January 2002 - Haskell-Cafe

how to implement the differential operation with Haskell?
by Zhe Fu 09 Jan '02

09 Jan '02

Is there any built-in functions in Haskell to implement diffential operation? Or can anyone give me some advices about how to implement them with Haskell? Thanks.

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State Transformer
by Jorge Adriano 07 Jan '02

07 Jan '02

Hi, I'm studying, among other things, Genetic Algorithms and Neural Networks and I decided I'd use haskell to code some simple GAs and NNs along with my study. Well, maybe it was not such a good idea after all, because I've been spending way more time learning more Haskell then GAs and NNs :( Anyway, I was coding some simple GA, and as you probably know I need to use random values. The most elegant way I could think of was to generate some infinite list of random values and pass them around as arguments to the functions that need those values. I called data which wraped this list Environment, and at first it seemed a nice way to solve the problem. Well, now I think it gets kind of weird because some functions will end up to have type something -> (otherthing, Environment), to update those lists... it's just ugly. Beside those lists I'd also like to control some statistics like the number of mutations, n. of crossovers, best fitness value in each generation, etc... I figured out that there should be a better way to do this then just chaging all the signatures and passing all this values around. Monads! (right?) Till then I had just read what I needed to be able to use the IO Monad. Seems to me like having a State Transformer monad its the best way to do it. Now I've read a great deal of Richard Birds Book chap 10 (Monads), as well as the "Monads for the Haskell Working Programmer"[1] by Theodore Norvell. I was going to try to make my own simple examples using a ST. A State Monad seemed something like would most probably be in some Standard Library, or at least in some GHC library. And it was (section 4.31.ST in the hslibs documentation) I wanted to use this ST, but then I noticed it was different from the one described in tutorial[1]. I was expecting the ST Monad ghc module to provide an apply function, analogue to the > applyST :: StateTrans s a -> s -> (s, a) > applyST (ST p) s = p s in the tutorial. I also expected to have general functions to access and change State. I can't implement them myself since the ST constructor is (obviously) not exported. But this ST module seems to work in a completely diferent way. >From what I can tell it is not suposed to be applyed to an initial state, instead it starts with an 'empty' state... State is controled with Referencies (mutable variables). Ok, now my problem, how do I use this? I can't really see how to change this referencies from within some function. (Got an example in the end to explain better waht I mean with that [Example1]) I'd also appreciate some coments on: Using a ST monad (good idea, bad?) Using the Ghc ST monad? Chromosomes defined as arrays? - either IArray or Diff array got to give it some more thought... (don't want Ints + bitwise operations right now...) Well, any other comments or hints that you think that might be usefully are welcome. I've already checked out the paper from the TAIGA project[2], it's not exactly done the way I'm thinking about doing it, but I got some usefull tips from there, like the use of a Monad to control statistics. One of my main problems so far as been *knowing what do I need to know*! I don't know anyone that codes in haskell, not having anyone to talk to and share ideas doesn't helps much either. Things get complicated where you (you - the guy that comes from the imperative paradigm) less expects it too... the space leaks, using monads to control state... if you still have not read about this stuff, IMO, it is easy to feel like you already know enough to do some solve some kind of problems when you actually don't. Any newbie to C or Pascal can make a few randoms here and there, and keep track of statistics... when you already spent some time with haskell you don't even question whether you already know enough to do something like *that*. Only when you start to work, and thing start to get messy, you begin to think that *maybe you need something you still don't know about*, and then you got to find out what it is... Documentation, I also feel like it could be more and better... the ST module in ghc for instance... would it be that hard to put at least some simple example there? No, just the type signatures... Well, this is just my opinion anyway. Thanks for your atention, and happy 2002 ;-) J.A. [Example1] How can I do this for instance, with the Ghc ST Monad: ------------------------------------------------------------------------ -- the State Trans defined as in the tutorial newtype StateTrans s a = ST( s -> (s, a) ) instance Monad (StateTrans s) where -- (>>=) :: StateTrans s a -> (a -> StateTrans s b) -> StateTrans s b (ST p) >>= k = ST( \s0 -> let (s1, a) = p s0 (ST q) = k a in q s1 ) -- return :: a -> StateTrans s a return a = ST( \s -> (s, a) ) applyST :: StateTrans s a -> s -> (s, a) applyST (ST p) s = p s -- just change the state putST :: (Char,Int) -> StateTrans (Char,Int) () putST (c',n') = ST(\(c,n)->((c',n'),())) -- get the state getST :: StateTrans (Char,Int) (Char,Int) getST = ST(\(x,y)-> ((x,y), (x,y))) testfunc :: StateTrans (Char,Int) Int testfunc = do foo bar (c,n)<-getST return n foo = do (c,n)<-getST putST ('a',n+1) bar = do (c,n)<-getST putST (c,n+2) tryTestFunc = applyST testfunc ('x',0) ------------------------------------------------------------------------ I is something like this? ---------------------------------------- stupidFunc runST ( do { initstate <- newSTRef ('x',0) -- and now what? }) ----------------------------------------- [1] http://www.engr.mun.ca/~theo/Misc/haskell_and_monads.htm [2] http://www.scms.rgu.ac.uk/staff/db/Taiga/ http://www.scms.rgu.ac.uk/staff/db/Taiga/APGA2000.ps.gz

