In ghci:
Data.Semigroup> 0.1 :: Sum Double
<interactive>:2:1: error: [GHC-39999]
• No instance for ‘Fractional (Sum Double)’
arising from the literal ‘0.1’
• In the expression: 0.1 :: Sum Double
In an equation for ‘it’: it = 0.1 :: Sum Double
Is this an oversight, or is there a principled reason not to make this instance? (I know that addition is not associative for Double -- but instance Monoid (Sum Double), the actually problematic instance for that objection, already exists anyway!)
@NaïmFavier answered this in their comment with a reference to ticket #18161, as Emily Pillmore said:
It'd be confusing for me to divide two Sum values. I think I'm leaning harder in the "no" direction because intent matters.
It thus to some extent depends on whether you see Sum as a construct that holds values, and then is only used to fold these, and then retrieve the value the result holds, or as not much more as a "marker interface" [wiki] and thus that Sum a is noth much more than an a that is marked as a sum, and where typeclasses a is a member of, also imply that Sum a should be a member of.
For the last one, that thus should mean that one essentially tries to implement it as:
instance C a => C (Sum a) where
-- …for as much Cs as possible, except the ones for which the "marker" is used (in this case thus Monoid and Semigroup).