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

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 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 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 _______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe

_______________________________________________ Haskell-Cafe mailing list Haskell-C...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe