December 2010 - Beginners

Architecture and monads
by Amy de Buitléir 08 Dec '10

08 Dec '10

Until recently, the choice of whether or not to use monads was easy. I didn't understand them, so I avoided them! But now that I think I understand enough to begin to use them, I have a different problem: I don't have the experience to know when using them is a good idea, and when it's overkill. This is my first serious Haskell project, and I would appreciate any advice. Here's the scenario: I'm developing a virtual world with alife creatures. At one extreme, I guess I could use the State monad for everything that has state, right down to the neurons in their brains. (I assume that would be overkill.) At the other extreme, I could do without the State monad altogether, and just have functions that take a creature, let it have an experience, and return a new creature. I guess somewhere in-between there's a happy medium. Perhaps I should us the State monad for the creatures, but not for anything lower-level than that. Or maybe use it for the creatures and their brains. One issue that complicates things slightly is that a *lot* of the processing I'm doing requires random numbers. The neurons don't require any randomness, but the brain as a whole does. And reproduction requires randomness down at the DNA level. The code works, but I feel it's a bit messy because of needing randomness in so many places, and I'd like to clean up the design a bit. Generally speaking, is it best to go for the "bon-bon" approach, with a pure functional core, and a monadic layer on the outside?

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whats up
by David Schonberger 08 Dec '10

08 Dec '10

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Inferring functions from parameter types
by Russ Abbott 07 Dec '10

07 Dec '10

Is there any theoretical reason why GHC (or Haskell) doesn't permit functions to be inferred from their parameter types? For example, if I write null Data.Set.empty I get a diagnostic saying Ambiguous occurrence `null' It could refer to either `List.null', imported from Data.List at ... or `Map.null', imported from Data.Map at ... or `IntSet.null', imported from Data.IntSet at ... Why doesn't GHC figure it out? More generally it would be nice to be able to define: f :: Type1 -> ResultType1 f :: Type2 -> ResultType2 and expect GHC to select the appropriate f based on the argument and implied result types. Even if the type can't always be determined, why not do it when possible and issue a complaint when it can't? * -- Russ *

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defining 'init' in terms of 'foldr'
by Paul Higham 07 Dec '10

07 Dec '10

Not sure if this thread is still active but I also struggled with this same exercise. I offer the following solution as a thing to shoot at: myInit :: [a] -> [a] myInit ys = foldr snoc [] $ (\(x:xs) -> xs) $ foldr snoc [] ys where snoc = (\x xs -> xs ++ [x]) Note that snoc is defined at the top of the same page as the exercise in Simon's book. ::paul

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gtk2hs website
by Venanzio Capretta 06 Dec '10

06 Dec '10

Hi everybody, I was starting to learn how to use gtk2hs. Unfortunately, the web site for it has suddenly disappeared. It used to be at "http://www.haskell.org/gtk2hs/". The information on the Haskell page "http://www.haskell.org/haskellwiki/Gtk2Hs" is not enough to understand how the library works. Does anybody know how I can access the old site? Thanks, Venanzio

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Exception back trace
by Russ Abbott 05 Dec '10

05 Dec '10

Hi, I am in the midst of debugging and got an exception: *** Exception: Prelude.minimum: empty list That's all it said. There was no information about where the exception occurred. Is it possible to ask for more information, preferably including a stack trace? Thanks. * -- Russ *

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Re: [Haskell-beginners] defining 'init' in terms of 'foldr'
by Hein Hundal 05 Dec '10

