I am looking at the proc notation de-sugar and I see results like this when using a Free Arrow (mostly copied from [1]): line2 = proc n -> do Effect getURLSum *** Effect getURLSum -< n Seq [Pure ] (Seq [Pure ] (Seq [Pure ] (Seq [Pure ](Par <Effect > {Effect } ) ) ) ) while this is so much simpler: line2 = Effect getURLSum *** Effect getURLSum Par <Effect > {Effect } Those `Seq [Pure ]` sequences come from application of (.) and I have noticed many similar inefficiencies in the Arrow preprocessor. Eventually the goal would be to optimize when possible, for example I started looking into this in order to use `concurrently` for (***) when in IO. There was a rewrite mentioned here [2]. The deSugar/DsArrows.hs [3] looks convoluted. Any progress or work needed? Or are Arrows not used much and not worth the effort? [1] http://stackoverflow.com/questions/12001350/useful-operations-on-free-arrows [2]https://mail.haskell.org/pipermail/haskell-cafe/2013-August/109795.html [3]https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsArrows.hs -Tom