On 2013-01-05, Simon King <simon.k...@uni-jena.de> wrote: > Hi John, > > On 2013-01-05, john_perry_usm <john.pe...@usm.edu> wrote: >> thinking of how java interfaces & multiple inheritance work: >> wouldn't it be possible to have an interface orabstract class >> that defines an abstract function abs, along with associated >> functions that require an abs functions, then declare in the >> class header that it implements it, and define the function so? > > The category framework offers such functionality. For example, one > could define the following in sage.categories.rings: > > from sage.misc.abstract_method import abstract_method > class Rings(Category): > ... > class ElementMethods: > @abstract_method(optional=True) > def abs(self): > """ > Return the absolute value. Implement only if that > makes sense in this ring. > """ > pass > > and then, for example: > > sage: from sage.structure.element import Element > sage: from sage.structure.parent import Parent > sage: class MyRingElement(Element): > ....: def _mul_(self, other): > ....: return self > ....: > sage: class MyRingElementWithAbs(MyRingElement): > ....: def abs(self): > ....: return self > ....: > sage: class MyRing(Parent): > ....: def __init__(self, has_abs=None): > ....: if has_abs: > ....: self.Element = MyRingElementWithAbs > ....: else: > ....: self.Element = MyRingElement > ....: Parent.__init__(self, category=Rings()) > ....: def _element_constructor_(self, *args, **kwds): > ....: return self.element_class(parent=self) > ....: > sage: R = MyRing() > sage: RAbs = MyRing(has_abs=True) > sage: a = R() > sage: b = RAbs() > sage: b.abs > <bound method MyRing_with_category.element_class.abs of Generic > element of a structure> > sage: b.abs() > Generic element of a structure > sage: hasattr(a, 'abs') # yes, still available by tab completion > True > sage: a.abs # but that's good: > NotImplemented
this is still a hack, as R() might in principle not have anything to do with abs(). In such a case NotImplemented is misleading, and it better be that hasattr(a, 'abs')==False > sage: a.__class__.abs? > ... > Definition: a.__class__.abs(self) > Docstring: > Return the absolute value. Implement only if that makes sense in > this ring. > ... > > I guess the main problem is that the abs() method of > sage.structure.element.RingElement has been introduced long before the > category framework was created. But we could consider to move abs() from > the base class RingElement to the element class of the category of > rings. The question is how to deal with old code, that tries to > implement abs() by means of __abs__(). > > Best regards, > Simon > > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To post to this group, send email to sage-devel@googlegroups.com. To unsubscribe from this group, send email to sage-devel+unsubscr...@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel?hl=en.
