warn :: String → IO Int warn = return 1 << putStrLn -- causes an error -- = \msg → return 1 << putStrLn msg -- works just fine -- = \msg → putStrLn msg >> return 1 -- works just fine (<<) :: Monad m ⇒ m b → m a → m b b << a = a >>= \_ → b Why do I get this compile-time error?? How can one define << ? cetin@linux-d312:~/lab/test/qths/p> ghc -fglasgow-exts -O2 -o d64x --make demo2.hs system.hs [1 of 2] Compiling Netman.System ( system.hs, system.o ) system.hs:23:14: No instance for (Num (IO Int)) arising from the literal `1' at system.hs:23:14 Possible fix: add an instance declaration for (Num (IO Int)) In the first argument of `return', namely `1' In the first argument of `(<<)', namely `return 1' In the expression: return 1 << putStrLn