at some point!) is definitely disturbing. It means one always has to argue about sage with in mind that every parent is essentially a global variable, and one you don't necessarily get to initialize yourself either! For this particular example the symptom can be remedied by ensuring that membership testing on `IntegralDomain()` also returns mathematical truth rather than "known fact by construction", which can get fouled up if membership testing for `Field()` changes. However, the underlying cause (post #13370 -- before that it's simply that IntegralDomain looks in a cache that it doesn't update itself) that something people might expect to be a constant, category membership, apparently isn't! Insisting that "parent in given_category", or for that matter "is_subcategory(parent.category(),given_category)" cannot be relied on but instead always has to be computationally investigated seems to seriously reduce its usefulness. -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To post to this group, send email to sage-devel@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel?hl=en. For more options, visit https://groups.google.com/groups/opt_out.
