- Beginners - Haskell.org

New to Haskell: constructing Data Types
by Tyler Hayes 05 Feb '11

05 Feb '11

As an exercise, I'm attempting to create an abstract syntax tree for a very small language: data Expression = Variable String | Integer | Expression BinOp Expression deriving Show data Statement = Assignment Variable Expression --error! | If Expression Statement Statement | While Expression Statement | Compound Statement deriving Show data BinOp = Add | Mult | LessThan | GreaterThan deriving Show However, there is a problem with having "Variable" as an argument to the "Assignment" constructor of the "Statement" data type: Prelude> :l test.hs [1 of 1] Compiling Test ( test.hs, interpreted ) test.hs:8:28: Not in scope: type constructor or class `Variable' Failed, modules loaded: none. An 'assignment' is a variable followed by an expression in this example, so what can I do to fix this? Thanks, Tyler

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lazy IO by example
by Tim Baumgartner 05 Feb '11

05 Feb '11

Hi community, In the following code, the main function only wants to print the first 5 numbers of a huge list. But since the computation is not lazy, this doesn't work in a satisfactory way. content :: Int -> IO [Int] content i = do fs <- files i ds <- directories i fss <- mapM content ds return $ fs ++ concat fss files i = return [1..i] directories i = return [1..i-1] main = content 1000000 >>= print . take 5 Now I'd like to know if it's possible to make this "IO" lazy, using unsafeInterleaveIO. I tried to do it, failed. Perhaps someone can help. I guess another weakness of this code is the bad performance of "dfs ++ concat fss", but currently I don't care about that. Thanks for any help Tim

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Informal proof of bijective mapping between Naturals and Natural pairs
by Lyndon Maydwell 04 Feb '11

04 Feb '11

Hi Beginners. I was musing over weather Naturals were isomorphic to Natural pairs and wrote the following "proof". I don't have any formal mathematical training beyond calculus, and was wondering if this was an okay approach to take, if there were any better techniques available, if my terminology makes sense, and if my use of Haskell could be improved. Any pointers would be greatly appreciated! The code follows, and is also available as a gist: https://gist.github.com/804248 ---------- import Control.Monad.Logic import Data.Maybe import Data.List import Data.Numbers.Primes import Test.QuickCheck -- This program demonstrates a mapping between the pairs of natural numbers and a subset of co-primes. -- The property should hold for all sized lists, not just pairs. Ordering is preserved. -- Question: Are we able to compress the range to create a bijection? -- Answer: Yes! We can use the breadth-wise indces of the products, rather than the products themselves. -- Pair to Number x_p = (evens !!) y_p = (odds !!) xy_p x y = x_p x * y_p y xy_c x = ns_nc . xy_p x -- Number to Pair p_xy n = (x,y) where x = fromJust $ findIndex (`divides` n) evens y = fromJust $ findIndex (`divides` n) odds c_xy = p_xy . nc_ns -- Sparse Number to Compact Number numbers = odds >>- \x -> evens >>- \y -> return (x*y) -- I don't really understand LogicT... :-( ns_nc n = fromJust $ findIndex (==n) numbers -- Compact Numner to Sparse Number nc_ns = (numbers !!) -- Helpers evens = map (primes !!) [0,2..] odds = map (primes !!) [1,3..] x `divides` y = y `mod` x == 0 -- Id referencing properties prop_p1 = forAll (elements [(x,y) | x <- [1..6], y <- [1..7]]) f where f (x,y) = (x,y) == c_xy (xy_c x y) prop_p2 = forAll (elements [1..100]) f where f c = c == uncurry xy_c (c_xy c)

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Lujvo forming with cmavo rafsi
by Alex Rozenshteyn 03 Feb '11

