Talk:What a Monad is not

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Revision as of 17:15, 19 June 2014 by Dfeuer (talk | contribs) (In what sense is Maybe commutative?)

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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.

Although I agree that the common issue is too mix monad and impurity, and the impurity question is only releated to IO.

"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)

produces Nothing whereas

do
  y <- undefined
  x <- Nothing
  return (x+y)

produces undefined. --Dfeuer (talk) 17:15, 19 June 2014 (UTC)