05 Dec '10

I am trying to understand the differences between two different solutions to the problem of expressing init in terms of foldr. The two solutions are called heinInit and chaddiInit. The heinInit does not work on infinite lists. The chaddiaiInit function does work on infinite lists. heinInit v = fromJust $ foldr f Nothing v where f a Nothing = Just [] f a (Just v) = Just (a:v) chaddaiInit v = fromJust $ foldr f Nothing v where f x xs = Just (case xs of Nothing -> []; Just ys -> x:ys) Here is my guess at the "trace" of the chaddaiInit code. 1) head (chaddaiInit [1..]) 2) head (fromJust $ foldr f Nothing [1..]) 3) head (fromJust $ f 1 (foldr f Nothing [2..])) 4) head (fromJust $ Just (case (foldr f Nothing [2..]) of Nothing -> []; Just ys -> 1:ys)) 5) head (case (foldr f Nothing [2..]) of Nothing -> []; Just ys -> 1:ys) 6) head (case (f 2 (foldr f Nothing [3..])) of Nothing -> []; Just ys -> 1:ys) 7) head (case (Just (case (foldr f Nothing [3..]) of Nothing -> []; Just ys -> 2:ys)) of Nothing -> []; Just ys -> 1:ys) 8) head (1:(case (foldr f Nothing [3..]) of Nothing -> []; Just ys -> 2:ys)) 9) 1 Here is my guess at the "trace" of the heinInit code. myinit v = fromJust $ foldr f Nothing v where f a Nothing = Just [] f a (Just v) = Just (a:v) 1) head (heinInit [1..]) 2) head (fromJust $ foldr f Nothing [1..]) 3) head (fromJust $ f 1 (foldr f Nothing [2..]) 4) head (fromJust $ f 1 (f 2 (fold f Nothing [3..]) 5) head (fromJust $ f 1 (f 2 (f 3 (fold f Nothing [4..]) And it just keeps going. Is this the right idea? Are my "traces" a reasonable estimate of how Haskell evaluates those expressions? Cheers, Hein

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Named Pipes Et Cetera
by matthew coolbeth 05 Dec '10

05 Dec '10

Hello, all. Three questions: 1> What, if anything, can I do in a Haskell program to do blocking reads from a Linux FIFO? For the curious, there is a small program below that doesn't block, but immediately terminates because of an EOF 2> How could I have found the answer to this question on my own? (The documentation that Google found for me was not completely adequate) 3> Is this sort of question generally regarded as too unixy or not haskelly enough to be appropriate for this mailing list? Thanks. --BEGIN PROGRAM-- module Main () where import Control.Monad import System.IO import System.Posix.Files pipeString = "/tmp/dld.fifo" main :: IO () main = do createNamedPipe pipeString $ unionFileModes ownerReadMode ownerWriteMode pipe <- openFile pipeString ReadMode forever (hGetLine pipe >>= putStrLn) -- mac

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Re: [Haskell-beginners] defining 'init' in terms of 'foldr'
by Hein Hundal 05 Dec '10

05 Dec '10

> From: Daniel Fischer <daniel.is.fischer(a)googlemail.com> > Subject: Re: [Haskell-beginners] defining 'init' in terms > of 'foldr' > On Saturday 04 December 2010 23:20:51, Paul Higham wrote: > > Not sure if this thread is still active but I also > > struggled with this > > same exercise. I offer the following solution as > > a thing to shoot at: > > > > myInit :: [a] -> [a] > > myInit ys = foldr snoc [] $ (\(x:xs) -> xs) $ foldr snoc [] ys > > where snoc = (\x xs -> xs ++ [x]) > > init === reverse . tail . reverse > only holds for finite lists, for infinite lists xs, reverse > xs = _|_, but > init xs = xs. > Also, it's inefficient, but that's not the point of the > exercise. Here is my incomplete beginner solution: --Start Code import Data.Maybe myinit v = fromJust $ foldr f Nothing v where f a Nothing = Just [] f a (Just v) = Just (a:v) --End Code This worked for the finite lists that I tried, but it did not work for infinite lists. I was surprised that foldr works with infinite lists. If I set v = take 5 $ foldr (\x xs -> (x*x):xs) [] [1..] then the value of v is [1,2,9,16]. Is there another way to create the init function with foldr that works for infinite lists? Hein

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Maybe perform an action
by Tim Baumgartner 05 Dec '10

05 Dec '10

Hi Haskellers, I have another simple question in order to improve my coding style. I'm using the following function, e.g. when a dialog returned Maybe NiceStuff: performMaybe :: Monad m ⇒ Maybe a → (a → m b) → m () performMaybe x action = when (isJust x) (action (fromJust x) >> return ()) example use: main = performMaybe (Just "Hello") print Now I wonder if others use such a function as well, if it's already defined in the standard libraries (didn't find it using Hoogle) and perhaps how to implement it a little nicer. Regards Tim

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