" The only technical catch is that you aren't allowed to change the type of the contents in the function. That means if you start with something of type A in th

" The only technical catch is that you aren't allowed to change the type of the contents in the function. That means if you start with something of type A in the container, you have to get a container containing something of type A at each stage of the pipeline." I don't think this is correct. What doesn't change is the overall type constructor (e.g. if you start with a list, you'll still end up with a list... If you start with an IO action, you'll still have an IO action). But the type that is wrapped by the container can and usually does change. e.g. if you "map" over a list of A's, you'll get a list of whatever type your map function returns - still a list, but a list of B's, not a list of A's. fmap :: (a->b) -> [a] -> [b].