Restricting to Integral only would be overconstraining - Sum Rational
is perfectly well behaved as a monoid. I could go either way on
whether Double and Float should be excluded - I'm actually slightly
sympathetic to the notion that they shouldn't even be Num.
On Fri, Sep 19, 2014 at 6:58 AM, Jason Choy
The Monoid instances for Sum and Product do not respect the Monoid laws.
The instances are:
Num a => instance Monoid (Sum a) Num a => instance Monoid (Product a)
Float and Double are instances of the Num typeclass, however, floating point addition and multiplication are not associative. Here's an example:
(Sum 1234.567 `mappend` Sum 45.67834) `mappend` Sum 0.0004 Sum {getSum = 1280.2457399999998}
Sum 1234.567 `mappend` (Sum 45.67834 `mappend` Sum 0.0004) Sum {getSum = 1280.24574}
Shouldn't these instances be constrained to Integral types?
Jason
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