implements bifoldable test

[solomon-b]

Resolves issue #48. We could reduce the number of type parameters by merging c and m, but I didn't want it to appear that bifoldr f g z t ≡ appEndo (bifoldMap (Endo . f) (Endo . g) t) z requires a monoid on c

[sjakobi]

Thanks! :)

Do you have a source for these laws?

[solomon-b]

They come from the Data.Bifoldable haddocks.

There is one more law I forget to include:

bifoldMap f g = bifold . bimap f g

This is only relevant if you have a Bifunctor. Would we put it in a separate test?

[sjakobi]

There is one more law I forget to include:

bifoldMap f g = bifold . bimap f g

This is only relevant if you have a Bifunctor. Would we put it in a separate test?

Yes, we can add this in the style of the existing foldableFunctor test.

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Regarding compatibility with GHC 7.10 and 8.0, I'll try to drop support for these soon, so we can merge your PRs without breaking CI.

[sjakobi]

Could you rebase?

[solomon-b]

Could you rebase?

done!