- Beginners - Haskell.org

question regarding standard libraries
by Jake Penton 03 Jun '11

03 Jun '11

I have a couple of questions about "standard libraries" as referred to in the Haskell 98 and 2010 reports. First, is there anything to infer from the use of the term "libraries"? What I see documented in the reports is a set of modules. Is this just a loose usage of the term libraries to mean a set of modules implemented somehow or other? Second, although I have only perused the 98 and 2010 reports briefly, what "standard" means in this context does not stand out for me. Part II of the 2010 report jumps right into the description of Control.Monad with no mention of the relationship of Part II to the rest of the report. Are the documented modules part of the language? Third, as I am learning Haskell by studying Real World Haskell I note that several of the modules discussed therein are not listed in the Haskell 2010 report. A notable example is Data.Map. What is the status of such modules? Are they just (maybe) available in particular implementations, or in privately donated packages? Finally, is there a good way to track changes in the implementation of various modules, whether "standard" or otherwise? For example I discover that Data.Map.lookup has changed (in ghc anyway) since RWH was written. Is this change agreed upon somewhere, or is it just dependent on whatever happens to be delivered with the Haskell implementation that I use? Thank heaven for a beginner's mailing list where I can ask dumb questions. And thanks in advance once again for your kind efforts to help me out. - J -

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Different ByteStrings, what now?
by Raphael Päbst 02 Jun '11

02 Jun '11

Hello once again! My forays into Haskell are once again stopped by my weak Haskell Fu. I am working with ProtocolBuffers and so far everything worked fine, but now I have encountered a problem. I am trying to serialize a message that contains, among other things, a blob of binary data. My problem now is to get that binary data into the message. The message requires the data type Text.ProtocolBuffers.Header.ByteString, whereas my data is of the type Data.ByteString. How can I turn one into the other or get my data into the right format in te first place? I tried to find a function to solve this, but I couldn't find it in the ProtoBuff documentation. Thanks Raf

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How to access non-exported function from Data.Set?
by Christoph Bussler 02 Jun '11

02 Jun '11

Hi, In Data.Set there is a function splitLookup splitLookup :: Ord a => a -> Set a -> (Set a,Maybe a,Set a) that is not exported, but would precisely give me the functionality that I need. Not sure why it is not exported, but is there a way to use it? Thanks, Christoph -------------------------------------------------------------------------- Christoph Bussler ChBussler(a)aol.com www.real-programmer.com www.linkedin.com/in/chbussler www.google.com/search?hl=en&q=Christoph+Bussler www.google.com/search?hl=en&q=Christoph%20Bussler&btnI=I%27m+Feeling+Lucky --------------------------------------------------------------------------

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Install on "foreign" server problem
by Lawrence Bottorff 01 Jun '11

01 Jun '11

I'm trying to install the binary of 7.0.3 on a Bluehost.com server where I have an account and ssh shell. I want to eventually run Yesod or happstack or some other sort of haskell on web. I've talked with their tech support and they're cool with it as long as it's in-place. Good. I try the ./configure and it errors saying things like: checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I'm not (nor ever was) much of a sysadmin type. Any advice? Actually, I'm just trying to do Web with Haskell on a server/cloud other than my own home box. I have a Web client and I'd like to get us off PHP. Any Haskell-friendly clouds/hosts to suggest? Olwe

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Problems with the Network package
by Raphael Päbst 01 Jun '11

01 Jun '11

Hey everyone! I'm still pretty inexperienced with Haskell and am now facing problems with the Network package. I tried to install it but this always fails, no matter if I try to install Network-2.3.0.3 or a previous version. The cabal install always fails with the error compiling dist\build\Network\Socket_hsc_make.c failed (exit code 1). Now I'm wondering, where I can find out what the problem is, what I can do about it and which additional information I need to solve it. I hope this is not too stupid a question, but my searches on the net haven't turned up anything useful so far and I'm pretty much stuck now. Raf

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Question on Real World Haskell ch.13 SymbolicManip
by Jake Penton 01 Jun '11

