May 2011 - Beginners - Haskell.org

More direct binding of IO result
by Christopher Howard 15 May '11

15 May '11

I understand that one can bind the unwrapped results of IO functions to variables, and pass them to functions, like so: main = do filecontents <- readFile "data.txt" putStrLn filecontents But does the syntax allow you to cut out the middle man, so to speak, and bind the results directly to the parameter? Like -- Tried this and it didn't work. main = do putStrLn (<- readFile "data.txt") -- frigidcode.com theologia.indicium.us

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hidden package error when using Snap
by Todd Porter 15 May '11

15 May '11

Hi, I am using the parseTime function from Data.Time.Format. The first parameter is a TimeLocale defined in System.Locale (old-locale-1.0.0.2). I first wrote a trivial little module to test my understanding in ghci: module Tbasic where import Data.Time import System.Locale parseDate :: String -> Maybe UTCTime parseDate s = parseTime defaultTimeLocale "%d/%m/%Y" s Works fine. I then tried using this same function in my Snap web application. The application builds (via cabal install) fine. However when I start up the application and make my first request I get the following error: A web handler threw an exception. Details: Compile errors: Could not find module `System.Locale': It is a member of the hidden package `old-locale-1.0.0.2'. Use -v to see a list of the files searched for. I have added old-locale to Build-depends in the applications .cabal file. old-locale-1.0.0.2 appears in the global list of packages when I run ghc-pkg list Unsure what to try next. To be honest I don't understand a compile error at runtime given I didn't activate dynamic recompilation (i.e. I last built using cabal install, not cabal install -fdevelopment). appreciate any advice, Todd

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do-notation question
by blackcat＠pro-ns.net 14 May '11

14 May '11

In The Yesod Book, I see the following code: defaultLayout contents = do PageContent title headTags bodyTags <- widgetToPageContent $ do addCassius ... How does this work? Is bodyTags drawn from widgetToPageContent and then used as the third argument to PageContent? tia, Lee

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Example Connection String to MySQL via ODBC?
by Christopher Howard 14 May '11

14 May '11

Hi. I'm trying to connect for the first time to a 5.1 MySQL database with HDBC via the ODBC driver. Would someone be kind enough to show me an example connect string to pass to connectODBC? I've tried to put together what I can from the "example" at Hackage and old posts I found on the Web, but all I ever get is *** Exception: <<Database.HDBC.SqlError>> If someone could give me a boilerplate example with the standard elements (host, database, user, password) that would probably help. Thank you. -- frigidcode.com theologia.indicium.us

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cabal question, asked in a foul mood
by Philippe Sismondi 13 May '11

13 May '11

What exactly *is* a cabal package? I an not asking about the purpose of a package, but very literally: is it a single file? a set of files? a hierarchy of directories? I apologize if this seems like a silly question. However, I have just read the first five pages of the cabal user's guide, and am none the wiser for it. That particular piece of documentation is in serious need of review by a good editor, to put it mildly. I cannot read any further in it without losing my patience completely. Thanks; sorry for the bit of ranting. - Phil -

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Takusen and inserting type Maybe with sqlite3
by Neil Jensen 13 May '11

13 May '11

Hello, I'm stumped on using Takusen to insert values of type Maybe into a sqlite database when the underlying database has a field type of REAL. When I bind a Maybe to an underlying sqlite field of type DATETIME or INTEGER, the value is stored correctly. However, when I bind a Maybe value to a database field of type REAL, the value is inserted as "Just ..." or "Nothing". I thought bindP would coerce the Maybe type either into the value or a database null. What am I missing? Here is an example of an inserted record in the database: sqlite> select * from summary; 20110331|100002|Just 0.37427717861180376|Just 2.198753718609092|Just 9.402720712996771|Just 17.50143032492688|Just 8.250653601136904|Nothing|Nothing|Just 6.190434991035554 Here is the function: storeSummary date account returns = do dbh <- openDb withSession dbh (do execDML(cmdbind "REPLACE INTO summary (date,account_id,one_mo,three_mo,one_yr,two_yr,three_yr,four_yr,five_yr,incept) \ \VALUES (?,?,?,?,?,?,?,?,?,?)" [bindP (Just date), bindP (Just account), bindP (returns !! 0),bindP (returns !! 1),bindP (returns !! 2),bindP (returns !! 3), bindP (returns !! 4),bindP (returns !! 5),bindP (returns !! 6),bindP (returns !! 7) ]) ) And the database: sqlite>.schema summary CREATE TABLE "SUMMARY" ("DATE" DATETIME NOT NULL ,"ACCOUNT_ID" INTEGER NOT NULL ,"ONE_MO" REAL,"THREE_MO" REAL,"ONE_YR" REAL,"TWO_YR" REAL,"THREE_YR" REAL,"FOUR_YR" REAL,"FIVE_YR" REAL,"INCEPT" REAL, PRIMARY KEY ("DATE","ACCOUNT_ID") ) Thanks in advance, Neil

