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Multiplexing I/O in Haskell
by Sergey V. Mikhanov 03 Sep '09

03 Sep '09

Hello community, Is there a good/proper way to do multiplexing I/O in Haskell, similar to what we are getting by using select()/poll() in Unix kernel? It does not seem impossible to implement this on top of existing IO library (though I haven't tried this actually, so I may be wrong). What interest me most here is whether we could use Haskell's implicit multithreading here (select()/poll() combination is all about multithreading)? Thanks in advance, Sergey

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Identical function and variable names and type inference
by aditya siram 03 Sep '09

03 Sep '09

Hi all, Recently I wrote a function that takes a unique identifier that I called 'id'. I then tried to apply the 'id' function to it and GHC did not like that. But it should. For example in 'test' I have told the compiler that the id argument is an Int. So type inference should be able to determine the first 'id' in 'id id' couldn't possibly be an Int, but it complains. So I explicitly told the compiler the type of 'id' in test1 - this didn't work either. The final function 'test3' works as expected. Is there something I am not understand about the way type inference is supposed to work? test :: Int -> Int test id = id id test1 :: Int -> Int test1 id = idFunc id where idFunc :: a -> a idFunc = id test2 :: Int -> Int test2 myid@id = id myid test3 :: Int -> Int test3 id = Prelude.id id thanks ... -deech

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Figuring out errors
by Edward Z. Yang 02 Sep '09

02 Sep '09

Hello all, I've been looking at [1] and trying to make tops and bottoms (pardon the pun) of error handling in Haskell. I am still uncertain of what to do. I recognize that there are different areas of code that may have different requirements for errors: * Pure code that is simple enough can probably get away with returning Maybe a * Pure code that has multiple failure modes (the canonical example is parsing) should return a Either e a. The type of e is a little difficult: many standard libraries seem to use String, but coming from Python this seems analogous to the long deprecated "string exceptions", which are quick and easy but not long-term maintainable due to lack of an easy way to match for them. This leads naturally into Either MyError, which is supported using throwError and catchError. * However, [1] specifically warns against using this technique in the IO monad, and I need a way to short circuit execution when a crucial pure operation fails (in Python, this would have just been an uncaught exception). This suggests using ErrorT on IO, however, [1] also claims that this is generally not a good idea, and suggests to use throwDyn (which actually looks like it's been renamed to throw) * Unfortunately, when I've tried to use this technique, I've run up against the fact that throw is implemented using bottom, so if I do a throw in a pure function, the exception might not actually surface up until I'm, say, attempting to print its return value to IO. Denizens on #haskell have instructed me to treat bottom as the bane of existence and opt for stacking ErrorT on IO (it would be nice to know if this was a good idea or bad idea) At which point, I am most thoroughly confused. Pointers, please! Cheers, Edward [1] http://www.randomhacks.net/articles/2007/03/10/haskell-8-ways-to-report-err…

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function - argument termination problem
by Γιάννης Μαντζουράτος 02 Sep '09

02 Sep '09

Hello, Is it possible to implement in haskell a function f (A) such that if A does not ever terminate then f always terminates, and if A always terminates then f does not ever terminate? I've been thinking it for a while, with no results unfortunately. the actual problem is that i can think of no way to force both ifs: a function that always terminates independently from its argument could be f (a) = 1, and a function that does not terminate even if its argument terminates could be: f (a) = f (a + 1), but i can't figure out a "hybrid" version... Is there any idea on this? Thanks in advance :-), yannis

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Using FiniteMap in ghci
by KwangYul Seo 02 Sep '09

02 Sep '09

Hello, I tried to load FiniteMap module in ghci and encountered an error message like the following: Prelude> import FiniteMap Could not find module `FiniteMap': it is a member of package ghc-6.8.2, which is hidden The error message says that it is hidden. How can I use the FiniteMap? Regards, Kwang Yul Seo

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Utter Newbie - simple problems, output - GHC vs GHCi
by Nigel Rantor 02 Sep '09

02 Sep '09

I am trying to get my head around Haskell but seem to keep butting against problems that have nothing to do with FP yet, but are simply to do with not understanding the tools. I've been trying a lot of code from multiple tutorials but I keep finding that the code simply does not work out of the box, and requires some other setup I am unaware of. I am currently on Debian, using GHC 6.8.2 installed using apt, so I assume that the toolchain is installed and working correctly. For example, the most recent tutorial I've been looking at is the "yet another haskell tutorial", here - http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf One of the exercises after talking about functions that act on lists is to determine the number of lowercase letters in a string. Fine, that makes complete sense to me. I figure something along the lines of: length( filter( Char.isLower "LoweR" ) ) should return the value 3 If I attempt this at the interactive GHC prompt I get the following: ------------------------------------------------------------------------------------------ wiggly@mink:~/src/ht$ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> show( length( filter( Char.isLower "LoweR" ) ) ) <interactive>:1:35: Couldn't match expected type `Char' against inferred type `[Char]' In the first argument of `GHC.Unicode.isLower', namely `"LoweR"' In the first argument of `filter', namely `(GHC.Unicode.isLower "LoweR")' In the first argument of `length', namely `(filter (GHC.Unicode.isLower "LoweR"))' Prelude> ------------------------------------------------------------------------------------------ If I attempt to put the code into a file and compile it I get the following: [Code] ------------------------------------------------------------------------------------------ module Main where import Char main = show( length( filter( Char.isLower "LoweR" ) ) ) ------------------------------------------------------------------------------------------ [Terminal] ------------------------------------------------------------------------------------------ wiggly@mink:~/src/ht$ ghc -o test ex3.hs ex3.hs:3:42: Couldn't match expected type `Char' against inferred type `[Char]' In the first argument of `isLower', namely `"LoweR"' In the first argument of `filter', namely `(isLower "LoweR")' In the first argument of `length', namely `(filter (isLower "LoweR"))' wiggly@mink:~/src/ht$ ------------------------------------------------------------------------------------------ This is one of the smallest examples I can think of posting for some help, and quite frankly, I'm feeling a bit dim because I just cannot understand why this doesn't work...I've tried in vain to mess with the code (I won't attempt to describe it, I'm not au fait enough with Haskell terminology yet, it would sound like gibberish *and* be incorrect) Any help would be awesome, equally, any ready-to-run examples/tutorials that people could recommend would likewise be awesome and beer-worthy[0]. Cheers, n [0] offer only applies to those in the London area or those environs where I find myself randomly

