January 2007 - Haskell-Cafe

Cannot understand liftM2
by Nicola Paolucci 13 Feb '12

13 Feb '12

Hi All, I'm loving learning Haskell quite a bit. It is stretching my brain but in a delightfull way. I've googled, I've hoogled but I haven't found a clear explanation for what exactly liftM2 does in the context below. Using the cool lambdabot "pointless" utility I found out that: > \x -> snd(x) - fst(x) is the same as: > liftM2 (-) snd fst I like the elegance of this but I cannot reconcile it with its type. I can't understand it. I check the signature of liftM2 and I get: Prelude> :t liftM2 Prelude> liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Can someone help me understand what's happening here ? What does a Monad have to do with a simple subtraction ? What is actually the "m" of my example ? I am sure if I get this I'll be another step closer to illumination ... Thanks, Nick

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Separate a string into a list of strings
by Sara Kenedy 18 Apr '08

18 Apr '08

Hi all, I want to write a function to separate a string into a list of strings separated by commas. Example: separate :: String -> [String] separate "Haskell, Haskell, and Haskell" = ["Haskell", "Haskell", "and Haskell"] If anyone has some ideas, please share with me. Thanks. S.

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Creating static libraries using GHC
by Tom Hawkins 16 Mar '07

16 Mar '07

I have a chunk of Haskell code I would like wrap up and distribute as a library. Is there a way to build a static library (*.a) that includes my code plus the Haskell runtime, which C programs can easily link against? Here is what I have tried so far... ghc --make -fffi MyLib # Builds MyLib.o and MyLib_stub.o. gcc -c -I/usr/local/lib/ghc-6.4.1/include MyLibWrapper.c ar -r libMyLib.a MyLib.o MyLib_stub.o MyLibWrapper.o This works fine when I use ghc to compile and link a C program... ghc main.c libMyLib.a But if I use gcc, it throws a lot of unresolved references. I have no problem compiling with ghc, but the folks using my library probably won't have it installed. I added -v to ghc to see how it calls gcc; it seems to link in different libraries based on what Haskell libraries are being used, and it undefines a bunch is symbol references. Are there any switches to have ghc return a single archive with everything included? Please excuse my ignorance of the linking process. Thanks for any help! -Tom

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Haskell Cookbook?
by Alexy Khrabrov 07 Mar '07

07 Mar '07

So I'm picking up Haskell bit by bit, and I found the code examples transpiring here most useful. Reflecting why it's harder to pick up Haskell than say Ruby or Python, here's what I found -- those languages deal with a typical domain available to any programmer -- his own computer/system/shell. The artifacts are files, directories, timestamps, etc. The stuff every programmer understands in their sleep. APIs. So I loved the shell-script beautification thread. That's how I learn about new modules and their functions -- as use cases. I also found some pieces of Haskell strewn around people's websites, blogs, forums; even at paste.lisp.org. I'm gathering them for my own purpose and trying to compile and run them. Wouldn't it be nice -- in case it doesn't exist already -- a[n O'Reilly[-like]] Haskell Cookbook? That would be the best way to learn Haskell. I've found a wikibook on Haskell, but I look for a big bag of small, self-contained programs. Perhaps you esteemed veterans can dig your small scripts and paste them into a wiki? Examples needed -- how to connect to a database; to a web service (e.g. Amazon); read a csv file; represent data equivalent to a directory listing; a text file/XML; etc... Cheers, Alexy

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rebinding >>= for restricted monads
by David Roundy 27 Feb '07

27 Feb '07

Hello fellow haskellers, I am wondering if anyone has an idea whether I'd run into trouble if I rebound >>= in order to provide a more restricted monad. My idea is to define a class: class WitnessMonad wm where (>>=) :: wm w w' a -> (a -> wm w' w'' b) -> wm w w'' b (>>) :: wm w w' a -> wm w' w'' b -> wm w w'' b return :: a -> wm w w' a fail :: String -> wm w w' a which obviously just adds a couple of phantom types indicating the monad state prior to and after each operation. I actually rather like this redefined monad, as it seems like it'd be useful in lots of places (e.g. a restricted IO monad that disallows use of a file handle after it's closed). It seems to me like this would be a pretty cool kind of monad. It'd allow us to encode typechecked "properties" in a monad, and there'd be very little restriction on how tricky those properties could be. And we'd get this all using the same do syntax we all love (and the new properties are by definition sequential, so a monad is precisely what we want, only at the type level). One trouble I see is how to move an ordinary monad into the WitnessMonad class. Of course, they differ in kind, so we need to do something, but I'm not sure what, and suspect that folks with experience using monad transformers will immediately see the answer. Of course, I want to be able to use both my fancy witnessed monad and IO in the same module, and I want to be able to use do notation for both of them. I suppose we could easily define data M m w w' c = M (m c) lift = M lower (M m) = m instance WitnessMonad (M m) where f >>= g = lift $ Prelude.(>>=) (lower f) (lower . g) ... but then we'd have to lift all our operations, so an ordinary IO function in a module that used my fancy rebound do notation would look like f :: IO Char f = lower $ do lift $ putStr "Hello world" lift $ getChar which seems rather heavy. Can anyone think of syntax-light way to allow my hypothesized rebound do-notation to also work with ordinary monads? -- David Roundy Department of Physics Oregon State University

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Re: [Haskell-cafe] State of OOP in Haskell
by dons＠cse.unsw.edu.au 26 Feb '07

