Re: [Haskell-beginners] Implementing a Local Propagation Network

Stephen, On Wed, May 19, 2010 at 6:55 PM, Stephen Blackheath [to Haskell-Beginners] wrote:

Patrick,

I think that looks like about the best way you could do it.  If I were doing it, I would probably write it without the state monad, with all the LPN ()'s becoming Network -> Network, and the code wouldn't be all that different, except that 'multiplier' and 'adder' would become slightly less readable.

If you did that, then in those cases (where your algorithm is described by a sequence of modifications) you can say 'flip execState network $ do ...'.  Then all the 'revokeWire sum name's have to become 'modify $ revokeWire sum name'.

For learning purposes, I tried what you suggested and you were absolutely right. With the arguments properly ordered, i.e. (... -> Network -> Network), the code remains pretty similar without the State monad. Proper ordering of arguments in combination with partial application and (.) is really fantastic! I also hadn't realized that you could create "monadic" code "on-the-fly" like this: adder :: NodeName -> WireName -> WireName -> WireName -> Network -> Network adder name ad1 ad2 sum net = flip execState net $ do if isJust v1 && isJust v2 then modify $ setWire sum name (fromJust v1 + fromJust v2) else modify $ revokeWire sum name if isJust v1 && isJust vs then modify $ setWire ad2 name (fromJust vs - fromJust v1) else modify $ revokeWire ad2 name if isJust v2 && isJust vs then modify $ setWire ad1 name (fromJust vs - fromJust v2) else modify $ revokeWire ad1 name where v1 = lookupWireValue ad1 name net v2 = lookupWireValue ad2 name net vs = lookupWireValue sum name net That's very neat. For the testing code I came up with 2 variants, I'm still not sure which one I like best: fc = input f 212 . addMultiplier "*" k l m . addAdder "+" j k i . addConstant "32" 32 j . addConstant "5/9" (5/9) l . addIO f i . addIO c m . foldr addWire networkMake $ wires where wires@[i, j, k, l, m] = ["i", "j", "k", "l", "m"] [f, c] = ["Fahrenheit", "Celsius"] fcm = flip execState networkMake $ do mapM_ (\w -> modify $ addWire w) wires modify $ addIO f i modify $ addIO c m modify $ addConstant "32" 32 j modify $ addConstant "5/9" (5/9) l modify $ addAdder "+" j k i modify $ addMultiplier "*" k l m modify $ input f 212 where wires@[i, j, k, l, m] = ["i", "j", "k", "l", "m"] [f, c] = ["Fahrenheit", "Celsius"] Thanks a lot for your insight, Patrick

There's no particular reason why that's better - it's just style.  If your main program is going to describe a whole lot of complex transformations that happen in sequence, then your way would likely be better than my suggested way.

Steve

On 20/05/10 07:12, Patrick LeBoutillier wrote:

...

Stephen,

Thanks for the advice, finally I ended up using a State Monad and names (String) as symbolic references.

Here is what I came up with: http://pastebin.com/gqkP2sWy

Here is some test code:

import LPN import Control.Monad.State

testfc = snd $ runState fc networkMake

fc :: LPN () fc = do   i:j:k:l:m:[] <- sequence $ map addWire ["i", "j", "k", "l", "m"]   f <- addIO "Fahrenheit" i   c <- addIO "Celsius" m   addConstant "32" 32 j   addConstant "5/9" (5/9) l   addAdder "+" j k i   addMultiplier "*" k l m   input f 212

At first it felt kind of messy, but as I kept refactoring and pushing stuff into the monad it became a lot cleaner and felt less heavy. I learned a lot about the State Monad doing this.

Thanks,

Patrick

On Mon, May 17, 2010 at 10:41 PM, Stephen Blackheath [to Haskell-Beginners] wrote:

...

Patrick,

If you want to implement it in a functional style, you have to use an association map of some sort.  Haskell only has values, but not any concept of a reference (unless you count things like IORef, but I am not counting those).  Generally speaking this is needed whenever you are dealing with a data structure that has cycles.  (Generally speaking because it's possible to make data structures lazily refer to themselves.)

People usually use IntMap, but there's a new package EnumMap on Hackage which is really powerful.  It's like IntMap only typesafe.  You will need a counter in your data structure as a source of unique ids.  You can also use value-supply (from Hackage), which is a great bit of code.

On the face of it, this seems cumbersome, but the way to do it is to create a data structure and access it through accessor functions like "add node", "delete node", "follow wire", etc.  This way you can abstract those details away.  People have done various directed/undirect graph packages and so on on Hackage - I can't recommend anything.

Stick with it - this approach does work.  I've done things like conversion of 3D models into triangle strips using this method, with very satisfying results.

Steve

On 18/05/10 12:59, Patrick LeBoutillier wrote:

...

Hi all,

After learning some Haskell recently, I decided to revisit a book about functional programming techniques for Perl: Higher Order Perl. I didn't fully understand the book at the time but now my Haskell experience has proved to be very insightful.

Towards the end of the book the author implements a local propagation network.

Here is the Perl source code: http://hop.perl.plover.com/Examples/Chap9/Local-Propagation/ The PDF of the specific chapter is here: http://hop.perl.plover.com/book/pdf/09DeclarativeProgramming.pdf

I would like to experiment with something similar in Haskell, but the way this network is designed is all about state and references:

- Wires have a values that can change over time; - Wires have references to nodes; - Nodes have references to wires;

I'm a bit stuck as to how to approach the "object has a list references to other objects" situation from Haskell. I tried this:

type Name = String data Node = Node Name [Wire] data Wire = Wire Name Node Double [Node]

But that doesn't seem like it would work since when I change a Wire I must find all "copies" of it (in the Node objects) and update them also. Perhaps I should just refer to Wires/Nodes by name and use an association list to lookup them up, but that seems cumbersome.

Anybody have any suggestions?

Thanks a lot,

Patrick

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-- ===================== Patrick LeBoutillier Rosemère, Québec, Canada