[Haskell-cafe] Sum and Product do not respect the Monoid laws

[Jason Choy]

>
> subject to certain caveats.  It's not unfair to say that
> floating point multiplication is (nearly) associative
> "within a few ulp".
>

I'm not disputing this.

However, you can't deny that this monoid law is broken for the floating
point operations:

mappend x (mappend y z) = mappend (mappend x y) z

Perhaps I'm being pedantic, but this law should hold for all x, y, z, and
it clearly doesn't.
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