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

Kevin Jardine kevinjardine at gmail.com
Fri Jul 30 03:59:37 EDT 2010

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?


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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