[Haskell-cafe] Arrow laws of Netwire

[KAction at gnu.org]

Hello! In process of adapting 'netwire-5.0.0' to my needs I discovered
following strange thing. Let us consider following simple program:


	{-# LANGUAGE Arrows #-}
	import           FRP.Netwire
	import Data.Monoid

        -- I almost sure this is correct, since it is copied
        -- from "Programming with Arrows", J. Hughes
	mapA :: (ArrowChoice a) => a b c -> a [b] [c]
	mapA f = proc input ->
	  case input of
	    [] -> returnA -< []
	    z:zs -> do y_ <- f -< z
	               ys_ <- mapA f -< zs
	               returnA -< y_:ys_

        mconcatA :: (ArrowChoice a, Monoid m) => a b m -> a [b] m
	mconcatA f = mapA f >>> arr mconcat

        -- Note the commented line.
	wire :: (Monad m, HasTime t s) => Wire s () m a Double
	wire = pure (Sum 1.0)
	       -- >>> arr (: []) >>> mconcatA returnA
	       >>> arr getSum
	       >>> integral 10

	main = testWire (countSession_ 1) wire

Problem is that, compiled with ghc-8.0.1 this program hangs if I
uncomment second line in body of ``wire`` function[1], which is wierd,
since assuming monoid and arrow laws, I believe

        -- (Arrow a, Monoid e) => a e e
	arr (: []) >>> mconcatA returnA = returnA

Is it false? Any suggestions?

.. [1] with that line commented program works and prints sequence of
       numbers, with every next over previous.