Hi folks, I am new to Haskell, coming from an object-oriented realm. As an exercise I am trying to develop an API for accessing/changing services configuration in a SysV-like init environment. Being object-oriented I have defined a model of runlevels/services and a parser that creates instances of these structures from the underlying files. That way my program is able to interrogate the current state of the services configuration. Now I would like to ask how to go about designing the part that will make changes to the config in respect to the following requirement. I want the API to allow making changes only to the model and then applying them back to the filesystem at once. This is to support UI where the user will make various changes and then hit "Apply" or "Cancel". I am imagining this as a sort of simple transactional behaviour of the in-memory model. At the moment I am thinking of passing the whole model around through the various API functions, that will make the changes, but there are many questions to be answered. I need to keep the original model state and a stack of the changes made so that at commit-time I can figure out what has been changed. Also some changes might affect each other (add service followed by remove service, etc.). Anyway the main question is how should such a scenario be approached. Can anyone throw in some ideas to get me started? It might well be that this whole idea of a model with changing state is wrong in respect to FP. If so, what would the right way be? Cheers vlcak