>>>>> Daniel Díaz <diaz.carr...@gmail.com> writes:
> While tinkering with the foldl package I found a way to convert some pipes
> from pipes-parse into folds from foldl.
You can go even a step further with producers, and encode them as a CPS-based
fold, i.e.:
newtype Source m a = Source
{ getSource :: forall r. Cont (r -> EitherT r m r) a }
deriving Functor
type Conduit a m b = Source m a -> Source m b
type Sink a m r = Source m a -> m r
I do this in the simple-conduit package, but am planning to part the idea over
to pipes as well. This is what a converter from conduit Producers to such a
representation looks like:
adaptFrom :: forall m a. MonadBaseControl IO m => C.Producer m a -> Source m a
adaptFrom (C.ConduitM m) = source go
where
go :: r -> (r -> a -> EitherT r m r) -> EitherT r m r
go z yield = f z m
where
f r (C.HaveOutput p c o) = yield r o >>= \r' -> f r' p `finally` lift c
f r (C.NeedInput _ u) = f r (u ())
f r (C.Done ()) = return r
f r (C.PipeM mp) = lift mp >>= f r
f r (C.Leftover p l) = yield r l >>= flip f p
And for folds from foldl:
fromFoldM :: Monad m => FoldM m a b -> Sink a m b
fromFoldM (FoldM step start done) src =
start >>= (\r -> sink r ((lift .) . step) src) >>= done
toFoldM :: Monad m => Sink a m b -> (forall r. FoldM m a r -> m r) -> m b
toFoldM s f = s $ source $ \z yield ->
lift $ f $ FoldM ((unwrap .) . yield) (return z) return
John
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