[Haskell-beginners] runtime error <<loop>> when using -O compile option

[Gerold Meisinger]

Hello!

I'm working on a computer game using Yampa and I get the following
runtime error:

$ myprog: <<loop>>

when compiling with

$ ghc --make MyProg.hs -o myprog -O
(without -O it works fine)

I stripped the bug down to the program below. What's funny is that the
error disappears under certain "odd circumstances" (marked as #1-#4). My
questions are:
1. How can I avoid this bug without introducing one of the "odd
circumstances"?
2. Why is it that I get this error?
3. How would you hunt down such a bug? Originally I got no clue where it
came from, so I just took the program apart piece by piece.

       {-# LANGUAGE Arrows #-}

       module Main (main) where

       import FRP.Yampa

       type ObjIn = Event () -- loop #1
       --type ObjIn = Bool -- no loop #1

       type ObjOut = (String, Int) -- loop #2
       --type ObjOut = Int         -- no loop #2

       type GameObj = SF ObjIn ObjOut

       testObj :: GameObj
       testObj = proc hit -> do
           returnA -< ("testObj", 1) -- loop #2
       --    returnA -< 1            -- no loop #2

       process :: [GameObj] -> SF () [ObjOut]
       process objs = proc _ -> do
           rec
               gamestate <- par logic objs
                   -< gamestate -- loop #3 (recursive definition!)
       --            -< [] -- no loop #3

           returnA -< gamestate

       logic :: [ObjOut] -> [sf] -> [(ObjIn, sf)]
       logic gamestate objs = map route objs
         where
           route obj =
               (if null (foo gamestate) then NoEvent else NoEvent, obj)
       -- loop #1
       --        (if null (foo gamestate) then False else False, obj)
       -- no loop #1

       foo :: [ObjOut] -> [ObjOut]
       foo [] = []
       foo objs = concat (collisions objs)
         where
           collisions [] = []
           collisions (out:objs') =
               [[out, out'] | out' <- objs, out `collide` out'] -- loop
       #4
       --        [[out, out'] | out' <- objs, True] -- no loop #4

       collide :: ObjOut -> ObjOut -> Bool
       collide (_, p) (_, p') = True -- loop #2
       --collide p p' = True         -- no loop #2


       main :: IO ()
       main = do
           putStrLn . show $ embed (process [testObj]) ((), [(1.0,
       Nothing)])

(Btw: I re-opened a bug report:
http://hackage.haskell.org/trac/ghc/ticket/2722#comment:10 )