01 Jun '11

Greetings. I post here from time to time as I work my way through Real World Haskell. I believe that I have understood most of what I have studied. However, today I ran into something that seems like magic. I hope it's ok to refer to RWH here, as I believe that the book is very commonly used for learning the language. I'll include the code that is puzzling me here anyway, though. On p.310 of RWH we have: data Op = Plus | Minus | Mul | Div | Pow deriving (Eq, Show) data SymbolicManip a = Number a | Arith Op (SymbolicManip a) (SymbolicManip a) deriving (Eq, Show) instance Num a => Num (SymbolicManip a) where a + b = Arith Plus a b a - b = Arith Minus a b a * b = Arith Mul a b negate a = Arith Mul (Number (-1)) a abs a = error "abs is unimplemented" signum _ = error "signum is unimplemented" fromInteger i = Number (fromInteger i) Ok, so far so good. But the following example seems quite astounding to me: ghci> (5 * 10 + 2)::SymbolicManip Int Arith Plus (Arith Mul (Number 5) (Number 10)) (Number 2) The book says to notice that haskell "converted" 5 * 10 + 2 into a SymbolicManip. Indeed! My understanding breaks down at this point. I suspect that it may be my weak grasp of the implications of lazy evaluation, and typeclasses generally. My (incorrect) intuition makes me think that the stuff inside the parentheses should be evaluated first, and then typed as a SymbolicManip. This would give the result Number 52. My question may be too vague to answer easily, but what I am hoping for is a bit of a narrative regarding how haskell knows that the pieces inside the parentheses are each of type SymbolicManip Int. Or something like that.... Thanks in advance. - Jake -

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How to avoid repeating code
by Federico Mastellone 31 May '11

31 May '11

Hi, I created a Data.MultiMap module based on Data.Map and Data.Set like this: data MultiMap k v = MultiMap (Map k (Set v)) and a Data.IntMultiMap module based on Data.IntMap and data.IntSet like this: data IntMultiMap = IntMultiMap (IntMap IntSet) For example the functions to add a value I wrote are: For MultiMap: addValue :: k -> v -> MultiMap k v -> MultiMap k v addValue k v (MultiMap m) = MultiMap $ Map.insertWith (\new old -> Set.insert v old) k (Set.singleton v) m For IntMultiMap: addValue :: Int -> Int -> IntMultiMap -> IntMultiMap addValue k v (IntMultiMap m) = IntMultiMap $ IntMap.insertWith (\new old -> IntSet.insert v old) k (IntSet.singleton v) m Both modules look almost the same, with the same documentation, same behavior, same function names but with different type signatures. Is there a way to make this simpler? The same thing happens to the modules that are using MultiMap and IntMultiMap, I have to write two versions of each. Thanks! -- Federico Mastellone Computer Science Engineer - ITBA ".. there are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult." Tony Hoare, 1980 ACM Turing Award Lecture.

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Please critique my first library
by Gideon 31 May '11

31 May '11

I've written a modest data binding library for haskell GUIs; the code is at http://code.accursoft.com/binding/. binding-core exposes functionality for binding IO objects to mutable variables and lists. bindnig-gtk and binding-wx are wrappers for Gtk2Hs and WxHaskell respectively. Examples of how to use these can be found in the tests directory of binding-core, and the demo directories of the GUI wrappers. Thank you!

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Handling a MonadError
by Christopher Howard 30 May '11

30 May '11

Hi. I'm trying to learn Data.ConfigFile. I want to use the function readfile: [DESCRIPTION] readfile :: MonadError CPError m => ConfigParser -> FilePath -> IO (m ConfigParser) Loads data from the specified file. It is then combined with the given ConfigParser using the semantics documented under merge with the new data taking precedence over the old. However, unlike merge, all the options as set in the old object are preserved since the on-disk representation does not convey those options. May return an error if there is a syntax error. May raise an exception if the file could not be accessed. [/DESCRIPTION] I understand that this function returns a monad, and I pull the value out of it with (<-). And I believe this is similar to a "Maybe" situation where I can do one thing if it provides a syntax error and another thing if it provides the ConfigParser. But I'm not sure what this would look like in actual code. Could somebody give me a small, simple example? -- frigidcode.com theologia.indicium.us

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Re: [Haskell-beginners] Install on "foreign" server problems
by Michael Snoyman 30 May '11

30 May '11

On Mon, May 30, 2011 at 11:53 PM, Lawrence Bottorff <galaxybeingplan9(a)gmail.com> wrote: >> Hi Lawrence >> >> I think a lot of us are finding that the easiest solution is to >> compile the binaries locally and upload to the server. In my case, >> I've tried having the compiler on a VPS, but compiling is very >> memory-intensive, and it can be a real pain whenever you need to >> upgrade libraries. >> >> My newest deployment is simply an EC2 micro instance running Ubuntu >> 32-bit, just like I have on my local system. I compile the app, copy >> the binary and the static files, and deployment is done. I'll likely >> be adding some notes about this to the Yesod book, and perhaps even >> put together some scripts to automate the process more. >> >> Michael > > So I would do an "in situ/in-place" binary install on a home machine and > simply move that whole installation directory onto the foreign machine. > Hmmm.... What about dependent libraries? If it can't find libc (or the > GL-mesa libraries) won't it break? I'm guessing from the instructions that I > would do a > # ./configure --prefix=/home/me/haskell/ > then a > # make in-place > Is this right? Then move that /haskell directory onto the foreign box? > Lb > What I meant was not to have GHC on the server at all. Build your web apps locally, and then upload the binaries to the server. You can even install a virtual machine locally with the same OS as what your server is using. Michael

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