January 2006 - Haskell-Cafe

I/O and utf8
by Andreas Kägi 12 Jan '06

12 Jan '06

hello i want to read a file encoded in utf8 and at a later time output portions of it on the console. Is there an easy way to do this in haskell? using the standard i/o functions i can read the file but the output gives me \1071 ... instead of the unicode characters.

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Haskell DB bindings
by oleg＠pobox.com 12 Jan '06

12 Jan '06

Krasimir Angelov wrote: > There are three active database libraries: HDBC, HSQL and Takusen. > It is quite disappointing from my point of view. Recently there was > the same situation with the GUI libraires. I think the dichotomy between lower-level Haskell libraries (whose API is closer/faithful to the underlying library) and higher-level, more idiomatic libraries will always exist, because there is need for both. Regarding GUI libraries, the bulk of wxHaskell is close to wxWindows. However, there are also more higher-level, more idiomatic interfaces built on the top of it. Takusen has its own lower-level layer -- which can be replaced with other suitable lower-level layer, for example, HSQL. By suitable I mean the layer should not improve the underlying database API too much, e.g., add finalizers to database and session handlers, because avoiding such finalizers was one of the main reason for the design of Takusen. It may be an interesting experiment to see if Takusen proper can be moved to use HSQL, for example, as the lower-level layer. Also, perhaps HSQL may find something useful to borrow from Takusen's lower-level layer.

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Re: [Haskell-cafe] multiline strings in haskell?
by Michael Vanier 12 Jan '06

12 Jan '06

Yes, just like that ;-) Thanks! Now if somebody has a string interpolation library, I'd be a pretty happy camper ;-) Mike > mvanier: >> Is there any support for multi-line string literals in Haskell? I've >> done a web search and come up empty. I'm thinking of using Haskell to >> generate web pages and having multi-line strings would be very useful. > > Do you mean like this: > > string = " line one\n\ > \ line two is here\n\ > \ line three is this line\n" > > > $ echo 'putStr string' | ghci A.hs > line one > line two is here > line three is this line > > -- Don

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Help compiling School of Expression graphics library?
by Rakesh Malik 12 Jan '06

12 Jan '06

I'm trying to build the GraphicsLib code in Linux, Fedora. I have the March '05 version of Hugs installed (and ghc, but that doesn't seem to be relevant to this). The first bit of puzzlement comes from the error messages I'm getting: ffihugs +G +L"X_stub_ffi.c" X.hs Warning: unknown toggle `G'; ignoring. Warning: unknown toggle `L'; ignoring. runhugs: Unable to initialise Hugs (Unrecognised option) Hugs is on my path ($HOME/hugs/bin), and I edited the Makefile to point the hugs_install home directory to $HOME/hugs. X_dir is correct, it has lib/libX11.a and include/X11/X.h. My gc version is: gcc (GCC) 4.0.0 20050519 (Red Hat 4.0.0-8) (I don't know whether that's relevant, but just in case.) Thanks for any help you can provide!

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Shootout favoring imperative code
by Scherrer, Chad 12 Jan '06

12 Jan '06

Several people on this list have said that the shootout favors imperative code. Is this really the case? Why is it Clean seems to have no trouble (for the incomplete set of benchmarks that are written in it)? http://shootout.alioth.debian.org/clean.php How difficult would it be to translate the Clean algorithms into Haskell? -Chad

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Intersection types for Haskell?
by Brian Hulley 12 Jan '06

