January 2014 - Beginners

foldM with tuple and maybe
by Miro Karpis 03 Feb '14

03 Feb '14

Hi, please can you help me with following? I have a following function: f1 :: (String, String) -> IO (Maybe String) which I would like to apply to a list of tuples.I have defined the function like this, but that gives me error (below): f2 :: [(String, String)] -> IO (Maybe String) f2 x = foldM f1 0 x error: Couldn't match expected type `IO (Maybe String)' with actual type `[b0] -> m0 (String, String)' In the return type of a call of `foldM' thanks, m.

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tree structure for resolving nested references
by Renah Scarowsky 30 Jan '14

30 Jan '14

Hi, I'm doing some xml processing where elements can reference each other in one particular way. Such a reference is resolved by copying the content of the referenced element to the content of the referencing element. For example, <A ref="B">foo</A>, bar would resolve to <A>bar</A>bar. The references can also be nested, for example: <A ref="B">foo</A>, bar<C>baz</C> would resolve to <A>baz</A>baz<C>baz</C>. I'm looking to collect all the elements with such references in some sort of (tree?) container for efficient resolution. The container should serve two purposes: 1) I need to check whether there are any cyclic references (for example, A references B which references C which references A) - which is invalid. 2) I need to traverse the tree upwards in order to copy the content from one element to another - beginning with the "leaves" (elements with non-nested references), working up the "branches" (elements with nested references) to the "roots" (elements with references that are not referenced by other elements). Obviously there can be many roots, as well as many leaves that are roots themselves. I'm not familiar with trees in Haskell at all. Can anyone provide some guidance on whether there are any existing containers that would be appropriate for checking cyclic dependencies and for reverse traversal? Thanks, Renah Scarowsky Suite Solutions Create>Manage>Deploy <http://www.suite-sol.com/> http://www.suite-sol.com

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breaking code to several lines
by Miro Karpis 29 Jan '14

29 Jan '14

Hi please,... I have one sql insert statement which is too long to be on one line. Is there a way that I can break it several lines? Or does it have to be everything in one line? here is the query: execute_ conn "INSERT INTO ttableXY (column1, column2, column3, column4, column5, column6, column7) VALUES ('var1', 'var2', 'var3', 'var4', 'var5', 'var6', 'var7')" Thanks, Miro

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Parallel and Concurrent Programming in Haskell - Rate-Limiting the Producer
by Kilian Gebhardt 29 Jan '14

29 Jan '14

Hi, I'm working through Simon Marlows Parallel and Concurrent Programming in Haskell. In Chapter 4, p. 69, or http://chimera.labs.oreilly.com/books/1230000000929/ch04.html#_rate_limitin… there is the task to extend a stream processing module based on Control.Monad.Par in such a way that the producer is rate-limited, i.e. the producer should not produce more than the consumer can consume. The book suggests the following type: data IList a = Nil | Cons a (IVar (IList a)) | Fork (Par ()) (IList a) I struggle to construct the functions for this type, as I don't know what to do with the IList in the Fork case. It seems more natural to me, to have an IVar (IList a) instead, whose value is provided by the computation in Par () in the following way: Fork (Par ()) (IVar (IList a)) streamFromListLimited :: NFData a => Int -> Int -> [a] -> Par (Stream a) streamFromListLimited f d xs = do var <- new fork $ loop f xs var return var where loop :: NFData a => Int -> [a] -> Stream a -> Par () loop _ [] var = put var Nil loop 0 (x:xs) var = do tail <- new put var (Fork (loop d xs tail) tail) loop n (x:xs) var = do tail <- new put var (Cons x tail) loop (n-1) xs tail Can someone give me a hint? Thanks, Kilian

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Error Message when trying to solve alphametic(SEND + MORE = MONEY)
by Sok Ha Chang 27 Jan '14

27 Jan '14

Hi. I’m new to Haskell. Use Haskell Platform on MacBook Air. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.3 I’m try to solve alphametic. SEND + MORE = MONEY Here is code… ---------------------------------------------------------------------------------------------------------- import Data.List decryp :: [Int] -> String -> String decryp (s:e:n:d:o:r:y:[]) a = if send + more == money then expr:a else a where send = s * 1000 + e * 100 + n * 10 + d more = 1000 + o * 100 + r * 10 + e money = 10000 + o * 1000 + n * 100 + e * 10 + y expr = show send ++ "+" ++ show more ++ "=" show money decrypSolver :: [String] decrypSolver = foldr decryp [] (permutations 7 [0, 2, 3, 4, 5, 6, 7, 8, 9]) ---------------------------------------------------------------------------------------------------------- Error Messages is ./decrypSolver.hs: line 10, column 60: Couldn't match expected type `(a0 -> String) -> Int -> [Char]' with actual type `[Char]' The function `"="' is applied to two arguments, but its type `[Char]' has none In the second argument of `(++)', namely `"=" show money' In the second argument of `(++)', namely `show more ++ "=" show money’ ---------------------------------------------------------------------------------------------------------- How can I fix this? Thank you. Sincerely, S. Chang

