December 2006 - 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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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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some way to reverse engineer lambda expressions out of the debugger?
by tphyahoo 12 Jan '07

12 Jan '07

I am a newbie learning haskell. (First forum post.) I am wondering if there is a trick to get debugging information about functions out of the environment (which for me, for now, is ghci). In this example, *UnixTools> :t map (*) [1,2] map (*) [1,2] :: (Num a) => [a -> a] This is very nice, but I would *really* like to see something like *UnixTools> explodeLambda( map (*) [1,2] ) [(\x -> 1*x),(\x -> 2*x)] Yes, maybe I'm dreaming, but I would like haskell to reverse engineer / pretty print lambda expressions for me. (Note that: *UnixTools> map ($ 5 ) [(\x -> 1*x),(\x -> 2*x)] [5,10] *UnixTools> map ($ 5) ( map (*) [1..2] ) [5,10] So these expressions really are the same, only it could be argued that the first expression is in some sense easier to read if you are debugging something complex. ) I would like to have something like "Data::Dumper" from perl, but of course, on steroids. Is something like this possible, or be worked on? Or probably never going to happen? Cheers, thomas. -- View this message in context: http://www.nabble.com/some-way-to-reverse-engineer-lambda-expressions-out-o… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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question about "How to Write a Haskell Program" tutorial
by Michael Vanier 09 Jan '07

09 Jan '07

First off, I apologize if this has come up before. As far as I can tell, the mailing list archives don't have a search function. I'm running ghc-6.6 and haddock-0.8, both compiled from source. I'm working my way through the "How to Write a Haskell Program" tutorial (which is a great idea, thanks guys!), and everything works fine until I get to the part about creating the haddock documentation. When I run: $ runhaskell Setup.hs haddock I get this: Preprocessing executables for haq-0.0... Running Haddock for haq-0.0... Warning: cannot use package haq-0.0: ghc-pkg failed With verbosity: $ runhaskell Setup.hs haddock -v I get this: Preprocessing executables for haq-0.0... Running Haddock for haq-0.0... /usr/local/packages/haskell-misc/bin/haddock --html --odir=dist/doc/html/haq --title=haq --use-package=haq-0.0 --use-package=base-2.0 dist/build/tmp/Haq.hs Warning: cannot use package haq-0.0: ghc-pkg failed Note that I'm installing into a non-standard location. I have no idea why haddock (called from cabal) is trying to use a package that is in the process of being created. Nevertheless, the documentation does get generated. Is this just something that should be ignored? Is it a Cabal bug? Mike

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Literate Haskell source files. How do I turn them into something I can read?
by Michael T. Richter 07 Jan '07

07 Jan '07

I'm trying to wrap my mind around the darcs source code as a preliminary to looking into GHC's guts. All of darcs is written as .lhs files which have bizarre mark-up in them which distracts me from the actual Haskell source I'm trying to figure out and get used to. Apparently the GHC compiler can take .lhs files, strip them with "unlit" (a utility which I finally found buried deep in the GHC installation -- off-path) and then compile them normally. The problem I have is that unlit leaves behind instead these huge gaping (and highly distracting) stretches of whitespace while it takes out the markup. Are there any tools which I can use to render .lhs files readable? I'm fine with having them converted into documented source (i.e. source code embedded in documentation) or as pure Haskell source (but without the huge whitespace gaps) -- but I can't figure out how to get either. -- Michael T. Richter Email: ttmrichter(a)gmail.com, mtr1966(a)hotpop.com MSN: ttmrichter(a)hotmail.com, mtr1966(a)hotmail.com; YIM: michael_richter_1966; AIM: YanJiahua1966; ICQ: 241960658; Jabber: mtr1966(a)jabber.cn "I think it is very beautiful for the poor to accept their lot [...]. I think the world is being much helped by the suffering of the poor people." --Mother Theresa

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less than helpful GHC error message
by Nicolas Frisby 07 Jan '07

07 Jan '07

Consider the terminal session at the bottom of this message. The extra "xc" in Test.hs was the result of me missing CTRL during an Emacs command (I'm guessing...). Unfortunately this took 30 minutes to find since I had far more than two modules and the error message doesn't point this out. Sure it's a silly mistake to make, but I'm guessing it's pretty common. I'm running GHC 6.6 on a PPC Mac. I know pragmas are wierd things, but could this error message be improved? Thanks, Nick $ head -n 10000 Main.hs Test.hs ==> Main.hs <== module Main where import Test main = test ==> Test.hs <== xc{-# OPTIONS -fglasgow-exts #-} module Test where test = putStrLn "weee" $ ghc --make Main <no location info>: file name does not match module name `Main'

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Re: [Haskell-cafe] Seeking advice on a style question
by Steve Schafer 07 Jan '07

