Right, everything is tailored to use with `foldl`, though I haven't
documented this yet.
In general everything marked with a final single quote is tailored for use
with `mapsM`
-- which is like `maps` but allows a monadic return type:
maps :: (Monad m, Functor f) => (forall x. f x -> g x) -> Stream f
m r -> Stream g m r -- as in pipes group
mapsM :: (Monad m, Functor f) => (forall x. f x -> m (g x)) ->
Stream f m r -> Stream g m r
thus
mapsM S.sum' :: (Monad m, Num a) => Stream (Stream (Of a) m) m r ->
Stream (Of a) m r
but more importantly
mapsM (L.purely S.fold' fld) :: Monad m => Fold a b -> Stream
(Stream (Of a) m) m r -> Stream (Of b) m r
\fld -> mapsM (L.purely S.fold' fld) . S.group
:: (Eq a, Monad m) => LB.Fold a b -> Stream (Of a) m r ->
Stream (Of b) m r
and better yet
\fld -> mapsM (L.impurely S.foldM' fld) . chunksOf 20
:: Monad m => L.FoldM m a b -> Stream (Of a) m r -> Stream (Of
b) m r
and so on.
The use of the single quote, and the duplication, are a little obnoxious,
of course.
(The same device is used in the bytestring modules.) I was thinking of
dropping the folds
that just return things like `m Int` in favor of the others, which are
`mapsM`-able,
and fit better with the idea of the package.
But the simple ones looked so good in ghci when I was testing examples...
It is also easy to write accumulating folds in various ways. I am trying to
think of a way of
presenting this that is easy to take in, so one doesn't get lost in the
rank 2 arguments and so on.
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