August 2009 - Haskell-Cafe

FFI: Problem with Signal Handler Interruptions
by Levi Greenspan 07 Aug '09

07 Aug '09

Dear list members, In February this year there was a posting "Why does sleep not work?" (http://www.haskell.org/pipermail/haskell-cafe/2009-February/055400.html) The problem was apparently caused by signal handler interruptions. I noticed the same (not with sleep though) when doing some FFI work and compiled the following test program: {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign.C.Types import Control.Concurrent sleep :: IO () sleep = c_sleep 3 >>= print fails :: IO () fails = sleep works :: IO () works = forkIO sleep >> return () main :: IO () main = fails >> works >> threadDelay 3000000 foreign import ccall unsafe "unistd.h sleep" c_sleep :: CUInt -> IO CUInt When compiled with GHC (using --make -threaded), it will print 3 immediately (from the "fails" function) and after 3 seconds 0 (from "works"), before it finally exits. man sleep(3) tells me that sleep returns 0 on success and if interrupted by a signal the number of seconds left to sleep. Clearly "fails" is interrupted by a signal (which seems to be SIGVTALRM). This was mentioned in the discussion from February. I would like to know why "fails" fails and "works" works, i.e. why is "sleep" not interrupted when run in a separate thread? And what can be done to make "sleep" work in the main thread? It wouldn't be wise to block SIGVTALRM, wouldn't it? Many thanks, Levi

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Seeking for an extention (functional incapsulation)
by Belka 07 Aug '09

07 Aug '09

Hello, cafe visitors! :) This is a double topic: 1. Can't find any good informative resource with descriptions of Haskell extensions. Could anybody please share good one if it exists? The only "good" one I found: http://hackage.haskell.org/trac/haskell-prime/wiki/HaskellExtensions But it's a bit too old and not that full... I undestand, that Haskell is kind of "boiling" language, in a sense of being neverending experiment. It develops all the time, extensions show up and drop out. So it's not that easy to support community with a fresh information about them. But on the other side, the property (of being "boiling" language) makes such information really important for community members... I think. :) 2. Consider situation: ----------------------------------- data SomeDataType = SomeDataType { sdtField1 :: SDT_Field1Type, sdtField2 :: SDT_Field2Type} sdtField3 :: SomeDataType -> SDT_Field3Type sdtField3 sdt = f (sdtField1 sdt) (sdtField2 sdt) ----------------------------------- I induced recently, that it would be very comfortable if I could perform in a way like this: ----------------------------------- data SomeDataType = SomeDataType { sdtField1 :: SDT_Field1Type, sdtField2 :: SDT_Field2Type, sdtField3 :: SDT_Field2Type, sdtField3 = f sdtField1 sdtField2} ----------------------------------- The situation is not that rare, when dealing with nonprimitive data constructions. Moreover would be really comfortable to reduce ----------------------------------- data SomeDataType = SomeDataType_111 { sdtField1 :: SDT_Field1Type, sdtField2 :: SDT_Field2Type} | SomeDataType_222 { sdtField1 :: SDT_Field1Type, sdtField2 :: SDT_Field2Type, sdtField5 :: SDT_Field5Type} sdtField3 :: SomeDataType -> SDT_Field3Type sdtField3 sdt = case sdt of {SomeDataType_111 -> f (sdtField1 sdt) (sdtField2 sdt) ; SomeDataType_222 -> g (sdtField1 sdt) (sdtField2 sdt) (sdtField5 sdt)} \/ \/ \/ \/ \/ \/ \/ \/ \/ \/ data SomeDataType = SomeDataType_111 { sdtField1 :: SDT_Field1Type, sdtField2 :: SDT_Field2Type, sdtField3 :: SDT_Field3Type, sdtField3 = f sdtField1 sdtField2} | SomeDataType_222 { sdtField1 :: SDT_Field1Type, sdtField2 :: SDT_Field2Type, sdtField5 :: SDT_Field5Type, sdtField3 :: SDT_Field3Type, sdtField3 = g sdtField1 sdtField2 sdtField5} ----------------------------------- Usable mechanics for realization would be: 1. Funtion similar to Data.Function.on (example: (*) `on` f = \x y -> f x * f y), but opposite - I called it under. t `under` f = \x y -> (x f) `t` (y f) 2. currying and uncurrying Is there any such extension? Belka -- View this message in context: http://www.nabble.com/Seeking-for-an-extention-%28functional-incapsulation%… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Looking to check on some capabilities of Data.Colour
by Jeff Heard 06 Aug '09

