Because the related change proposal(s) are never accepted nor implemented. Most recent one is quite recent though. https://mail.haskell.org/pipermail/libraries/2020-October/030862.html - Oleg On 23.12.2020 8.32, 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
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?