I have lot of ST actions that shall be bound strictly (they write to a buffer), but somewhere between these actions I like to have a "laziness break". I thought I could do this by temporarily switching to Lazy.ST, but this does not work. It follows a simplified example Prelude> :module Control.Monad.ST.Lazy Prelude Control.Monad.ST.Lazy> runST (Monad.liftM2 (,) (return 'a') (undefined::Monad m => m Char)) ('a',*** Exception: Prelude.undefined Prelude Control.Monad.ST.Lazy> Control.Monad.ST.runST (lazyToStrictST $ Monad.liftM2 (,) (return 'a') (undefined::Monad m => m Char)) *** Exception: Prelude.undefined I hoped to get the first answer also for the second command. It seems that conversion from lazy to strict ST also removes laziness breaks. It seems that I have to stick to unsafeInterleaveIO, but I like to know, why the above method does not work.