comprehension vs `map'

Dear GHC team, I would like to write [rl {ruleMode = AlwaysApply} | rl <- rules calc] (I) instead of map (\ rl -> rl {ruleMode = AlwaysApply}) $ rules calc (II) and instead of let rs = rules calc in [rl {ruleMode = AlwaysApply} | rl <- rs] (III). But is this reliable in GHC that the compiler converts (I) into something which is not not worse than III ? What about other implementations? Regards, ----------- Mechveliani mechvel@botik.ru