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IO vs. other monads

The IO monad is special​

  • IO is a primitive type, and (>>=) and return for IO are primitive functions,
  • there is no (politically correct) function runIO :: IO a -> a , whereas for most other monads there is a corresponding function, or at least some way to get an a out of the monad;
  • values of IO a denote side-effecting programs that can be executed by the run-time system.

Effectful programming​

  • IO being special has little to do with it being a monad;
  • you can use IO an functions on IO very much ignoring the presence of the Monad class;
  • IO is about allowing real side effects to occur; the other types we have seen are entirely pure as far as Haskell is concerned, even though they capture a form of effects.

IO, internally​

If you ask GHCi about IO by saying :i IO , you get

newtype IO a
= GHC.Types.IO (GHC.Prim.State# GHC.Prim.RealWorld
-> (# GHC.Prim.State# GHC.Prim.RealWorld, a #))
-- Defined in ‘GHC.Types ’

So internally, GHC models IO as a kind of state monad having the “real world” as state!