Monoid[Double] and Monoid[Float] are incorrect

[xuwei-k]

xuwei-k@6dbad2ddff10

[ceedubs]

I believe this was a conscious decision by @non with the assumption that people generally know about the gotchas with floating point arithmetic but still want to be able to add floats and doubles. Is that an accurate assessment, @non?

It seems to me like the options are either:

1. Move these into alleycats (but I suspect people using algebra without Cats still want these)
2. Be explicit about the fact that we know this is naughty but we are doing it anyway. In this case we should add some documentation about this.

I could see an argument that the blame lies as much with the Eq instances as it does with the Semigroup and Monoid instances.

[xuwei-k]

I don't mean it.

https://github.com/non/cats/blob/682a39423bca461ffb37a0ff7d12b2a6fda7099e/core/src/main/scala/cats/std/anyval.scala#L97-L98

def combine(x: Float, y: Float): Float = x + y
def empty: Float = 1F

the combine defined by +, but empty is 1F.
I think empty should be 0F or def combine(x: Float, y: Float): Float = x * y

[ceedubs]

@xuwei-k oh wow thanks for pointing that out. I can't believe we let that go for so long! I'll submit a fix.