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

Is there a compelling reason for: sum = getSum #. foldMap' Sum product = getProduct #. foldMap' Product rather than: sum = foldl' (+) 0 product = foldl' (*) 1 A quick ghci session with today's GHC head yields: λ> import qualified Data.Foldable as F λ> :set +s λ> F.sum [0..10000000] 50000005000000 (2.98 secs, 1,612,368,368 bytes) λ> F.foldl' (+) 0 [0..10000000] 50000005000000 (0.28 secs, 880,065,112 bytes) The `foldl'` variant looks substantially more efficient (at least for lists), is there some important context in which `foldMap'` is preferable? -- Viktor.