- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Is there a nicer way to do this?
by Don Stewart 07 Jul '08

07 Jul '08

mfeathers: > Don Stewart wrote: > >mfeathers: > >> > >>segment :: Int -> [a] -> [[a]] > >>segment 0 _ = [] > >>segment _ [] = [] > >>segment n x = (take n x) : segment n (drop n x) > > > >The first set of parens can go, > > > > segment n x = take n x : segment n (drop n x) > > > >>I did a version of this which used splitAt but I wasn't sure whether it > >>was going to buy me anything re performance that would justify its > >>ugliness. > > > >Besides, > > > > splitAt n xs = (take n xs, drop n xs) > > Thanks. That is odd, though. It makes me wonder what to expect re > optimization. Would the compiler/runtime know that splitAt could be > done in a single pass? Not with that definition. It would require some moderately unusual fusion combining the take and drop into a single fold with the (,) on the inside, rather than on the outside. However, GHC actually implements splitAt as: splitAt (I n) ls | n < 0 = ([], ls) | otherwise = splitAt' n ls where splitAt' :: Int -> [a] -> ([a], [a]) splitAt' 0 xs = ([], xs) splitAt' _ xs@[] = (xs, xs) splitAt' m (x:xs) = (x:xs', xs'') where (xs', xs'') = splitAt' (m - 1) xs So there may be some benefit. -- Don

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Compiling GHC 6.8.3 on OS X with readline support
by Chris Eidhof 07 Jul '08

07 Jul '08

Hey all, When compiling GHC 6.8.3 on OS X, I ended up with a GHCi without readline support. That makes interacting quite hard, especially because the Backspace-key didn't even work. With some help on irc from Baughn and by reading a blog post from Paul Brown, I managed to get readline working after all. After installing readline from fink, I had to pass the following options to configure: ./configure --with-readline-includes=/sw/include/readline --with- readline-libraries=/sw/lib After that make and sudo make install worked just fine, and I had readline again! I wrote this down in case anybody else has had the same problems. -chris

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Re: [Haskell-cafe] OpenSSL.Digest linking error: EVP_mdc2
by Donn Cave 07 Jul '08

07 Jul '08

Quoth "Ken Takusagawa" <ken.takusagawa.2(a)gmail.com>: | $ ghc --make test-hopenssl.hs -lcrypto | [1 of 1] Compiling Main ( test-hopenssl.hs, test-hopenssl.o ) | Linking test-hopenssl ... | /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In | function `s2O8_info': | (.text+0xf3a): undefined reference to `EVP_mdc2' | /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In | function `s2TQ_info': | (.text+0x1435): undefined reference to `EVP_mdc2' | collect2: ld returned 1 exit status I see the following in the Configure file for openssl-0.9.8h: my $default_depflags = "-DOPENSSL_NO_CAMELLIA -DOPENSSL_NO_CMS -DOPENSSL_NO_GMP -DOPENSSL_NO_MDC2 -DOPENSSL_NO_RC5 -DOPENSSL_NO_RFC3779 -DOPENSSL_NO_SEED -DOPENSSL_NO_TLSEXT ";my $default_depflags = "-DOPENSSL_NO_CAMELLIA -DOPENSSL_NO_CMS -DOPENSSL_NO_GMP I don't know why this would be, but I infer that the function hopenssl was looking for is commonly missing in SSL builds. Donn Cave, donn(a)avvanta.com

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Definition of hidden instance members (bug in GHC or Hugs+Yhc)
by Neil Mitchell 07 Jul '08

07 Jul '08

Hi, This is either a GHC bug, or a Yhc+Hugs bug - I'm not sure which, but the compilers disagree: import Prelude hiding ((==)) data Foo = Foo instance Eq Foo where (==) a b = True GHC says: Temp.hs:14:4: `==' is not a (visible) method of class `Eq' Yhc and Hugs both successfully compile the module. Does anyone know which compiler(s) are in the wrong, and need bugs filing? Thanks Neil

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Santana on my evil ways
by John D. Ramsdell 07 Jul '08

07 Jul '08

My son's nickname is Rama, so let me adopt it. I am a functional programmer, even when I use languages such as C. Scheme facilitated my development into a functional programmer, however, I appreciate the benefits of pure function programming at times. Yet when I use Haskell, I hear reminders of my Scheme past cast in the music of Santana. The words I hear are set to "Eval Ways": You've got to change your evil ways... Rama Before I stop respecting you. You've got to change... Rama And every word that I say, it's true. You use strange syntax and typing And offset rules You don't mutate locations You use strange do's This can't go on... Lord knows you got to change. John

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Type families versus functional dependencies question
by Alexey Rodriguez 07 Jul '08

