[skybrian]

The way I think of it is that an IO value is a program where all the Haskell code runs in callbacks. This is a bit like a JavaScript program where everything runs in a callback. The major difference is that callbacks written in Haskell are constrained to be pure functions.

The IO type is rather rigid in that there is always a next callback, which implicitly contains the entire program state as part of the function closure. In a reactive programming language like Elm you can have many functions that may be the next callback depending on what the next event is, along with all the callbacks that run as part of the signal graph. Purity is about how constrained the callbacks are, not about the overall structure of the program.