On Tue, Apr 17, 2012 at 4:10 PM, Chris Smith
One thing that Gabriel and Michael have been talking about, though, that seems to be missing here, is a way for a pipe to indicate that it's finished with its upstream portion, so that upstream finalizers can be immediately run without waiting for the downstream parts of the pipe to complete.
Yes, that is indeed a limitation of pipes-core, but it's still possible to achieve early finalization when needed, by explicitly returning a "continuation" pipe. The following operator (>!>) :: Monad m => Pipe a b m r -> Pipe b c m (Pipe a c m r) -> Pipe a c m r p1 >!> p2 = do r0 <- (Left <$> p1) >+> (Right <$> p2) case r0 of Left r -> return r Right p' -> p' could be used to compose such a pipe. Alternatively, one can use 'Control.Pipe.Zip.controllable' together with 'loopP' (there's a similar example in pipes-extra/Examples/finalizers/complex.hs). Unfortunately, both approaches are not compositional. They require you to structure the whole pipeline around them. I suspect it might be possible to incorporate these ideas into the Pipe type without losing associativity, but I don't have any motivating examples where this feature would actually be useful. Do you have anything in mind, Chris? BR, Paolo