07 Jan '07

On Tue, 26 Dec 2006 20:21:45 -0800, you wrote: >How would this example look if you named only multiply-used expressions? >I'd like to see it in a more conventional pointful style with nested >expressions. I'm still wondering whether the awkwardness results from your >writing style or is more inherent. Showing the real variable names may also >help also. This is what it looks like "for real": > process :: Item -> MediaKind -> MediaSize -> Language -> SFO > process item mediaKind mediaSize language = > let pagemaster = loadPagemaster item mediaKind mediaSize; > questions = stripUndisplayedQuestions mediaKind $ > appendEndQuestions item pagemaster $ > coalesceParentedQuestions $ > validateQuestionContent $ > loadQuestions item; > (numberedQuestions,questionCategories) = numberQuestions pagemaster questions; > numberedQuestions' = coalesceNAQuestions numberedQuestions; > (bands,sequenceLayouts) = buildLayout mediaKind language numberedQuestions'; > bands' = resolveCrossReferences bands; > groupedBands = groupBands bands'; > pages = paginate item mediaKind mediaSize pagemaster groupedBands; > pages' = combineRows pages; > sfo = pages' sequenceLayouts; > in sfo These are the function signatures: > loadPagemaster :: Item -> MediaKind -> MediaSize -> Pagemaster > loadQuestions :: Item -> [Question] > validateQuestionContent :: [Question] -> [Question] > coalesceParentedQuestions :: [Question] -> [Question] > appendEndQuestions :: Item -> Pagemaster -> [Question] -> [Question] > stripUndisplayedQuestions :: MediaKind -> [Question] -> [Question] > numberQuestions :: Pagemaster -> [Question] -> ([NumberedQuestion],[QuestionCategory]) > coalesceNAQuestions :: [NumberedQuestion] -> [NumberedQuestion] > buildLayout :: MediaKind -> Language -> [NumberedQuestion] -> ([Band],[SequenceLayout]) > resolveCrossReferences :: [Band] -> [Band] > groupBands :: [Band] -> [[Band]] > paginate :: Item -> MediaKind -> MediaSize -> Pagemaster -> [[Band]] -> [Page] > combineRows :: [Page] -> [Page] > createSFO :: [Page] -> [SequenceLayout] -> SFO MediaKind, MediaSize and Language are simple enumerations; everything else is a complex structure. Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

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State separation/combination pattern question
by Reto Kramer 03 Jan '07

03 Jan '07

I'm looking for a pattern to use for "state separation" in my application. I need to write two "stateful libraries". One is a partitioned in- memory store, the other a disk based store with an in-memory cache fronting it. Both store modules need the IO and State monad. My aim is to write the libraries independently from one another in such a way that they are not aware of the context they will be used in. Both stores are called from a breadth first search loop. The example below extracts the essence of my current approach. foo is function in one of the store modules, bar is from the other type of store. Both manipulate their type of state (StateA, StateB). They extract their slice of the "global" application level state (AppStateRec). What I don't like is that the code in foo and bar know about the AppStateRec and that they need to get the "a", or "b" slot respectively. The store modules should not have such coupling with their client code. I've tried to thread the two states (StateA and StateB) using a chain of StateT ... StateT ..., but couldn't really make that work. It seems rather arbitrary in this case which state to make the inner/ outer one and depending on this ordering the "lifts" have to go with one or the other set of store calls. What I'm really looking for is not so much the chaining of StateT compositions, but rather the isolation of StateA from StateB while they both flow from the search loop into the respective library calls (foo, bar) transparently to the application programmer. I'm hoping there's a way to have the loop be in a State monad whose content is the sum of the two states that are needed for the foo and bar call made to the stores from inside the loop. The calls sites for foo and bar should then extract the right component of the global state and thread only that state through into the modules. Sounds like magic, but how close can I get? I've been unable to find a pattern on the WIKI or the web that refers to this type of "state composition" (or I may not have recognized the match). I trust many of you have run into this and there's an obvious and straight forward best way to address this type of state handling. Thanks, - Reto -------------------------- -- ghci -fglasgow-exts ... -- type StateA = [Integer] type StateB = [Integer] data AppStateRec = AppStateRec { a :: StateA, b :: StateB } deriving Show foo :: MonadState AppStateRec m => m () foo = do st <- get put $ st { a = 1:(a st) } bar :: MonadState AppStateRec m => m () bar = do st <- get put $ st { b = 2:(b st) } type Eval a = StateT AppStateRec Identity a exec :: Eval () exec = do foo bar foo foo bar go = runIdentity $ runStateT exec AppStateRec { a = [], b = [] } Prints: ((),AppStateRec {a = [1,1,1], b = [2,2]})

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