Comparison in the Gap interface raises an error

[SimonKing]

On sage.math with sage-3.4.1, one has

sage: gap('DihedralGroup(8)')==gap('DihedralGroup(8)')
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
...
RuntimeError: Gap produced error output
Error, no 1st choice method found for `LT' on 2 arguments

   executing $sage1 < $sage2;

The problem seems to be that Gap is unable to compare:

sage: gap('DihedralGroup(8)=DihedralGroup(8)')
false

Perhaps it would make sense to try and implement a __cmp__ method that is more sophisticated than what is done in Gap?

At least it should be made sure that the __cmp__ method of the Gap interface does not raise an error.

[SimonKing]

I see no way for a really satisfying solution, as long as GAP can not even compare two objects whose definitions are identical.

However, the errors being raised by GAP when comparing elements are now caught in a try-except clause. We have, as doc tests:

sage: gap('DihedralGroup(8)')==gap('DihedralGroup(8)')
False    # sorry, this is what GAP claims.
sage: gap('SymmetricGroup(8)')<gap('AlternatingGroup(8)')
True
sage: gap('SymmetricGroup(8)')>gap('AlternatingGroup(8)')
False
sage: gap('SymmetricGroup(8)')==gap('SymmetricGroup(8)')
True

All but the first of these examples worked before. But the first resulted in an error, which is now fixed.

[SimonKing]

I just found that this ticket needs review since 8 months. Fortunately the patch still works fine. So, any volunteer?

[wdj]

Replying to SimonKing:

I just found that this ticket needs review since 8 months. Fortunately the patch still works fine. So, any volunteer?

I have spring break coming up and can try to review it then if no one beats me to it.

[wdj]

This patch applies to 4.7.a1 and passes sage -testall. The patch does as claimed (adding some try-except statements) and contains the proper additional examples in the docstrings. Positive review from me.

Thanks Simon!

[jdemeyer]

Please change the commit message (using hg qrefresh -e) such that the ticket number appears on the first line.

[SimonKing]

Avoid an error being raised when comparing GAP elements. Add doctest.

[SimonKing]

Replying to jdemeyer:

Please change the commit message (using hg qrefresh -e) such that the ticket number appears on the first line.

Done.

[jdemeyer]

Replying to SimonKing:

Done.

Thanks!