September 2011 - Beginners

how to catch the error thrown by openFile?
by Ovidiu Deac 25 Sep '11

25 Sep '11

Thanks a lot! It took me a while to understand what you meant but eventually I got it. This is the final version and it looks much better: f :: MyMonad String f = do liftIO $ putStrLn "Please enter a file name: " name ← liftIO getLine hFile ← ErrorT $ (openFile name ReadMode ↠ return.Right) `catch` λ_ → return.Left $ KnownErr "open failed" content ← liftIO $ hGetContents hFile return content Somehow I understand but I don't have the feeling yet why I apply ErrorT to the whole catched expression instead of liftIO. Do you have a nice explanation for it? Ovidiu On Sat, Sep 24, 2011 at 10:21 PM, Daniel Fischer <daniel.is.fischer(a)googlemail.com> wrote: > On Saturday 24 September 2011, 19:52:24, Ovidiu Deac wrote: >> Given this function >> >> f :: MyMonad String >> f = do >> liftIO $ putStrLn "Please enter a file name: " >> name ← liftIO getLine >> hFile ← liftIO $ openFile name ReadMode >> content ← liftIO $ hGetContents hFile >> return $ content >> >> I would like to catch the error thrown by openFile and translate it to >> my own exception type. >> >> So first I tried this: >> hFile ← (liftIO $ openFile name ReadMode) `catch` λ_ → throwError >> (KnownErr "open failed") > > catch has type (IO a -> (e -> IO a) -> IO a) [where e = IOError if you're > using Prelude.catch, and e can be any instance of Exception if you're using > Control.Exception.catch (recommended)]. > > But (liftIO ioAction) has type (MyMonad a), so that can't be used as an > argument to catch. > Nor can (\_ -> throwError (KnownErr "...")). > >> >> ..but it fails because the error handler is returning a different type >> then the "try block". Right? > > No, it fails because both have the wrong type for catch. > It would typecheck with catchError instead of catch, but that wouldn't > catch the error :( > >> >> Then I tried this: >> f = do >> liftIO $ putStrLn "Please enter a file name: " >> name ← liftIO getLine >> hFile ← (liftIO $ openFile name ReadMode ↠ return.Right) >> `catch` λ_ → return (Left $ KnownErr "open failed") > > Close. If you replace liftIO with ErrorT in that line and move the first > open parenthesis past the $, then it works and does what you want. Without > changing the following two lines (or, better, do change them; > do x <- act > return x > > is the same as just > > act > ). liftIO adds a layer of Either, ErrorT doesn't. > > hFile <- ErrorT $ liftM Right (openFile name ReadMode) > `catch` (\_ -> return $ Left (KnownErr "open failed")) > liftIO $ hGetContents hFile >

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Functional programming principles at higher levels?
by Christopher Howard 25 Sep '11

25 Sep '11

Caveat: I have a lot to learn about functional programming. And it is probably going to take me years to get there, because of all the higher mathematics I need to catch up on. Having said that... It seems to me like there are at least two core ideas to functional programming: 1. Seeing a program as an expression that is evaluated. 2. Referential transparency. And these lead to or involve various other concepts: writing code that can easily be refactored; viewing computations as a set of dependencies to be resolved rather than steps to follow; mathematical rather than procedural approaches to solving problems; the exclusion or minimization of state. But has anyone attempted to apply these principles to programming at higher levels than just writing the code? Say, overarching software architecture? Or resource management models? I've been reading lately about various higher level software approaches and models like RESTful architectures and MVC frameworks and RM-ODP, and it is difficult to see what I should grab onto. -- frigidcode.com theologia.indicium.us

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how more efficient read/write a file ?
by anyzhen 24 Sep '11

24 Sep '11

consider this : i want load a 4G file(or some bigger file) ,and the process data operation just like 010 to 101( XOR bits ) , is it some efficient function down it ? such hPutStr hPutChar is Char layer , is exist bit layer operations? thanks for any help jiangzhen3s(a)qq.com

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how to catch the error thrown by openFile?
by Ovidiu Deac 24 Sep '11

24 Sep '11

Given this function f :: MyMonad String f = do liftIO $ putStrLn "Please enter a file name: " name ← liftIO getLine hFile ← liftIO $ openFile name ReadMode content ← liftIO $ hGetContents hFile return $ content I would like to catch the error thrown by openFile and translate it to my own exception type. So first I tried this: hFile ← (liftIO $ openFile name ReadMode) `catch` λ_ → throwError (KnownErr "open failed") ..but it fails because the error handler is returning a different type then the "try block". Right? Then I tried this: f = do liftIO $ putStrLn "Please enter a file name: " name ← liftIO getLine hFile ← (liftIO $ openFile name ReadMode ↠ return.Right) `catch` λ_ → return (Left $ KnownErr "open failed") case hFile of Right handle → do content ← liftIO $ hGetContents hFile return $ content Left error → throwError error ...which also fails with Couldn't match expected type `ErrorT MyType IO t0' with actual type `IO a0' In a stmt of a 'do' expression: hFile <- (do { handle <- liftIO $ openFile name ReadMode; return (Right handle) }) `catch` \ _ -> return (Left $ KnownErr "open failed") Any help is appreciated. Thanks! Full code here: import Control.Monad.Error import System.IO data MyType = UnknownErr | KnownErr String deriving (Show) instance Error MyType where noMsg = UnknownErr strMsg str = KnownErr str type MyMonad = ErrorT MyType IO main = do r ← runErrorT f reportResult r f :: MyMonad String f = do liftIO $ putStrLn "Please enter a file name: " name ← liftIO getLine hFile ← liftIO $ openFile name ReadMode content ← liftIO $ hGetContents hFile return $ content reportResult (Right c) = putStrLn ("The content is " ⊕ c) reportResult (Left e) = putStrLn ("Error: " ⊕ (show e))

