The important issue here is that, when using =$, $=, and =$=, leftovers will discarded. To see this more clearly, realize that the first line of sink is equivalent to:

out1 <- C.injectLeftovers CT.lines C.>+> CL.head

So any leftovers from lines are lost once you move past that line. In order to get this to work, stick the consume inside the same composition:

sink = C.injectLeftovers CT.lines C.>+> do

out1 <- CL.head

out2 <- CL.consume

return (out1, T.unlines out2)

Or:

sink = CT.lines C.=$ do

out1 <- CL.head

out2 <- CL.consume

return (out1, T.unlines out2)

Michael

On Sat, Oct 27, 2012 at 9:20 PM, Myles C. Maxfield <myles.maxfield@gmail.com> wrote:

Hey,
Say I have a stream of Data.Text.Text objects flowing through a
conduit, where the divisions between successive Data.Text.Text items
occur at arbitrary boundaries (maybe the source is sourceFile $=
decode utf8). I'd like to create a Sink that returns a tuple of (the
first line, the rest of the input).

My first attempt at this looks like this:

sink = do
out1 <- CT.lines C.=$ CL.head
out2 <- CL.consume
return (out1, T.concat out2)

However, the following input provides:

runIdentity $ CL.sourceList ["abc\nde", "f\nghi"] C.$$ sink
(Just "abc","f\nghi")

But what I really want is
(Just "abc", "\ndef\nghi")

I think this is due to the auto-termination you mention in [1]. My
guess is that when CT.lines yields the first value, (CL.head then also
yields it,) and execution is auto-terminated before CT.lines gets a
chance to specify any leftovers.

How can I write this sink? (I know I can just use CL.consume and
T.break (== '\n'), but I'm not interested in that. I'm trying to
figure out how to get the behavior I'm looking for with conduits.)

Thanks,
Myles

[1] http://hackage.haskell.org/packages/archive/conduit/0.5.2.7/doc/html/Data-Conduit.html