April 2010 - Haskell-Cafe

Trouble with the FFI, Cabal, and GHCI -- installed libraries have invalid FFI pointers
by John Millikin 27 Apr '10

27 Apr '10

I have two modules. One is a library, installed with Cabal: ------------------------------------------------------------------------- -- Data/Text/IDN/StringPrep.hs {-# LANGUAGE ForeignFunctionInterface #-} module Data.Text.IDN.StringPrep where import Foreign foreign import ccall "stringprep.h &stringprep_nameprep" profileNameprep :: Ptr () ------------------------------------------------------------------------- The other is the main program, either interpreted directly or compiled and run: ------------------------------------------------------------------------- -- TestIDN.hs {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Data.Text.IDN.StringPrep foreign import ccall "stringprep.h &stringprep_nameprep" stringprep_nameprep :: Ptr () main = do putStrLn $ "stringprep_nameprep = " ++ show stringprep_nameprep putStrLn $ "profileNameprep = " ++ show profileNameprep ------------------------------------------------------------------------- Now, given that both modules import the exact same C symbol, I would expect that the pointers would be the same when running. This is the case for compiled code: ------------------------------------------------------------------------- $ ghc --make TestIDN [1 of 1] Compiling Main ( TestIDN.hs, TestIDN.o ) Linking TestIDN ... $ ./TestIDN stringprep_nameprep = 0x000000000067e3e0 profileNameprep = 0x000000000067e3e0 ------------------------------------------------------------------------- However, if I use ghci / runhaskell, the pointers are different: ------------------------------------------------------------------------- $ rm TestIDN TestIDN.o TestIDN.hi $ runhaskell TestIDN.hs stringprep_nameprep = 0x00007feebe5fe4e0 profileNameprep = 0x0000000040a908bc ------------------------------------------------------------------------- Specifically, the pointer loaded from the library ('profileNameprep') is invalid -- attempting to pass it to libidn will result in errors, and trying to poke around in it yields segmentation faults. I'm using GHC 6.12.1, on Linux. This problem occurs with both dynamic and static libraries. Does anybody have suggestions about how to solve or work around it?

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Re: Sorting Types
by John Creighton 27 Apr '10

27 Apr '10

> I was wondering if it is possible to sort types in hakell and if so what > language extension I should use. Not sure if > this is possible but here is my attempt: > > (I'm aware I don't need so many pragmas > > {-# LANGUAGE GADTs #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE TypeSynonymInstances #-} > data Z=Z deriving (Show) > data S i=S i deriving (Show) > data family N a > type family Add n m > type instance Add Z m = m > type instance Add m Z = m > type instance Add (S n) (S m) = S (S (Add n m)) > --14 > type family LT a b > data Cat=Cat > data Dog=Dog > data Fish=Fish > type family Sort a --19 > data And a b=And a b > > type instance LT Dog Z = Cat > type instance LT Fish Z = Dog > type instance LT a (S i) = LT (LT a Z) i > type instance Sort (And a (LT a i))=And (LT a i) a > > I get the following error: > > Illegal type synonym family application in instance: And a (LT a i) > In the type synonym instance declaration for 'Sort' > Failed, modules loaded: none, > > >

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The instability of Haskell libraries
by John Goerzen 27 Apr '10

27 Apr '10

It is somewhat of a surprise to me that I'm making this post, given that there was a day when I thought Haskell was moving too slow ;-) My problem here is that it has become rather difficult to write software in Haskell that will still work with newer compiler and library versions in future years. I have Python code of fairly significant complexity that only rarely requires any change due to language or library changes. This is not so with Haskel. Here is a prime example. (Name hidden because my point here isn't to single out one person.) This is a patch to old-locale: Wed Sep 24 14:37:55 CDT 2008 xxxxx(a)xxxxx.xxxx * Adding 'T' to conform better to standard This is based on information found at http://en.wikipedia.org/wiki/ISO_8601#Combined_date_and_time_representations diff -rN -u old-old-locale/System/Locale.hs new-old-locale/System/Locale.hs --- old-old-locale/System/Locale.hs 2010-04-23 13:21:31.381619614 -0500 +++ new-old-locale/System/Locale.hs 2010-04-23 13:21:31.381619614 -0500 @@ -79,7 +79,7 @@ iso8601DateFormat mTimeFmt = "%Y-%m-%d" ++ case mTimeFmt of Nothing -> "" - Just fmt -> ' ' : fmt + Just fmt -> 'T' : fmt A one-character change. Harmless? No. It entirely changes what the function does. Virtually any existing user of that function will be entirely broken. Of particular note, it caused significant breakage in the date/time handling functions in HDBC. Now, one might argue that the function was incorrectly specified to begin with. But a change like this demands a new function; the original one ought to be commented with the situation. My second example was the addition of instances to time. This broke code where the omitted instances were defined locally. Worse, the version number was not bumped in a significant way to permit testing for the condition, and thus conditional compilation, via cabal. See http://bit.ly/cBDj3Q for more on that one. I could also cite the habit of Hackage to routinely get more and more pedantic, rejecting packages that uploaded fine previously; renaming the old exception model to OldException instead of introducing the new one with a different name (thus breaking much exception-using code), etc. My point is not that innovation in this community is bad. Innovation is absolutely good, and I don't seek to slow it down. But rather, my point is that stability has value too. If I can't take Haskell code written as little as 3 years ago and compile it on today's platform without errors, we have a problem. And there is a significant chunk of code that I work with that indeed wouldn't work in this way. I don't have a magic bullet to suggest here. But I would first say that this is a plea for people that commit to core libraries to please bear in mind the implications of what you're doing. If you change a time format string, you're going to break code. If you introduce new instances, you're going to break code. These are not changes that should be made lightly, and if they must be made (I'd say there's a stronger case for the time instances than the s/ /T/ change), then the version number must be bumped significantly enough to be Cabal-testable. I say this with a few hats. One, we use Haskell in business. Some of these are very long-term systems, that are set up once and they do their task for years. Finding that code has become uncompilable here is undesirable. Secondly, I'm a Haskell library developer myself. I try to maintain compatibility with GHC & platform versions dating back at least a few years with every release. Unfortunately, this has become nearly impossible due to the number of untestable API changes out there. That means that, despite my intent, I too am contributing to the problem. Thoughts? -- John

