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.