On Jul 17, 2008, at 10:31 PM, William Stein wrote: > On Fri, Jul 18, 2008 at 2:11 AM, John H Palmieri > <[EMAIL PROTECTED]> wrote: >> >> >> >> On Jul 17, 4:51 pm, Carl Witty <[EMAIL PROTECTED]> wrote: >>> On Jul 17, 3:30 pm, John H Palmieri <[EMAIL PROTECTED]> wrote: >>> >>> >>> >>>> I have several different vector space bases for an algebra A, and I >>>> have implemented this by having a class MyAlgebra, instances of >>>> which >>>> have an attribute _basis_name which is a string naming the >>>> basis. I >>>> have __cmp__ defined so that the basis is ignored: any two >>>> instances >>>> are canonically isomorphic, and there is good coercion between >>>> them. >>> >>>> Now, the attribute _basis_name just affects the _repr_ and _latex_ >>>> methods for elements of the algebra; it has no other effect. My >>>> question is this: when I multiply elements from algebras with two >>>> different bases, I can't figure out how the parent of the result is >>>> determined. Some documentation suggests that the parent should be >>>> determined by the left factor, but that is not consistently what's >>>> happening. >>> >>>> I think I would like to force the result to have the same parent as >>>> the left-hand factor, but by the time elements get to the _mul_ >>>> method, their parents have been changed (via coercion), so I can't >>>> find out what that parent is. (Perhaps something is getting >>>> cached in >>>> the coercion process, so things are getting coerced to the cached >>>> algebra, not the left-hand parent?) What should I do? Does it >>>> sound >>>> like I'm doing something wrong, and if so, what should I >>>> investigate? >>> >>> Currently it's built-in to coercion that if two parents are equal, >>> then it doesn't matter which parent it picks. (And the decision is >>> baked into various caches, etc., so the decision can depend on the >>> previous history of commands in this session.) >>> >>> I've raised this issue as a problem a few weeks ago (seehttp:// >>> groups.google.com/group/sage-devel/browse_thread/thread/4520ed...), >>> but nothing came of it yet. >>> >>> Even if this gets fixed, I don't think you can fix it so that >>> coercion >>> uses the parent of the left-hand value by changing your code; this >>> would require changes inside the base coercion code. >> >> Okay, thanks. Here's something from Section 2.8 of the Sage >> Programming Guide: >> >> ARITHMETIC __add__, __mul__, ...:: When doing a binary operation, if >> the parents are not identical (in the sense of is ), determine if >> precisely one _coerce_ map is defined; if so, apply it and do the >> arithmetic operation. If both are defined, the parents are >> canonically >> isomorphic, so use the left one. If neither are defined, raise a >> TypeError. (Whether or not there is a coerce map between objects >> should be cached for efficiency.) >> >> I guess you're saying that this is wrong (the part that says "so use >> the left one"). > > Argh. It was right when I wrote that documentation and implemented > the second version of the coercion model. :-) Whoever implemented > the third/fourth version(s) of the coercion model will I hope also > update the programming guide with correct statements (hint, hint > Robertwb).
Yep, sorry. Do we want this behavior? (This could be done without too much trouble.) - Robert --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-devel@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sage-devel URLs: http://www.sagemath.org -~----------~----~----~----~------~----~------~--~---