12 Jan '06

Hi - I'm wondering if there is any possiblility of getting intersection types into Haskell. For example, at the moment there is no (proper) typing for: f g x y = (g x, g y) Ideally, I'd like to be able to write: f:: (a -> b & c -> d) -> a -> c -> (b,d) or f :: (a -> b a) -> c -> d -> (b c, b d) which is perhaps clearer and prevents bad types such as (Int -> String & Int -> Char) by construction. While it may be impossible (?) to infer such a type for f, would it be possible to make use of such an annotation (esp since Haskell with GHC extensions already has arbitary rank polymorphism)? Also, as a second point, could functional dependencies in type classes be written using a similar syntax eg instead of class Insert t c a | c a -> t where insert :: t -> c a -> c a we could write: class Insert (h (c a)) c a where insert :: h (c a) -> c a -> c a or class Insert t@(h (c a)) c a where -- re-using as-pattern syntax insert :: t -> c a -> c a to avoid having to have a special syntax just for functional dependencies and/or to be able to write more complicated fundeps more succinctly? Regards, Brian Hulley

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FunctionalJ - a library for Functional Programming in Java
by Graham Klyne 11 Jan '06

11 Jan '06

A colleague alerted me to this, which I thought might be of interest here: http://www.theserverside.com/news/thread.tss?thread_id=38430 (I have already found that my Haskell experiences have influenced my Python programming; maybe there's also hope for my Java?) #g -- Graham Klyne For email: http://www.ninebynine.org/#Contact

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rank-2 :(
by Bulat Ziganshin 11 Jan '06

11 Jan '06

Hello , those rank-2 types wil make me mad :) encodeHelper :: (MRef m r, Binary m a, BitStream m (StringBuffer m r)) => a -> m String encodeHelper x = do h <- newStringBuffer "" stringBufferDefaultCloseFunc put_ h x getStringBuffer h encode x = runST (encodeHelper x) i can't force `encode` to compile, either with or without any type signatures. actually, signature for `encodeHelper` was developed by Hugs, but now i don't know how to proceed further just now tried to force Hugs tell me its type. mission impossible :) DataAltBinaryClass> :t (runST (encodeHelper 1)) ERROR - Cannot justify constraints in application *** Expression : encodeHelper 1 *** Type : ST c [Char] *** Given context : () *** Constraints : (Num b, Binary (ST c) b) Binary class in my library includes monad in its header. May be it's the cause of problems? class Binary m a where -- | Convert value to sequence of bits and write it to the stream put_ :: (BitStream m h) => h -> a -> m () but it's needed because MArray types, for example, can be serialized only in the appropriate monad: instance Binary IO (IOArray a) where ... -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Fannkuch timings
by Daniel Fischer 11 Jan '06

11 Jan '06

On my 1.2 GHz Duron (SuSE linux), I get significantly different timings than those on the wiki. Sebastian Sylvan's, Kimberley Burchett's and Bertram Felgenhauer's all take roughly thrice as long as posted (that's rather consistent, but the factor is surprisingly large). Cale Gibbard's takes 18.44 secs for N = 9, which is rather bad, I think. Really surprising (to me) are the following (N = 10) Algo Wiki Here Clean imperative 2.1 s 4.54 s Fastest Pure 1.8 s 8.65 s Fastest Impure 1.4 s 1.77 s. So the ratio for the fast impure version is approximately 1.6GHz/1.2GHz, which is natural, but the ratio for clean impure is 2.2 and for the fast pure version, it's even 4.8! All are compiled with -O2 -optc-O3. Does that mean my system is really poor in producing binaries from pure Haskell? And why would that be so? Cheers, Daniel

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hPutStrLn and hFlush
by Gracjan Polak 11 Jan '06

11 Jan '06

Hi all, A bit strange behaviour with hPutStrLn. Consider following program: main = do handle <- openFile "output.txt" WriteMode hPutStrLn handle (unlines contLines2) -- hFlush houtput where contLines2 = flip map [1..2000] $ \x -> show x ++ " been there done that" Outputs file which ends with following lines: 1989 been there done that 1990 been there done that 1991 been there done that 1992 been there done that 199 (END) So the output is truncated. When I uncomment hFlush, file is fully written. Is this expected/documented behaviour? Platform: WinXP, GHC version 6.4.1 -- Regards, Gracjan

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