about immutability and reference frame: I guess I do not understand the details of what you describe. Anyway, I will not be against immutability if it does not mess up with `__eq__` and `__hash__` so it depends on the implementation.
about Vector: if it is not a subclass of Expr it will not be useful outside of mechanics. For instance symplify(sin(asin(phi))*unit_vector) will just raise an error instead of return phi*unit_vector. On 11 June 2013 02:39, Gilbert Gede <gilbertg...@gmail.com> wrote: > Stefan, > I think I disagree about Reference Frames being immutable. If you are going > to create a series of Reference Frames when performing successive rotations, > you are going to need to need to be able to modify the original frame to > point to the new frames. There is a similar issue for setting the angular > velocity of a Reference Frame after it is created. > > Also, do you think that Vector should be a subclass of Expr? > > -Gilbert > > > On Mon, Jun 10, 2013 at 5:45 AM, Stefan Krastanov > <krastanov.ste...@gmail.com> wrote: >> >> > @Stefan. I agree about the tree traversals part. However, I still feel >> > there must be a way for the user to check if two scalar fields expressed in >> > different ways are mathematically equivalent. Could you suggest a way? >> > Maybe >> > just another function? >> >> This is described in one of the first pages of the sympy >> documentation: Checking for equality in general is mathematically >> impossible. In practice most of the time one does `simplify(expr_a - >> expr_b)` and sees whether he gets 0. In numerics one just plots the >> difference. >> >> > Also, have a look at the __eq__ function of the current Vector class. >> >> The current Vector class is not really a part of SymPy. One of the >> reasons that these two projects are useful is that they will make >> mechanics and sympy more inter-operable so stuff like `simplify` can >> be used seamlessly in mechanics. >> >> > I would really like atleast _some_ way of checking mathematical >> > equality. >> >> As I mentioned above, there is already a way for that and the issues >> surrounding it are described in the docs (well, the documentation can >> be better, that is clear :) >> >> >> Anyway, given all these issues I can not imagine an api similar to the >> currently suggested one working well with sympy. Both ScalarField and >> Vector will break most of sympy if they eagerly eat all expressions >> that are added to them. Unrelated to that, ScalarField is ill defined >> because all the confusion around reference frames and coordinate >> systems and reuse of Symbols. Reference Frames and/or Coordinate >> Systems are represented as mutable objects which is nonsensical >> mathematically (one does not change how to references relate (one >> might want to parametrize something wrt time, but this is not the >> same))... >> >> @Sachin and Prasoon, can you work together on the next iteration of >> the design that is meant to fix these issues? When would you be able >> to show it? >> >> Multiple times I have suggested partial/fragmented designs similar to >> `diffgeom`. While some parts of `diffgeom` have no place here, some of >> the ideas (the representation of general fields based on sympy >> expressions containing base fields) could be helpful in preserving all >> the invariants important for sympy. Do you want me to provide a more >> complete suggestion on which you can base yours, or do you prefer to >> continue on your own for the moment? >> >> -- >> You received this message because you are subscribed to the Google Groups >> "sympy" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to sympy+unsubscr...@googlegroups.com. >> To post to this group, send email to sympy@googlegroups.com. >> Visit this group at http://groups.google.com/group/sympy?hl=en-US. >> For more options, visit https://groups.google.com/groups/opt_out. >> >> > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sympy+unsubscr...@googlegroups.com. > To post to this group, send email to sympy@googlegroups.com. > Visit this group at http://groups.google.com/group/sympy?hl=en-US. > For more options, visit https://groups.google.com/groups/opt_out. > > -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sympy+unsubscr...@googlegroups.com. To post to this group, send email to sympy@googlegroups.com. Visit this group at http://groups.google.com/group/sympy?hl=en-US. For more options, visit https://groups.google.com/groups/opt_out.
