Thanks a lot! Althaugh I have some understanding of the Haskell basics and the most important monads, I feel that I have to see more well designed code in order to become a good Haskeller. Can somebody make suggestions what materials are best to work through in order to achieve this? Are there easy research papers about Haskell programming? Or should I try the Monad.Reader? I'm looking for topics that either can be used directly in many situations or that show some functional principles that boost my creativity and functional thinking. Regards, Tim 2011/11/19 Felipe Almeida Lessa

On Sat, Nov 19, 2011 at 6:08 PM, Tim Baumgartner wrote:

...

I have not yet gained a good understanding of the continuation monad, but I wonder if it could be used here. What would a clean solution look like? Perhaps there are other things that need to be changed as well?

Your 'Interaction' data type is actually an instance of the more general "operational monad" (as named by Heinrich Apfelmus) or "prompt monad" (as named by Ryan Ingram). You will ready-to-use implementations on the packages MonadPrompt [1] and operational [2,3]. Reading their documentation you'll find some links about the development of these concepts. MonadPrompt uses a Cont-style implementation, while operational uses a simple abstract data type together with a viewing function that avoids O(n²) behavior.

If you find it hard to describe your 'Interaction' using operational (easiest to use package, IMO), please send us another e-mail.

Cheers!

[1] http://hackage.haskell.org/package/MonadPrompt [2] http://hackage.haskell.org/package/operational [3] http://heinrichapfelmus.github.com/operational/Documentation.html

-- Felipe.

I've read Martin Erwig and Steve Kollmansberger's *Probabilistic functional programming in Haskell*. Does someone know if the library they are talking about is available on hackage? 2011/11/21 Heinrich Apfelmus

Tim Baumgartner wrote:

...

Thanks a lot! Althaugh I have some understanding of the Haskell basics and the most important monads, I feel that I have to see more well designed code in order to become a good Haskeller. Can somebody make suggestions what materials are best to work through in order to achieve this? Are there easy research papers about Haskell programming? Or should I try the Monad.Reader? I'm looking for topics that either can be used directly in many situations or that show some functional principles that boost my creativity and functional thinking.

You may want to start with the Functional Pearls

http://www.haskell.org/**haskellwiki/Research_papers/**Functional_pearlshttp://www.haskell.org/haskellwiki/Research_papers/Functional_pearls

In particular, I recommend

* Richard Bird. A program to solve Sudoku. * Graham Hutton. The countdown problem. * Martin Erwig and Steve Kollmansberger. Probabilistic functional programming in Haskell. * Conor McBride and Ross Paterson. Applicative Programming with Effects.

Best regards, Heinrich Apfelmus

-- http://apfelmus.nfshost.com

______________________________**_________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/**mailman/listinfo/haskell-cafehttp://www.haskell.org/mailman/listinfo/haskell-cafe

Hi Heinrich, I read your article about the operational monad and found it really very enlightening. So I'm curious to work through the material you linked below. Thanks! Regards Tim 2011/11/21 Heinrich Apfelmus

Tim Baumgartner wrote:

...

Thanks a lot! Althaugh I have some understanding of the Haskell basics and the most important monads, I feel that I have to see more well designed code in order to become a good Haskeller. Can somebody make suggestions what materials are best to work through in order to achieve this? Are there easy research papers about Haskell programming? Or should I try the Monad.Reader? I'm looking for topics that either can be used directly in many situations or that show some functional principles that boost my creativity and functional thinking.

You may want to start with the Functional Pearls

http://www.haskell.org/**haskellwiki/Research_papers/**Functional_pearlshttp://www.haskell.org/haskellwiki/Research_papers/Functional_pearls

In particular, I recommend

* Richard Bird. A program to solve Sudoku. * Graham Hutton. The countdown problem. * Martin Erwig and Steve Kollmansberger. Probabilistic functional programming in Haskell. * Conor McBride and Ross Paterson. Applicative Programming with Effects.

Best regards, Heinrich Apfelmus

-- http://apfelmus.nfshost.com

______________________________**_________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/**mailman/listinfo/haskell-cafehttp://www.haskell.org/mailman/listinfo/haskell-cafe

Free Monads. It's amazing to be confronted again with notions I learned more than ten years ago for groups. I have to admit that I'm probably not yet prepared for a deeper understanding of this, but hopefully I will return to it later ;-) Is Cont free as well? I guess so because I heard it's sometimes called the mother of all monads. Regards Tim 2011/11/21 David Menendez

On Sat, Nov 19, 2011 at 3:29 PM, Felipe Almeida Lessa wrote:

...

On Sat, Nov 19, 2011 at 6:08 PM, Tim Baumgartner wrote:

...

I have not yet gained a good understanding of the continuation monad, but I wonder if it could be used here. What would a clean solution look like? Perhaps there are other things that need to be changed as well?

Your 'Interaction' data type is actually an instance of the more general "operational monad" (as named by Heinrich Apfelmus) or "prompt monad" (as named by Ryan Ingram).

Both of which are just disguised free monads. For reference:

data Free f a = Val a | Wrap (f (Free f a))

foldFree :: Functor f => (a -> b) -> (f b -> b) -> Free f a -> b foldFree v w (Val a) = v a foldFree v w (Wrap t) = w $ fmap (foldFree v w) t

instance Functor f => Monad (Free f) where return = Val m >>= f = foldFree f Wrap m

To use Free, just find the signature functor for Interaction by replacing the recursive instances with a new type variable,

data InteractionF a b x = ExitF b | OutputF b x | InputF (a -> x)

instance Functor (InteractionF a b) where fmap f (ExitF b) = ExitF b fmap f (OutputF b x) = OutputF b (f x) fmap f (InputF g) = InputF (f . g)

roll :: InteractionF a b (Interaction a b) -> Interaction a b roll (ExitF b) = Exit b roll (OutputF b x) = Output b x roll (InputF g) = Input g

type InteractionM a b = Free (InteractionF a b)

runM :: InteractionM a b b -> Interaction a b runM = foldFree Exit roll

exit :: b -> InteractionM a b c exit b = Wrap (ExitF b)

output :: b -> InteractionM a b () output b = Wrap (OutputF b (Val ()))

input :: InteractionM a b a input = Wrap (InputF Val)

-- Dave Menendez http://www.eyrie.org/~zednenem/

2011/11/21 David Menendez

Here's how you might implement your monad using Cont,

type InteractionM a b = Cont (Interaction a b)

exit b = Cont $ \k -> Exit b output b = Cont $ \k -> Output b (k ()) input = Cont $ \k -> Input k runM m = runCont m Exit

That's what I originally wanted to know. I guess I struggled with the definition of output. Oh, there's so much more to learn... Thanks, Tim