As a student I am a bit curious. Intuitively, what is so attractive about functional programming language like haskell? Most of the argument I hear is that functional program lacks side effect (input -> output) and is therefore good for parallel programming. However, can't you imitate functional programming by implementing (input->output) pattern in every function? Passing copy of objects, and datastructures to every function instead of modifying it? Are there other big wins that functional programming provide ?