03 Feb '11

Specifically, I want to form a lujvo meaning "omnicide" (to ta'o this is due to the tvtropes trope "Omnicidal Maniac" ta'onai toi). {ro zei catra} or {rolcatra} seems to be wrong. {ro zei se catra} seems like it's all the victims, so maybe selrolselcatra? I'm not quite clear on what it means to be a lujvo if you don't have a tanru as the basis. -- Alex R

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Sqlite3 - INSERT statement question
by Patrick Lynch 03 Feb '11

03 Feb '11

Good morning, I'm using "Real World Haskell" and a Windows Vista PC and I have Sqlite3 installed...[note: I had to change test1.db to /users/user/test1.db in order to get this to work, otherwise, neither the database nor the table could be created -- also, note c:/users/user/test1.db gives a syntax error], ghci doesn't like c:/]: I tried to use the following example from the book but it failed...see following: Prelude> :m Database.HDBC Database.HDBC.Sqlite3 Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "/users/user/test1.db" ... Prelude Database.HDBC Database.HDBC.Sqlite3> run conn "INSERT INTO test VALUES (?, ?)" [toSql 0, toSql "zero"] <interactive>:1:43: No instance for (Data.Convertible.Base.Convertible t SqlValue) arising from a use of `toSql' at <interactive>:1:43-49 Possible fix: add an instance declaration for (Data.Convertible.Base.Convertible t SqlValue) In the expression: toSql 0 In the third argument of `run', namely `[toSql 0, toSql "zero"]' In the expression: run conn "INSERT INTO test VALUES (?, ?)" [toSql 0, toSql "zero"] I'd appreciate any advice...

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ghci loading of hs files...
by Patrick Lynch 03 Feb '11

03 Feb '11

...I'm using the book "Real World Haskell"... ...In Chapter 22, it uses two files, namely: ...\ch22\PodDB.hs and ...\ch22\PodTypes.hs. ...This is the calling code in PodDB.hs: {-- snippet all --} module PodDB where import Database.HDBC import Database.HDBC.Sqlite3 import PodTypes ...I was instructed to start ghci and invoke the command: ...> :load C:\Users\User\Downloads\rwh-examples2\examples\ch22\poddb.hs ...this gives the following error message: C:\Users\User\Downloads\rwh-examples2\examples\ch22\poddb.hs:6:7: Could not find module `PodTypes': Use -v to see a list of the files searched for. Failed, modules loaded: none. ...I then tried the command: :l C:\Users\User\Downloads\rwh-examples2\examples\ch22\poddb.hs C:\Users\User\Downloads\rwh-examples2\examples\ch22\podtypes.hs ...it loaded both files... ...is there someway to modify the code in PodDB.hs that would allow me to simply invoke the command, as mentioned by the author, :load PodDB.hs? Thank you

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Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista
by Patrick Lynch 03 Feb '11

