accumulate package:varying

accumulate :: Monad m => (c -> b -> c) -> c -> VarT m b c
varying Control.Varying.Core
Accumulates input values using a folding function and yields that accumulated value each sample. This is analogous to a stepwise foldl. 
>>> testVarOver (accumulate (++) []) $ words "hey there man"
"hey"
"heythere"
"heythereman"

>>> print $ foldl (++) [] $ words "hey there man"
"heythereman"