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Re: [Haskell-beginners] Just started working with Haskell. Need some help
by Pyro Crane 22 Jan '14

22 Jan '14

Hello again. Thank you all for your very helpful replies (below) Here is the error I got when I tried to run my script : Could not find module `Graphics.GD' Locations searched: Graphics/GD.hs Graphics/GD.lhs As with the previous errors, I went on Google, to find out which module and/or package I needed to install. Apparently, I need something called "GD" When I tried : cabal install gd I got the following : Resolving dependencies... Configuring gd-3000.7.3... cabal-1.16.0.2: Missing dependencies on foreign libraries: * Missing (or bad) header file: gd.h * Missing C libraries: gd, expat Failed to install gd-3000.7.3 cabal-1.16.0.2: Error: some packages failed to install: gd-3000.7.3 failed during the configure step. The exception was: ExitFailure 1 Upon further inquiries via google, I found somewhere the following : This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. I thought I did this already. But, apparently, I did something wrong?? -----Original Message----- From: beginners-request <beginners-request(a)haskell.org> To: beginners <beginners(a)haskell.org> Sent: Sat, Jan 18, 2014 11:35 pm Subject: Beginners Digest, Vol 67, Issue 19 Message: 1 Date: Sat, 18 Jan 2014 12:25:01 -0500 (EST) From: Pyro Crane <pyrocrane1(a)aol.com> To: beginners(a)haskell.org Subject: [Haskell-beginners] Just started working with Haskell. Need some help Message-ID: <8D0E2895A21535B-1058-8B60(a)webmail-d276.sysops.aol.com> Content-Type: text/plain; charset="us-ascii" Hi, I just started working with Haskell Running it on a RHEL linux operating system I was able to download and install it, along with libraries and packages (or, at least, what I thought were the libraries and packages) But, when I try to run a simple script, I keep getting the error : "Could not find module XXXXXX".............. I understand what this error means ---- there is a package/library missing. I tried using the "Cabal Update" command, but this did not solve the problem. So, I am now fixing this problem the primitive way, as follows : (a) I use Google to first find out what the missing module is (b) then, based on what google says, I am able to determine the name of the actual package that needs to be installed (c) I use "Cabal Install" to install the package (d) I try to run the script again (e) I get the same error, but this time, it cannot find a different module (f) I go back to Google to locate the missing module etc, etc, etc, etc Obviously, this is as slow as it is ridiculous............... especially when one considers that there are probably hundreds of these modules/packages And, now, I've come up against a module/package, which I am unable to locate in Google. It's missing from my installation, and Google does not know what it is, or where I can find it Is there any way to simply install ALL required packages??? I thought I already did this, but obviously I missed something Thanks

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Follow @HaskellTips
by Kim-Ee Yeoh 21 Jan '14

21 Jan '14

@HaskellTips is a twitter account just launched by Jon Fischoff, a member of the community. It looks to have useful tidbits of Haskell programming folklore that anyone who's done enough hacking in it acquires. Eventually. But following the twitter account should give you a leg-up in the process. If you prefer an email feed, here's one way: 1) Use StreamSpigot to get a web page like this: http://www.streamspigot.com/tweet-digest/digest?usernames=HaskellTips&outpu… 2) Use your favorite website-change-detection service to get deltas via email. For e.g. search "web page changes". If you're stumped by anything @HaskellTips, feel free to discuss it here! -- Kim-Ee

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Introduction to FFI by wrapping MurmurHash3
by Courtney Robinson 19 Jan '14

19 Jan '14

Just thought I'd share a lib I've put together and the blog post I wrote about it http://haskell.zcourts.com/ffi/2014/01/19/haskell-foerign-function-interfac… The package is available on Hackage http://hackage.haskell.org/package/Dish And API docs for MurmurHash3 at http://hackage.haskell.org/package/Dish-0.0.0.4/docs/Data-Dish-Murmur3.html It is a wrapper around MurmurHash3 at the moment but I've got some more hash related utilities I'll be adding. I'm still pretty new to Haskell so would appreciate any feedback or pointers on any gotchas I may have overlooked.

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Eg.
by willie ekaputra 19 Jan '14

19 Jan '14

Eg. how to make the translation formula ? u take a coordinate and multiply it with p Element R, so that it is translated , like 1, 3 to 2, 6 , I use a 2 as multiplicator , which is from real numbers.How to express this real number ?That is why " any number will just do ! " Regards , Wili.

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"any real numbers will just do"
by willie ekaputra 19 Jan '14

19 Jan '14

how to express that Expression in Hasski ? Tq. regards, Willie. "Sanctus Dominus ."

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