03 Feb '11

...just to put the 'finishing touch' on this -- Daniel Fischer pointed out how to use the Windows back slash for this command [he pointed it out that GHCI consider the backspace to be an escape character] - note the use of the double back slashes: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "c:\\users\\user\\test1.db" Prelude Database.HDBC Database.HDBC.Sqlite3> Thanks Daniel... ----- Original Message ----- From: Patrick Lynch To: Patrick Lynch ; beginners(a)haskell.org Sent: Thursday, February 03, 2011 1:01 PM Subject: Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista ...oh dear, guess what -- I told you a lie...ghci does except either c:/users/user/test1.db or /users/user/test1.db but not the Windows folder separator of backslash '\'...please see following: ...this works: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "c:/users/user/test1.db" Prelude Database.HDBC Database.HDBC.Sqlite3> ...so does this: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "/users/user/test1.db" Prelude Database.HDBC Database.HDBC.Sqlite3> ...this doesn't work, note the back slash...also, this will only work, that is to create the database automatically and then subsquently create a table, if you user the 'users' folder...if you use 'program files, etal' you will not be able to create the database...bummer: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "c:\users\user\test1.db" <interactive>:1:27: lexical error in string/character literal at character 'u' ---- Original Message ----- From: Patrick Lynch To: Patrick Lynch ; beginners(a)haskell.org Sent: Thursday, February 03, 2011 11:24 AM Subject: Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista ...this seems to work: ghci> conn <- connectSqlite3 "/users/user/test1.db" ...this allows me to create a database in this directory, named test1.db, and then I can create a table in this database... ...I'm now working on writing some queries... ...wish me luck ----- Original Message ----- From: Patrick Lynch To: Kathleen Lynch ; beginners(a)haskell.org Sent: Thursday, February 03, 2011 10:08 AM Subject: Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista ...could this be the problem...in Windows Vista I cannot change the file name in the directory c:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci but can change it in c:\users\user directory... ...> ren "c:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci\test1.db" test2.db Access is denied. ...but I can see the contents of this file: ...> type test1.db hello, world! ...is it possible to specify the directory c:\users\user in ghci so that Sqlite3 will look there to open the database and then to create a table in it? Good day ----- Original Message ----- From: Kathleen Lynch To: beginners(a)haskell.org Sent: Wednesday, February 02, 2011 5:28 PM Subject: [Haskell-beginners] Can't find or open a Sqlite3 database in Windows Vista ...seems like I 'kicked over another bee hive'...plenty of messages in regard to this when you do google search--->[sqlite3.operational error unable to open database file in windows vista] ...i'm getting the following error messages when I try to create a database in Sqlite3: *Sandbox> dbh <- connect "poddbtest.db" *** Exception: SqlError {seState = "", seNativeError = 21, seErrorMsg = "prepare 6: BEGIN: unable to open database file"} *Sandbox> dbh <- connect "test1.db" *** Exception: SqlError {seState = "", seNativeError = 1, seErrorMsg = "step: SQL logic error or missing database"} Both come from Chapters 21 and 22 in "Real World Haskell" I can't invoke Sqlite3 from the command line using---> Sqlite3 myDatabase.db... I created blank files in folder C:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci -- seemed to get me a bit further along...at least I was able to connect... Is it possible to specify the folder that the database should be created in? Is it possible to run Sqlite3 from the command line - when it has been installed using the Haskell wiki? Has anyone had better success with MySql - I've used it in the past and it was very reliable? Thank you ------------------------------------------------------------------------ _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista
by Patrick Lynch 03 Feb '11

