Using GHCi I found it informative to see that IO indeed is a kind of state
monad. Here's a GHCi session to show that:
Prelude> :m GHC.Prim
Prelude GHC.Prim> :i IO
newtype IO a
= GHC.IOBase.IO (State# RealWorld -> (# State# RealWorld, a #))
-- Defined in GHC.IOBase
instance Monad IO -- Defined in GHC.IOBase
instance Functor IO -- Defined in GHC.IOBase
So every "IO a" action takes the RealWorld as input, and outputs
the RealWorld and some extra value "a" :)
2008/12/23 Adrian Neumann
Am 23.12.2008 um 15:16 schrieb Hans van Thiel:
Hello All,
I just saw somewhere that one of the purposes of monads is to capture side effects. I understand what a side effect is in C, for example. Say you want to switch the contents of two variables. Then you need a third temporary variable to store an intermediate result. If this is global, then it will be changed by the operation.
But the two variables have also changed. After all they have different values after the switch. You see, even locally changing a variable is a side-effect. It changes the state of the program. Pure Haskell programs on the other hand have no notion of state, there are no variables which can change their value. Every time you want to manipulate something you're actually generating an new copy. You mustn't think of a haskell program as a series of changes to some state.
However when you *do* want state you can simulate it with a monad. The IO Monad is a special case here, since its actions don't change your program, they change the "world" the program is running in (writing files etc.). getLine etc are functions when you think of them as taking a hidden parameter, the state of the world. So getChar would become
getChar :: World -> (Char,World)
but the world stays hidden inside the IO Monad.
Regards,
Adrian
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