I've felt need to use State monads in two distinct situations: ... And I've seen two distict aproaches. Using a State monad like one provided with GHC (1), or a state monad like the one defined in the paper "Moands for the Working Haskell Programmer" (2). In (1), I can see how I can take advantage of the monad and make updates in place using referencies. But then you'll have to either: - pass the referencies as a parameter to the functions where you use them (needless too say that this defeats the (my) main purpose of using a state monad which is in case A, keep track of data with minimal signature change ) You can do this conveniently using implicit parameters bound to your references: that way you don't have to add parameters to all your function definitions just to pass in a reference. (The types do change, though). I would probably use one implicit parameter bound to a record of references. ... Another state issue, sometimes I have some values that I want to keep constant through the whole algorithm. Example: (some very simple NNs for instance) - learning rate - activation function - maximum number of steps - minimum error required So I'll just declare them as 'constants'. But what if I decide I want the user to be able to chose? Now I got two options: - pass them around as values to all the functions - And signatures get HUGE - just pass them to a higher level function that will encapsulate the functions that use them... which is ugly and complicates everything because you can't test the lower level functions in the interpreter. Am I missing something here or is this really the best you can do? Implicit parameters solve this too. Read http://www.cse.ogi.edu/~mbs/pub/implicit_parameters/ I've written a short article on global variables in Haskell, which explains this particular application. http://www.md.chalmers.se/~rjmh/Globals.ps John Hughes