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

[Kevin Jardine]

Oops, I should have written

IO ByteString

as the State stuff is only *inside* execState.

But a monad none the less?

Kevin

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?
>
> Kevin
>
> On Jul 30, 9:49 am, Anton van Straaten <an... at appsolutions.com> wrote:
>
> > C K Kashyap wrote:
> > > In the code here -
> > >http://hpaste.org/fastcgi/hpaste.fcgi/view?id=28393#a28393
> > > If I look at the type of modifiedImage, its simply ByteString - but
> > > isn't it actually getting into and back out of the state monad? I am of
> > > the understanding that once you into a monad, you cant get out of it? Is
> > > this breaking the "monad" scheme?
>
> > modifiedImage uses the execState function, which has the following type:
>
> >    execState :: State s a -> s -> s
>
> > In other words, it applies a State monad value to a state, and returns a
> > new state.  Its entire purpose is to "run" the monad and obtain the
> > resulting state.
>
> > A monadic value of type "State s a" is a kind of delayed computation
> > that doesn't do anything until you apply it to a state, using a function
> > like execState or evalState.  Once you do that, the computation runs,
> > the monad is "evaluated away", and a result is returned.
>
> > The issue about not being able to escape that (I think) you're referring
> > to applies to the functions "within" that computation.  A State monad
> > computation typically consists of a chain of monadic functions of type
> > (a -> State s b) composed using bind (>>=).  A function in that composed
> > chain has to return a monadic value, which constrains the ability of
> > such a function to escape from the monad.
>
> > Within a monadic function, you may deal directly with states and
> > non-monadic values, and you may run functions like evalState or
> > execState which eliminate monads, but the function still has to return a
> > monadic value in the end, e.g. using "return" to lift an ordinary value
> > into the monad.
>
> > Anton
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