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Network.Curl POST body not showing up
by Michael Xavier 13 May '11

13 May '11

Hi all, I was hoping one of you was pretty proficient with Network.Curl to help me out, as I am not. I need to post JSON to an HTTPS endpoint using HTTP Basic Authentication. The endpoint will not accept the request unless the content-type is set to application/json. If I were using the curl utility, I'd do something like curl -H "Content-Type: application/json" -d "{blah:42}" -u 'myuser:mypass' https://example.com/api.json I'd really like to use curl's curlGetString because the return type is perfect, the code and the body. I've tried curlGetString, curlPost and curlMultiPost and all combinations of CurlOption, but I can either get it to send the correct content type header with empty content or send the post body with the content type defaulted to urlencoded or multipart/form-data. My code right now looks something like: doPost :: (ToJSON a) => T.Text -> T.Text -> T.Text -> a -> CampfireM (CurlCode, String) doPost key sub path pay = liftIO $ curlGetString (url path sub) opts where opts = method_POST ++ curlOpts key ++ [CurlVerbose True, post, ct] post = CurlPostFields [T.unpack encPay] ct = CurlHttpHeaders ["Content-Type: application/json"] encPath = encodePath path encPay encPay = encodePayload pay The post fields will not be in the form of "key=value", but rather a JSCODE: * Server auth using Basic with user 'SECRET_KEY' > POST /rooms/287132/speak.json HTTP/1.1 Authorization: Basic SECRET_KEY Host: example.com Accept: */* Content-Type: application/json Content-Length: 103 * The requested URL returned error: 500 * Closing connection #0 * HTTP response code said error It would seem to me from this output that no actual content was sent. What am I doing wrong? I'd really like to use curlGetString instead of some of the more complex curl functions. Thanks! -- Michael Xavier http://www.michaelxavier.net -- Michael Xavier http://www.michaelxavier.net

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Closing a channel
by Lyndon Maydwell 13 May '11

13 May '11

Hi Beginners. I'm trying to implement a program that accumulates a list of items through many IO actions, then use these in another part of the program as a list. I've found Control.Concurrent.Chan which seems to provide a nice abstraction with getChanContents :: Chan a -> IO [a], but there doesn't seem to be a way for the source to close the channel. Are channels the right abstraction for something like this? Should look in to (itter|enumer)at(or|ee)s instead... They seem to encompass this model, but look quite involved.

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Compiling with HDBC
by Christopher Howard 13 May '11

13 May '11

Hi. I'm very new to Haskell, but I'm trying to compile a simple program that uses HDBC with ODBC. I am working on an amd64 Gentoo Linux system. I've installed hdbc and hdbc-odbc with the unixOBDC bindings. So far, I created a small hello world type program. It doesn't use any HDBC functionality yet, but I inserted the following two lines at the top to see if I could compile correctly: import Database.HDBC import Database.HDBC.ODBC However, when I try to compile I get this: $ ghc Main.hs Main.o: In function `s1cY_info': (.text+0x1ef): undefined reference to `__stginit_HDBCzmodbczm1zi1zi3zi0_DatabaseziHDBCziODBC_' Main.o: In function `s1cY_info': (.text+0x1fb): undefined reference to `__stginit_HDBCzm2zi2zi6zi1_DatabaseziHDBC_' collect2: ld returned 1 exit status Though, these errors do not show up when I run the .hs file with runhaskell. I'm guessing I just don't know how to link to the correct library or libraries. I tried adding -lodbc to the ghc line but that did not help. In /usr/lib I see libodbc.so, as well as the directories hdbc-1.1.3 and hdbc-odbc-1.1.3.0. I would appreciate any guidance. -- frigidcode.com theologia.indicium.us

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Deserializing data with ProtocolBuffers
by Raphael Päbst 12 May '11

12 May '11

Hey! I am currently working on a project, where I need to serialize data with ProtocolBuffers. I have managed to compile a .proto file and serialize messages with messagePut. However, when I try to deserialize a ByteString again, I get the following error: ambiguous type variable `msg0' in the constraints: (Text.ProtocolBuffers.WireMessage.Wire msg0) arising from a use of "messageGet" at... (Text.ProtocolBuffers.Reflections.ReflectDescriptor msg0) arising from a use of "messageGet" at ... Probable fix: add a type signature that fixes these type variables in the expression "messageGet bs" in an equation for `IT': "IT = messageGet bs" I'm trying to call messageGet from the prompt in ghc. What exactly am I doing wrong? And how can I fix this? Many Thanks Raphael

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