Hi Haskell gurus, I'm very puzzled on some code I saw in GHC Monad.StateT (which is about state monad transformers) source and hope you can kindly give me some insight into this. newtype StateT s m a = S (s -> m (a,s)) instance MonadPlus m => MonadPlus (StateT s m) where mzero = lift mzero mplus m1 m2 = do s <- peek let m1' = runState s m1 m2' = runState s m2 ???????----> ~(a,s') <- lift (mplus m1' m2') poke s' return a To illustrate my puzzle, say m is of List type and runState s m1 = m1' = [(a1, s1)] runState s m2 = m2' = [(a2, s2)] With the definition of lift (also in the StateT.hs file) as instance Trans (StateT s) where lift m = S (\s -> do x <- m return (x,s)) I got lift (mplus m1' m2') = lift ([(a1,s1), (a2,s2)]) = S (\s -> [ ((a1,s1),s), ((a2,s2),s)]) I'm puzzled over this line: ~(a,s') <- lift (mplus m1' m2') I think ~(a,s') shall be the value of the (StateT s m a) which is generated by "lift (mplus m1' m2')". My questions are - why use lazy pattern? - how is ~(a,s') extracted from (StateT s m a)? This looks like magic to me. In the example I have (a1,s1) and (a2,s2) in the lifted monad, but it seems (a,s') only represents one at a time. It looks like how data is pulled out of a List monad. Your help is highly appreciated. Thanks, Fan