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?
I know that unsafePerformIO can do this, but I thought that was a bit of a hack. I'm still trying to understand how monads interact with types so I am interested in this as well. Kevin On Jul 30, 10:11 am, Kevin Jardine <kevinjard...@gmail.com> wrote: > 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...@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...@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 > > > _______________________________________________ > > > 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 > > _______________________________________________ > Haskell-Cafe mailing list > haskell-c...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