07 Jul '08

Hi guys, We are having trouble with the following program that uses type families: > class Blah f a where > blah :: a -> T f f a > class A f where > type T f :: (* -> *) -> * -> * the following function does not type: > wrapper :: forall a f . Blah f a => a -> T f f a > wrapper x = blah x GHC gives the error: Couldn't match expected type `T f1 f1 a' against inferred type `T f f a' In the expression: blah x In the definition of `wrapper': wrapper x = blah x Maybe it is a problem with ambiguous types, namely "f" appears only in applications of "T". But that is not the case, there is a "naked" f appearing as the argument of "T f". But perhaps the type checker does not want to unify those two f's precisely because they are the arguments of "T f". I have tried to encode the above program using FunDeps, because this procedure helps me understand type errors. But in this case I get no type error! > class A' (f :: * -> *) (g :: (* -> *) -> * -> *) | f -> g where > class Blah' f a where > blah' :: A' f g => a -> g f a > wrapper' :: forall a f g . (Blah' f a,A' f g) => a -> g f a > wrapper' x = blah' x So my question is whether my encoding is correct. And if it is, why isn't the type families version working? Thanks! Alexey

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not for me, guv!
by Evan Laforge 07 Jul '08

07 Jul '08

So I wrote a big email about this mysterious message I was getting, but then after some research and poking and debugging tracked it down to a dangling pointer in C that was causing a callback to point to an invalid address, which somehow worked anyway 99% of the time. That all said, I'm not totally convinced that "freeHaskellFunctionPtr: not for me, guv! 0x23143f0" is the clearest error message ever. Now, if it were in a more contemporary American idiom such as "wtf this ptr ain't from MY house lol!!!" I probably would have found the bug in half the time. Anyway, I fixed my bug and I'm happy about that, but I didn't want to waste a chance to send an email with this title...

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The state of database libraries
by Chris Eidhof 07 Jul '08

07 Jul '08

Hey everyone, I'm figuring out how to do databases in Haskell (on OS X). So far, I've tried the following approaches: 1. hdbc. I'd like to connect to MySQL, so I need the ODBC backend. I couldn't get this to work under OS X, while I installed myodbc, which seems to be broken. 2. hsql. The packages on hackage don't compile, so I grabbed the darcs version. hqsl itself installed perfectly, but when I try to compile hsql-mysql, it does not recognize that hsql is already installed and tries to recompile. Compiling fails, so I generated a .tar.gz of hsql using cabal sdist, put it in the .cabal/packages directory in the right place, and finally, it does compile. Now hsql-mysql starts compiling, but it finally fails with: MySQL.hsc:270:0: error: ‘MYSQL_NO_DATA’ undeclared (first use in this function) MySQL.hsc:270:0: error: (Each undeclared identifier is reported only once MySQL.hsc:270:0: error: for each function it appears in.) So what is the state of hsql? And haskelldb? Are they actively maintained? Thanks, -chris

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OpenSSL.Digest linking error: EVP_mdc2
by Ken Takusagawa 07 Jul '08

07 Jul '08

I'm using hopenssl 1.0 from hackage. What am I doing wrong here? $ cat test-hopenssl.hs import OpenSSL.Digest main=undefined $ ghc --make test-hopenssl.hs -lcrypto [1 of 1] Compiling Main ( test-hopenssl.hs, test-hopenssl.o ) Linking test-hopenssl ... /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In function `s2O8_info': (.text+0xf3a): undefined reference to `EVP_mdc2' /tmp/ken/lib/hopenssl-1.0/ghc-6.8.2/libHShopenssl-1.0.a(Digest.o): In function `s2TQ_info': (.text+0x1435): undefined reference to `EVP_mdc2' collect2: ld returned 1 exit status $ dpkg-query -S /usr/lib/libcrypto.a libssl-dev: /usr/lib/libcrypto.a $ dpkg-query -l libssl-dev Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Cfg-files/Unpacked/Failed-cfg/Half-inst/trig-aWait/Trig-pend |/ Err?=(none)/Hold/Reinst-required/X=both-problems (Status,Err: uppercase=bad) ||/ Name Version Description +++-======================-======================-============================================================ ii libssl-dev 0.9.8g-10.1 SSL development libraries, header files and documentation $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 $ uname -a Linux localhost 2.6.24-1-686 #1 SMP Thu May 8 02:16:39 UTC 2008 i686 GNU/Linux

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Is there a nicer way to do this?
by Michael Feathers 07 Jul '08

07 Jul '08

segment :: Int -> [a] -> [[a]] segment 0 _ = [] segment _ [] = [] segment n x = (take n x) : segment n (drop n x) I did a version of this which used splitAt but I wasn't sure whether it was going to buy me anything re performance that would justify its ugliness. Michael

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