Hello! I have a program that is using ST.Strict, which works fine. However, the program needs to be extended, and to do that, lazy evaluation is needed. As a result of that, I have switched to ST.Lazy to be able to do stuff like foo y = do x <- something xs <- foo (y+1) return (x:xs) However, when running the program compiled with ST.Lazy, the following is outputted: [tobsi@wobsi]$ ./runnerLazy looper.hex runnerLazy: <<loop>> The very same program compiled with ST.Strict outputs: [tobsi@wobsi]$ ./runner looper.hex 83298556 The code that is actually computing stuff is this: loopSys :: Int -> CPU s Int loopSys cc = do instr <- fetch if instr == 0xEA --NOP then return cc else do c <- execute instr loopSys $! (cc+c) The CPU type looks as follows: type CPU s a = ReaderT (SysEnv s) (ST s) a The program is run like runReaderT (loopSys 0) which in turn is being runST'd and then printed Does anyone know why the program just outputs <<loop>> when compiled under ghc 6.10.2, and runs perfectly fine under ghc 6.8.2? The program is compiled with --make and -O2 Tobias Olausson tobsan@gmail.com