03 Feb '11

...oh dear, guess what -- I told you a lie...ghci does except either c:/users/user/test1.db or /users/user/test1.db but not the Windows folder separator of backslash '\'...please see following: ...this works: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "c:/users/user/test1.db" Prelude Database.HDBC Database.HDBC.Sqlite3> ...so does this: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "/users/user/test1.db" Prelude Database.HDBC Database.HDBC.Sqlite3> ...this doesn't work, note the back slash...also, this will only work, that is to create the database automatically and then subsquently create a table, if you user the 'users' folder...if you use 'program files, etal' you will not be able to create the database...bummer: Prelude Database.HDBC Database.HDBC.Sqlite3> conn <- connectSqlite3 "c:\users\user\test1.db" <interactive>:1:27: lexical error in string/character literal at character 'u' ---- Original Message ----- From: Patrick Lynch To: Patrick Lynch ; beginners(a)haskell.org Sent: Thursday, February 03, 2011 11:24 AM Subject: Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista ...this seems to work: ghci> conn <- connectSqlite3 "/users/user/test1.db" ...this allows me to create a database in this directory, named test1.db, and then I can create a table in this database... ...I'm now working on writing some queries... ...wish me luck ----- Original Message ----- From: Patrick Lynch To: Kathleen Lynch ; beginners(a)haskell.org Sent: Thursday, February 03, 2011 10:08 AM Subject: Re: [Haskell-beginners] Can't find or open a Sqlite3 database inWindows Vista ...could this be the problem...in Windows Vista I cannot change the file name in the directory c:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci but can change it in c:\users\user directory... ...> ren "c:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci\test1.db" test2.db Access is denied. ...but I can see the contents of this file: ...> type test1.db hello, world! ...is it possible to specify the directory c:\users\user in ghci so that Sqlite3 will look there to open the database and then to create a table in it? Good day ----- Original Message ----- From: Kathleen Lynch To: beginners(a)haskell.org Sent: Wednesday, February 02, 2011 5:28 PM Subject: [Haskell-beginners] Can't find or open a Sqlite3 database in Windows Vista ...seems like I 'kicked over another bee hive'...plenty of messages in regard to this when you do google search--->[sqlite3.operational error unable to open database file in windows vista] ...i'm getting the following error messages when I try to create a database in Sqlite3: *Sandbox> dbh <- connect "poddbtest.db" *** Exception: SqlError {seState = "", seNativeError = 21, seErrorMsg = "prepare 6: BEGIN: unable to open database file"} *Sandbox> dbh <- connect "test1.db" *** Exception: SqlError {seState = "", seNativeError = 1, seErrorMsg = "step: SQL logic error or missing database"} Both come from Chapters 21 and 22 in "Real World Haskell" I can't invoke Sqlite3 from the command line using---> Sqlite3 myDatabase.db... I created blank files in folder C:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci -- seemed to get me a bit further along...at least I was able to connect... Is it possible to specify the folder that the database should be created in? Is it possible to run Sqlite3 from the command line - when it has been installed using the Haskell wiki? Has anyone had better success with MySql - I've used it in the past and it was very reliable? Thank you -------------------------------------------------------------------------- _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Can't find or open a Sqlite3 database in Windows Vista
by Kathleen Lynch 03 Feb '11

03 Feb '11

...seems like I 'kicked over another bee hive'...plenty of messages in regard to this when you do google search--->[sqlite3.operational error unable to open database file in windows vista] ...i'm getting the following error messages when I try to create a database in Sqlite3: *Sandbox> dbh <- connect "poddbtest.db" *** Exception: SqlError {seState = "", seNativeError = 21, seErrorMsg = "prepare 6: BEGIN: unable to open database file"} *Sandbox> dbh <- connect "test1.db" *** Exception: SqlError {seState = "", seNativeError = 1, seErrorMsg = "step: SQL logic error or missing database"} Both come from Chapters 21 and 22 in "Real World Haskell" I can't invoke Sqlite3 from the command line using---> Sqlite3 myDatabase.db... I created blank files in folder C:\Program Files (x86)\Haskell Platform\2010.2.0.0\winghci -- seemed to get me a bit further along...at least I was able to connect... Is it possible to specify the folder that the database should be created in? Is it possible to run Sqlite3 from the command line - when it has been installed using the Haskell wiki? Has anyone had better success with MySql - I've used it in the past and it was very reliable? Thank you

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How to use the yaml package?
by Xavier Shay 02 Feb '11

02 Feb '11

Hello, I want to use YAML in my app I have run: cabal install yaml I have added it to my build-depends line in my .cabal file: Build-Depends: base,time,yaml $ runhaskell Setup.lhs configure Configuring xtdo-0.1... Setup.lhs: At least the following dependencies are missing: yaml -any Oh no! I tried just importing Text.Libyaml in both my app and GHCi but neither error gave any clues. I also found references to HsSyck, data-object-yaml and YAML::Tiny libraries, but not sure what I should be using. Right now I'm feeling totally lost, not being able to find examples of code using yaml, nor documentation for how I would use the libraries to play around with it. Also not comfortable I understand the interaction between cabal, runhaskell, ghc, and my build file. Any pointers? References to documentation I should have read also appreciated. Cheers, Xavier (I don't know how relevant it is, but here is my minimal project I am trying to add it to: https://github.com/xaviershay/xtdo-hs)

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