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RE: Compiler error using IO
by Ashley Yakeley 07 Jan '02

07 Jan '02

>| while :: IO Bool -> IO () -> IO () >| >| while test action = do res <- test >| if res then do action >| while test action >| else return () > >Haskell uses "layout" to reduce the need for parentheses. For your code to >parse, "if" should align with "res" vertically, as in: > > ... = do res <- test > if res ... The "layout" rules drive me nuts. You might prefer using parentheses and semicolons, as I do: -- (not tested) while :: IO Bool -> IO () -> IO (); while test action = do { res <- test; if res then do { action; while test action; } else return (); }; -- ...and then you can put the whitespace wherever you want. -- Ashley Yakeley, Seattle WA

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(no subject)
by Noriyuki Sugita 05 Jan '02

05 Jan '02

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Compiler error using IO
by Tom Bevan 03 Jan '02

03 Jan '02

When trying to right IO I continually run across this sort of compile error which I just don't understand. Any help would be very muich appreciated. Function: while :: IO Bool -> IO () -> IO () while test action = do res <- test if res then do action while test action else return () Error: io.hs: 112 lines, 2463 characters. Reading file "io.hs": Parsing ERROR io.hs:110 - Last generator in do {...} must be an expression Tom

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Intro to Functional Prog. by Bird, Section 2.5
by Shawn P. Garbett 02 Jan '02

02 Jan '02

I've been working on understanding Haskell using hugs98 and playing with some of the examples in Bird's book. In section 2.5, page 46 of the second edition, he gives an example of dependent types: But I get this error I've been unable to solve: ---------------------------------------------------------------------------------------------------------------- --Type checking --ERROR sec2-5.hs:8 - Type error in application --*** Expression : case' (Left' f,Right' g) --*** Term : (Left' f,Right' g) --*** Type : (Either' (d -> e) f,Either' g (h -> i)) --*** Does not match : (a -> b,c -> b) data Either' a b = Left' a | Right' b case' :: (a->g, b->g) -> Either' a b -> g case' (f, g) (Left' x) = f x case' (f, g) (Right' y) = g y plus' :: (a->b , g->d) -> Either' a g -> Either' b d plus' (f,g) = case' (Left' f, Right' g) ----------------------------------------------------------------------------- Please note that the version I've entered is correct by the errata published for this section.

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RE: haskell sockets
by Simon Marlow 02 Jan '02

02 Jan '02

> i want to write/read data to the soccerserver of the robocup. > the sample > code below works, but i want to have a handle for writing to > the server > instead of SocketPrim.sendTo. how can i do that ? SocketPrim.socketToHandle gives you back a Handle which you can use to read/write to/from using Haskell's normal IO operations. Note that you need to call SocketPrim.connect, even for a Datagram socket, to fix the endpoint for the socket. Cheers, Simon

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