26 Feb '07

deliverable: > Well, I'm thinking in terms of OOD/OOA/OOP -- Design, Architecture, > Programming. That's about the only way to model a bog system. Say I > have a stock market model -- I'll have a database of tickers, a > simulator to backtest things, a trading strategy, etc. > > Do Haskell modules provide enough encapsulation to design a system in > terms of them? What are the design/architecture units in Haskell if > not OO-based? Haskell's pretty big on encapsulation (we like our strong statically checked guarantees after all) Some of the encapsulation mechanisms used in large (>5k loc up to 100k + Haskell programs I've hacked on): modules qualified modules type classes to specify generic interfaces existential types to hide implementations completely (and statically) existentials using typeclasses to package the interface monads for state/effect encapsulation abstract data types data types with smart constructors first class modules via existentials purity laziness So the same kind of encapsulation as OO stuff provides (existentials are essentially OO objects), and a few more besides. Monads in particular are perfect for encapsulating interesting effects, and having that encapsulation statically enforced. Probably some of the OO refugees can think of other mechanisms. Cheers, Don

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Saving the AST generated by Template Haskell
by Alfonso Acosta 21 Feb '07

21 Feb '07

Hi all, I'm using Template Haskell to design a small subset of a Hardware Description DSEL (Domain Specific Embedded Language). My language supports higher order as the user can supply small functions as arguments. I chose to parse them with TH because it allows me to use plain Haskell for the function syntax (instead of reinventing the wheel) but mostly because gives parsing for free. The AST of the functions must be kept by the embedded compiler so that it can be later translated to a target language by one of the potential backends of the embedded compiler. The problem is ... how to store that AST? Let me show an example ---- import Language.Haskell.TH.Syntax -- sample function from the DSLE library hdMapSY :: (HDPrimType a, HDPrimType b) => HDFun (a -> b) -> HDSignal a -> HDSignal b -- We keep the function's AST (to make program transformations in the backends) newtype HDPrimFun = HDPrimFun [Dec] deriving Show -- Type safety layer, we keep the function to make sure TH checks the type (and possible further simulations) data HDFun a = HDFun [Dec] a deriving Show -- Helper constructor function, which suffers from the Saving-the-AST problem -- mkMetaAST currently returns a phony value mkHDFun :: Q [Dec] -> Q Exp mkHDFun qd = do dx <- qd metaASTnm <- newName "metaAST" let funnm = getFunName dx metaAST = mkMetaAST dx metaASTdec = ValD (VarP metaASTnm) (NormalB metaAST) [] return $ LetE (metaASTdec:dx) (AppE (AppE (ConE $ mkName "HDFun") (AppE (ConE $ mkName "HDPrimFun") (VarE metaASTnm))) (VarE funnm)) where getFunName :: [Dec] -> Name getFunName [FunD nm _] = nm getFunName _ = error "mkHDFun: toy example, just and exactly one dec!" -- This function should create an AST expression from the AST -- but it would be a big pain to code mkMetaAST :: [Dec] -> Exp mkMetaAST _ = AppE (ConE (mkName "LitE")) (LitE (StringL "big pain to code!")) --- An example program coded in the DSLE could could be something like .. myCircuit :: HDSignal Int -> HDSignal Int myCircuit = hdMapSY ($mkHDFun [d| f input = input+1 |]) So the question is .. How can mkHDfun save the AST of "f input = input+1" (for which it needs to create and return a metaAST) without the effort of having to create boiler plate code for the whole TH library types? Did anyone do something similar before? I workaround would be saving the String of the AST with show, but Dec nor Exp belong to the Read class, :S Thanks in advance, Alfonso Acosta

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trouble installing ghc 6.6: xargs: /usr/bin/ar: terminated by signal 11
by tphyahoo 20 Feb '07

20 Feb '07

I'm having trouble installing ghc 6.6. On ubuntu, virtual server (user mode linux). Something seems to be killing the process, no idea why. Anyone seen this? I looked around with ps to see if anything looked suspicious. I do have a lot of screens open, but I don't see why that should matter. There is no instance of haskell or gnci running. after running configure with no problem..... ~$ cd ghc-6.6/ ~/ghc-6.6$ sudo make &>makeOut Password: ~/ghc-6.6$ tail -n8 makeOut (echo Control/Concurrent_stub.o System/CPUTime_hsc.o System/Time_hsc.o ; /usr/bin/find Control/Applicative_split Control/Arrow_split Control/Concurrent_split Control/Concurrent/Chan_split ...long mess... Text/PrettyPrint/HughesPJ_split Text/Printf_split Text/Read_split Text/Read/Lex_split Text/Show_split Text/Show/Functions_split -name '*.o' -print) | xargs /usr/bin/ar q libHSbase.a /usr/bin/ar: creating libHSbase.a xargs: /usr/bin/ar: terminated by signal 11 make[2]: *** [libHSbase.a] Error 125 make[2]: *** Deleting file `libHSbase.a' make[1]: *** [all] Error 1 make[1]: Leaving directory `/home/thartman/ghc-6.6/libraries' make: *** [stage1] Error 2 ~/ghc-6.6$ -- View this message in context: http://www.nabble.com/trouble-installing-ghc-6.6%3A-xargs%3A--usr-bin-ar%3A… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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State of OOP in Haskell
by Alexy Khrabrov 16 Feb '07

16 Feb '07

...In the tradition of the "letters of an ignorant newbie"... What's the consensus on the OOP in Haskell *now*? There're some libraries such as OOHaskell, O'Haskell, and Haskell~98's own qualified type system with inheritance. If I have GHC, which way to do anything OOP-like is considered "right" today? Cheers, Alexy

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IO is not a monad
by Yitzchak Gale 12 Feb '07

12 Feb '07

<troll> Prelude> let f .! g = ((.) $! f) $! g Prelude> let f = undefined :: Int -> IO Int Prelude> f `seq` 42 *** Exception: Prelude.undefined Prelude> ((>>= f) . return) `seq` 42 42 Prelude> ((>>= f) .! return) `seq` 42 42 </troll> Regards, Yitz

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