[Haskell-cafe] Re: Can we come out of a monad?

[Anton van Straaten]

 >> On Jul 30, 9:59 am, Kevin Jardine <kevinjard... at gmail.com> wrote:
 >>
 >>> The original poster states that the type of modifiedImage is "simply
 >>> ByteString" but given that it calls execState, is that possible?
 >>> Would it not be State ByteString?

 >> Oops, I should have written
 >>
 >> IO ByteString
 >>
 >> as the State stuff is only *inside* execState.
 >>
 >> But a monad none the less?

State is a pure monad that doesn't involve IO.  It works by threading a 
state value through the monadic computation, so old states are discarded 
and new states are passed on, and no actual mutation is involved.  This 
means there's no need to bring IO into it.

If you look at the type signature of execState, you'll see that unless 
the state type 's' involves IO, the return type can't involve IO.

It can help to run little examples of this.  Here's a GHCi transcript:

Prelude> :m Control.Monad.State
Prelude Control.Monad.State> let addToState :: Int -> State Int (); 
addToState x = do s <- get; put (s+x)
Prelude Control.Monad.State> let mAdd4 = addToState 4
Prelude Control.Monad.State> :t mAdd4
m :: State Int ()
Prelude Control.Monad.State> let s = execState mAdd4 2
Prelude Control.Monad.State> :t s
s :: Int
Prelude Control.Monad.State> s
6

In the above, addToState is a monadic function that adds its argument x 
to the current state.  mAdd4 is a monadic value that adds 4 to whatever 
state it's eventually provided with.  When execState provides it with an 
initial state of 2, the monadic computation is run, and the returned 
result is 6, which is an Int, not a monadic type.

> Or is it possible to call a function in a monad and return a pure
> result? I think that is what the original poster was asking?

If you use a function like execState (depending on the monad), you can 
typically run a monadic computation and get a non-monadic result. 
However, if you're doing that inside a monadic function, you still have 
to return a value of monadic type - so typically, you use 'return', 
which lifts a value into the monad.

> I know that unsafePerformIO can do this, but I thought that was a bit
> of a hack.

IO is a special monad which has side effects.  unsafePerformIO is "just" 
one of the functions that can run IO actions, but because the monad has 
side effects, this is unsafe in general.  With a pure monad like State, 
there's no such issue.

Anton