Brian Brunswick wrote:
One thing that I keep seeing people say (not you), is that monads /sequence/ side effects. This is wrong, or at least a limited picture.
/All/ of the above structures are about combining compatible things things together in a row. /None/ of them force any particular order of evaluation - that all comes from the particular instance. So its only a particular feature of IO that it sequences the side effects. Others don't - we can have a lazy State monad that just builds up big thunks.
I am a bit astonished. Let's take the simplest example: Maybe. The effect in question is the premature abortion of a computation (when Nothing is returned). And of course Maybe sequences these effects, that's what you use it for: the _first_ action to be encountered that returns Nothing aborts the computation. Clearly sequencing goes on here. Similar with the Error Monad (i.e. Either Err, for some Err type). I won't talk about List Monad because I always had difficulty understanding the List Monad. What about State? The effect is reading/writing the state. Again, the State Monad takes care that these effects get sequenced, and again that's what you expect it to do for you. And so on... This is -- of course -- not a proof, so maybe there /are/ Monads that don't sequence (their) effects. I'd be most interested to see an example, if there is one, to bring myself nearer to the -- unattainable -- goal of full enlightenment wrt Monads. Cheers Ben