Why are `sum` and `product` defined via foldMap' and not foldl'?

[Tom Ellis]

On Wed, Dec 23, 2020 at 01:32:38AM -0500, Viktor Dukhovni wrote:
> Is there a compelling reason for:
> 
>     sum = getSum #. foldMap' Sum
>     product = getProduct #. foldMap' Product
> 
> rather than:
> 
>     sum = foldl' (+) 0
>     product = foldl' (*) 1
[...]
> The `foldl'` variant looks substantially more efficient (at least for
> lists), is there some important context in which `foldMap'` is
> preferable?

Have you benchmarked with optimisations on?  I would not be surprised
if a small amount of inlining brings them to the same speed.

Tom