Talk:What a Monad is not
Nicolas Pouillard Comments
I disagree with: "So ordering is not in any way essential to what a monad is."
There is commutative monads, great! Other than that the order is important. Moreover there is nothing wrong to see >>= as a sequencing operator.
I also disagree the "IO is impure" paragraph: Indeed only IO will trigger visible side effects, but this is only due to the common evaluation of IO primitives. Moreover I would say that only the runtime system is impure because it does reduce 'main :: IO ()', other than that we are just building a computation plan in a pure way.
"Monads are not values"
Yes they are! But they are not of the type of their arguments. For instance "return 3" is a value that has (or can have) type "IO Int". But it is not a value of type Int. —Ashley Y 23:44, 23 November 2009 (UTC)
[Nicolas Pouillard]: The point was that the IO monad is not a value, of course there is values of type "IO Int". However the record/dictionary is a value and one could say that it is the monad.
In what sense is Maybe commutative?
It seems to me that
Maybe is not really a commutative monad, as claimed here and at Monad.
do x <- Nothing y <- undefined return (x+y)
do y <- undefined x <- Nothing return (x+y)