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haskell code to html
by I. J. Kennedy 01 Sep '09

01 Sep '09

Is there a decent utility out there for syntax coloring haskell code?(haskell to html)

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Parsec newbie question
by Patrick LeBoutillier 31 Aug '09

31 Aug '09

Hi all, I'd like to use parsec to parse IP addresses. So far I've written a (tiny) parser for bytes: byte :: GenParser Char st Word8 byte = do n <- many1 digit return (read n) The function works fine, but it accepts numbers greater than 255. How do I encapsulate this condition in the parser so that it fails (with en appropriate error message) in this case? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Type Class Woes ..
by Tom Poliquin 31 Aug '09

31 Aug '09

I've been writing Haskell programs (even useful ones :-) ) for awhile and I thought it was time to experiment with my own type classes. I chose the (contrived toy) problem of computing the volume of various fruits. First I wrote the code using algebraic data types .. (shown below); then I 'converted' it to use type classes (also shown below) The type class version gives me the errors .. Main.hs:12:16: Not in scope: type constructor or class `Banana' Main.hs:15:16: Not in scope: type constructor or class `Watermelon' .. which makes sense since Banana and Watermelon are 'data constructors' and not 'type constructors'. The problem is I'm not sure how to get around it .. If I try putting data Orange = Orange data Banana = Banana .. then what does 'initFruit' return .. ? it can't return [Fruit FruitType] anymore .. It seems like this should be a simple problem. I'm clearly not 'getting it'. Any help greatly appreciated, Tom ---------- Algegraic Data Type Version (works) ---------- module Main where data Fruit a = F {radius::Double, len::Double, fType::a} data FruitType = Orange | Apple | Banana | Watermelon deriving Show initFruit :: [Fruit FruitType] initFruit = [ (F 3.0 0.0 Orange), (F 3.0 0.0 Apple), (F 3.0 2.0 Banana), (F 40.0 20.0 Watermelon) ] volume :: Fruit FruitType -> Double volume F{radius=r,len=l,fType=Orange} = (4.0/3.0) * pi * r * r * r volume F{radius=r,len=l,fType=Apple} = (4.0/3.0) * pi * r * r * r volume F{radius=r,len=l,fType=Banana} = pi * (r * r) * l volume F{radius=r,len=l,fType=Watermelon} = (4.0/3.0) * pi * (2.0 * r) * l * (0.5 * l) ---------- -- Main -- ---------- main = do fruit <- return $ initFruit mapM (\f@(F{fType=t}) -> putStrLn ("Volume -> " ++ show t ++ " = " ++ show (volume f))) fruit Volume -> Orange = 113.09733552923255 Volume -> Apple = 113.09733552923255 Volume -> Banana = 56.548667764616276 Volume -> Watermelon = 67020.64327658225 ---------- Type Class Version (fails) ---------- module Main where data Fruit a = F {radius::Double, len::Double, fType::a} data FruitType = Orange | Apple | Banana | Watermelon class Volume a where volume :: (FruitType a) => Fruit a -> Double -- default spherical fruit .. volume F{radius=r,len=l} = (4.0/3.0) * pi * r * r * r instance Volume Banana where volume F{radius=r,len=l} = pi * (r * r) * l instance Volume Watermelon where volume F{radius=r,len=l} = (4.0/3.0) * pi * (2.0 * r) * l * (0.5 * l) initFruit :: [Fruit FruitType] initFruit = [ (F 3.0 0.0 Orange), (F 3.0 0.0 Apple), (F 3.0 2.0 Banana), (F 40.0 20.0 Watermelon) ] ---------- -- Main -- ---------- main = do fruit <- return $ initFruit mapM (\f@(F{fType=t}) -> putStrLn ("Volume -> " ++ show t ++ " = " ++ show (volume f))) fruit Main.hs:12:16: Not in scope: type constructor or class `Banana' Main.hs:15:16: Not in scope: type constructor or class `Watermelon'

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pretty-printing data
by Michael Mossey 31 Aug '09

31 Aug '09

For debugging purposes I'm interested in pretty-printing data; to start with, lists of algebraic data types. Basically I'd like 'show' with the ability to put each entry of a list on a separate line, and indented. Note that the algebraic data might have an inner list as one of its elements, so this is a non-obvious formatting problem. I believe I can make instances of Show, can I not? Is there something called showList which I can use to code my own method of showing lists of a particular type? My understanding is that I can't make [a] an instance of Show; hence they provided showList. So basically, I'm wondering if a pretty-printing (for data!) library already exists, or whether I should try to make instances of Show. Thanks, Mike

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