Hi! Now, i'm getting familiar with monads, but there is a little discredit about one-way monads. For example, in monads presented in Monads for Functional Programming by Philip Wadler, they are all two-way monads, internal data can be extracted from the monad, and these data, call them state, are in deed _local_ to a function using the monadic computation, so the state there is just a little magic, and they are really just functions(as in a pure language). An other example is Random. I've figured out, that when initializing a generator, it uses the actual timestamp and cputime to initialize. If it wouldnt happen, two distinct uses of Random in a program would result the same random number sequence. Here comes IO and one-way monads, where the internal state can not be extacted, and seems, that the internal data is global to the program. Hows that could be? Is it just because main::IO() or because the implementation of IO uses external C functions and there are no internal state in the monad itself at all? And why one can not write a function that makes an IO computation and the return type does not include and IO contructor? It is a feature of the language and specific to IO, or anybody could write a monad with this property(one-way), assuming the implementation does not use external languages? Or the one-way property is just that, there is no such functions, that allow extracting internal data? Also, is there any other monad, that has the one-way property? Thanks, -- Zsolt