Proposal: Add a strictly accumulating scanl' to Data.List

Joachim Breitner wrote, in https://ghc.haskell.org/trac/ghc/ticket/9345#comment:6, a strictly accumulating version of scanl, to be named scanl'. I was initially concerned about its safety for fusion, but am now convinced of its correctness and believe it should be added to Data.List. scanl' :: (b -> a -> b) -> b -> [a] -> [b] scanl' f a bs = build $ \c n -> a `seq` a `c` foldr (\b g x -> (let b' = f x b in b' `seq` (b' `c` g b'))) (\b -> b `seq` n) bs a