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Final call for participation: Summer school "Applied Functional Programming", Aug 16-27, Utrecht NL, Application deadline: May 1
by Andres Loeh 27 Apr '10

27 Apr '10

[Feel free to advertise the school to others who might be interested.] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Utrecht Summer School in Computer Science: Applied Functional Programming August 16-27, 2010 Utrecht, The Netherlands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The summer school is intended as a first or an advanced course on Haskell, consisting of lectures and projects to be solved by the participants, working in teams. We mainly target bachelor and master students, but others who are interested are also welcome to apply. The deadline for applications is *** Saturday, May 1, 2010 ***. More information: http://www.cs.uu.nl/wiki/USCS2010 If you have any questions, feel free to contact me. Cheers, Andres -- Andres Loeh, Universiteit Utrecht mailto:andres@cs.uu.nl mailto:mail@andres-loeh.de http://www.andres-loeh.de

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ANN: iteratee-parsec 0.0.3
by Maciej Piechotka 27 Apr '10

27 Apr '10

iteratee-parsec is a package allowing parsec parser using iteratee. Changes in 0.0.3: - Update to transformers 0.2.* (tested with 0.2.1.0) - Add stricter checks for version of dependencies Regards

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Haddock infix constructors in markup
by Ozgur Akgun 27 Apr '10

27 Apr '10

Hi all, If I have the following data type: data Expr = Num Int | Expr :+: Expr | Expr :-: Expr Haddock handles the infix constructors, and generates a very nice output (html in this case) However when I try to reference to one of the infix constructors in the markup of other functions it fails to hyperlink. -- | If the input is 'Num' does magic, if it is ':+:' does even more magic! someFunc :: Expr -> Expr In the output of this markup, the 'Num' is hyperlinked but the ':+:' is not. Is there a different syntax for infix constructors (and for infix functions presumably) or does haddock simple lack this feature? Best, -- Ozgur Akgun

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Curl & UTF8
by Rickard Karlsson 27 Apr '10

27 Apr '10

Hi, I'm trying to download a file in UTF-8 with libcurl(1.3.5) and GHC 6.12: >import Network.Curl >u = "http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-demo.txt" >main = curlGetString u [] >>= putStrLn . snd Which doesn't print the characters correctly. If i read the file from local storage with getFile it is displayed properly.

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Abstracted File Library?
by Kazu Yamamoto 27 Apr '10

27 Apr '10

Hello, I'm implementing a command which manipulates files both on Unix/Mac and Windows. I was very surprised because there is not "getStatusChangeTime" function. So, I wrote it with CPP. http://github.com/kazu-yamamoto/Mew/blob/master/bin/hs/Stat.hs Another problem is that since '\\' is used as the file separator of Windows, I cannot use regular expression naturally for file path. In many programming languages, '/' is only file separator for programmers. And my friend, who is an expert of Windows API, says to me that Windows API allows '/' as a file separator. Why don't we use '/' on Windows, too? Are there any abstracted file library to solve these problems? Or should I start to write such a library? --Kazu

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Proper Handling of Exceptional IEEE Floating Point Numbers
by Barak A. Pearlmutter 27 Apr '10

27 Apr '10

Comparison of exceptional IEEE floating point numbers, like Nan, seems to have some bugs in ghci (version 6.12.1). These are correct, according to the IEEE floating point standards: Prelude> 0 < (0/0) False Prelude> 0 > (0/0) False Prelude> 0 == (0/0) False But these are inconsistent with the above, and arguably incorrect: Prelude> compare (0/0) (0/0) GT Prelude> compare (0/0) 0 GT Prelude> compare 0 (0/0) GT Prelude> compare (0/0) (1/0) GT Prelude> compare (1/0) (0/0) GT I'd suggest that compare involving a NaN should yield error "violation of the law of the excluded middle" The min and max functions have strange behaviour with regard to NaN, especially when mixed with Infinity: Prelude> max (0/0) (1/0) NaN Prelude> max (1/0) (0/0) Infinity Prelude> min (0/0) (1/0) Infinity Prelude> max (0/0) 0 NaN Prelude> max 0 (0/0) 0.0 Hugs (Version: September 2006) has similar issues: Hugs> compare (0/0) (0/0) EQ Hugs> compare (0/0) 1 EQ Hugs> (0/0) == (0/0) False Hugs> min (0/0) 1 nan Hugs> min 1 (0/0) 1.0 Hugs> max (0/0) 1 1.0 Discuss? -- Barak A. Pearlmutter <barak(a)cs.nuim.ie> Hamilton Institute & Dept Comp Sci, NUI Maynooth, Co. Kildare, Ireland http://www.bcl.hamilton.ie/~barak/

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Re: What do _you_ want to see in FGL?
by Louis Wasserman 26 Apr '10

26 Apr '10

I'm a fan of making FGL more record-based, but definitely keeping the inductive graph style. My own biggest gripe with previous versions of FGL was that the graph implementations were severely underoptimized. In particular, PatriciaTree left open a *lot* of optimization. Ivan, would you like to set up a darcs repo for FGL-Prime or something at code.haskell.org, and start cracking? I'm really excited for this project. Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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