I am looking at the proc notation de-sugar and I see results like this when using a Free Arrow (mostly copied from [1]):

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