Add a isiterable trait

[davidanthoff]

I'm not sure this can be done, but I have a vague idea that with all the new work about world counters etc. it might be feasible to add a trait that checks whether all the functions required for iteration are implemented for a type? I remember we discussed this somewhere before, and back then this didn't work because of the infamous problem in julia that I think is now solved with the world counting stuff.

Having an iterable trait would allow me to go to a much more elegant design in Query.jl.

[mauro3]

I'm not sure how the word-age stuff could help here, but I know next to nothing about it. The new type-system might. You could try to use method_exists inside a generated function, like described in https://github.com/mauro3/SimpleTraits.jl#advanced-features.

[davidanthoff]

See #25.

Next step for me is to understand inheritance of traits, so that I can define a trait IsIterableTable, where I also check whether the element type of the iterator is a subtype of a specific type.

Thanks for the pointer to method_exists, one of those cases where I would have hunted forever, but that made it all really simple :)

[mauro3]

Cool :-)

There is no inheritance built into SimpleTraits but you can do it by hand, again using generated functions. See the BeautyAndBeast example in the README.

Although the IsIterableTable may not be suitable for base-traits.jl, please post it here if you piece it together.

[mauro3]

Fixed by #25.