Hi Eric, On 2014-06-02, Eric Gourgoulhon <egourgoul...@gmail.com> wrote: >> How about defining two parents (CU and CCU, with a coercion CCU --> CU=20 >> and a partial conversion CU --> CCU), and two element classes (SF and a= >=20 >> subclass CSF), and having CU(0) create an instance of CSF, but set the=20 >> instance's parent to CU?=20 >> > > Thank you for your anwser.=20 > Is this the standard way to implement algebraic substructures ? i.e. have a= >=20 > coercion B -> A and a partial conversion B -> A when B is a subspace (in=20 > the present case a subalgebra) of A ?
If B is a sub-structure of A, then the inclusion map is (by definition of a "sub-structure") a homomorphism. If you distinguish isomorphic sub-structures, then this homomorphism is canonical. Hence, it should be safe to use the embedding as a *coercion* B -> A. Note that registering a homomorphism as a coercion has implications: It will automatically be applied when you do arithmetic operations or comparison between elements of B and elements of A. Since B is a sub-structure of A, it is *always* possible to define a partial conversion A -> B (note the typo in your post: When we have coercion B -> A then we *cannot* additionally have a partial conversion in the same direction!): Its restriction to B is the identity, and it raises an error on the complement of B. Unlike a coercion, a conversion is not necessarily a homomorphism, and it will not be applied automatically. Best regards, Simon -- 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. For more options, visit https://groups.google.com/d/optout.