06 Aug '09

I was wondering if Data.Colour supported Double-valued colour components > 1.0 or less than 0. I'm looking to create an HDR image processing library, and Haskell has one of the most extensive and correct colour models around, thanks to Russell. With 16bpcc or 32bpcc images, however, I need to be sure to be able to correctly calculate colour values that fall outside the usual [0.0,1.0] gamut. Does Data.Colour support this functionality? -- Jeff

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A mistake in haskellwiki
by L.Guo 06 Aug '09

06 Aug '09

Hi haskellers: There is a mistake in http://www.haskell.org/haskellwiki/State_Monad It post two functions like this : evalState :: State s a -> s -> a evalState act = fst $ runState act execState :: State s a -> s -> s execState act = snd $ runState act Both the '$' operators should be '.'. Anyone would correct it ? Regards -------------- L.Guo 2009-08-06

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n00b question: defining datatype
by Iain Barnett 06 Aug '09

06 Aug '09

Hi, I'm trying to get my head around datatypes, and wondering how I might define a simple "Task" datatype in Haskell. data Task = Task { title :: String, completed :: Bool } Ok, that's straightforward, but sometimes tasks become a list of tasks themselves data Task = Task { title :: String, completed :: Bool, subtasks :: [Task] } But that's not really right, because obviously, some tasks don't have subtasks. So I try this: data Task = Task { title :: String, completed :: Bool } | TaskWithSubtasks { title :: String, completed :: Bool, subtasks :: [Task] } It's a bit more accurate, but it's repeating things, which is ok with a simple type. Could anyone suggest a better way to define this? If I was using C#, which I'm far more familiar with, I could overload the constructor and refer to the smaller constructor. Is there a way to do that in Haskell, or am I still thinking too OOP? Iain

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SQL Database in Haskell?
by Günther Schmidt 06 Aug '09

06 Aug '09

Hi all, is there an SQL Database in Haskell or is there a project trying to implement one? Günther

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Do you understand posix well?
by Maurício CA 06 Aug '09

06 Aug '09

I've beeing writing a low-level binding to posix that can be usefull if you want to use posix but has no time to learn FFI: http://hackage.haskell.org/package/bindings-posix However, my understandment of posix is barely nothing, and I see that many of its functionality is enabled or disabled by macros, and I don't know which ones are related to what functionality. So, if you know about that: would you be able to list all macros (or at least most important ones, if there are too many) that enable all (or most) of posix? In exchange, you get a posix binding that is actually comprehensive :) Thanks, Maurício

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About the import module
by xu zhang 06 Aug '09

06 Aug '09

Hi there, If I import a module and do not explicitly point out the entities I have imported. And I want the ghc to point out the entities automatically. Is there any method to do this? any methods to have the ghc point out the entities I import and export? Because there are so many files and I don't want to change them one by one, so I just want to find if there is a simple and automatic way to have the entities pointed out. Thank you in advance!

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Efficient functional idiom for histogram
by Paul Moore 05 Aug '09

05 Aug '09

How would I efficiently write a function in Haskell to count occurrences of unique elements in a (potentially very large) list? For example, given the list [1,2,3,4,5,3,4,2,4] I would like the output [[1,1], [2,2], [3,2], [4,3], [5,1]] (or some equivalent representation). Clearly, this won't be possible for infinite input, but I would like it to be as efficient as possible for (lazy) input lists containing many millions of elements. So an implementation based on group . sort is not going to work. In an imperative language like Python, I'd use a dictionary as an accumulator - something like for el in input: accums[i] = accums.get(i, 0) + 1 That's about as efficient as you can hope for (assuming an efficient dictionary implementation). How would I code something equivalent in Haskell? Thanks, Paul.

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Basic questions about concurrency in Haskell
by Thomas Witzel 05 Aug '09

05 Aug '09

Hello all, I'm new to Haskell, but have a good background in LISP/Scheme and do mostly C/C++ programming on a daily basis. I'm learning Haskell mainly because it provides facilities for concurrency on the language level, and I'm mainly interested in implementing parallel or massively parallel algorithms with Haskell. I have two questions that bother me. 1. Does the Haskell compiler make sure that there is no page sharing between threads, in order to avoid cache thrashing between cpus (a real killer on large SMP or ccNUMA systems) ? If so, are there functions/options to control this ? 2. I started with the very simple nfib example given in the manual for Control.Parallel (Section 7.18). On my systems using multiple cores makes the code actually slower than just using a single core. While the manual cautions that this could be the case for certain algorithms, I'm wondering whether this is the desired behaviour for this example. I'm using ghc 6.10.4 right now. Thank you for your help getting me started here. Thomas

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