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Question regarding to type classes
by Johannes Engels 24 Sep '11

24 Sep '11

Thank you very much, Antoine Latter and, particularly, Daniel Fischer for your excellent explanation! You have exactly understood what I intended and answered my question completely, so you really made my day! Johannes

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Question regarding to type classes
by Johannes Engels 24 Sep '11

24 Sep '11

Dear list members, as an exercise, I tried to define a type class for structures with ordered key values. The key values could be numbers, but also strings. For instance, I intend to define an address list containing records with names as key values. So far, my type class definition contains a function "key", which should extract the key values from the structures, and a function which compares the key values. So I tried: class StructsWithOrderedKeys a where -- no default definition for key key :: (Ord b) => a -> b (<?) :: a -> a -> Bool x <? y = (key x) < (key y) Here I get the following error message from GHCI: "Ambiguous type variable 'b' in the constraint: 'Ord b' arising from a use of 'key' ... Probable fix: add a type signature that fixes these type variable(s)" Could anybody explain what ambiguity arises here? As the arguments of (<?) are of the same type, I expected also the results (key x) and (key y) to be of the same type, which should be by the type constraint for "key" an instance of Ord. Why I am not allowed to use "key" in the definition of (<?) ? I would be grateful for any hint! Johannes

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apt cabal hackage
by Anuradha L 24 Sep '11

24 Sep '11

Can anyone point me to a tutorial or howto on haskell package management under debian/ubuntu? Some of the things that are making my head spin... How when to use hackage/cabal? And when apt? How to translate haskell internal names (ie names given to import) to package names ie either cabal or apt names? Global (/usr) vs local (under $HOME) packages and figuring out paths Thanks

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Invitation to connect on LinkedIn
by Mukesh Tiwari via LinkedIn 21 Sep '11

21 Sep '11

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Formal proof with haskell
by Sebastien Lannez 20 Sep '11

20 Sep '11

Dear all, I am currenlty learning Haskell. I want to use it to solve combinatorial problems. I want Haskell to prove a simple logical assertion (a tautology) ... Unfortunately, I failed to express it in Haskell. The logical proposition: proof1 x z = ( (not (x<z)) || ( (x<y) && (y<z) ) where y = (x+z)/2 It is easy to see that simple substitution lead to proof1 x z = (not (x<z)) || (x<z) = True Is Haskell smart enough to automatically solve such tautology ? ------------------------------------------ Sébastien Lannez Expert Optimisation ------------------------------------------ www: http://www.lannez.fr tel. (home): +33 9 81 16 50 74 tel. (office): +33 1 64 47 85 73 mob.: +33 6 31 82 32 19 ------------------------------------------

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Error in Installing Data.deriveTH
by mukesh tiwari 15 Sep '11

15 Sep '11

Hello all I am trying to install Data.deriveTH . When i tried cabal install derive , i got this error. ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/assignment_haskell/work$ cabal install deriveResolving dependencies...<command line>: cannot satisfy -package Cabal-1.10.2.0: Cabal-1.10.2.0-072fce39d68f011c6a364057a3d15270 is unusable due to missing or recursive dependencies: containers-0.3.0.0-ee442470d8dcc9e45f31677c400c5379 (use -v for more information)cabal: Error: some packages failed to install:derive-2.5.4 depends on haskell-src-exts-1.11.1 which failed to install.haskell-src-exts-1.11.1 failed during the configure step. The exception was:ExitFailure 1 ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/assignment_haskell/work$ cabal install haskell-src-extsResolving dependencies...<command line>: cannot satisfy -package Cabal-1.10.2.0: Cabal-1.10.2.0-072fce39d68f011c6a364057a3d15270 is unusable due to missing or recursive dependencies: containers-0.3.0.0-ee442470d8dcc9e45f31677c400c5379 (use -v for more information)cabal: Error: some packages failed to install:haskell-src-exts-1.11.1 failed during the configure step. The exception was:ExitFailure 1 ntro@ntro-HP-dx2480-MT-NA121PA:~/Mukesh/assignment_haskell/work$ cabal install containersResolving dependencies...No packages to be installed. All the requested packages are already installed.If you want to reinstall anyway then use the --reinstall flag. I tried to resolve this issue on #haskell and i got suggestion that it was due to conflict in global and local namespace [ see here for more detail http://hpaste.org/51376 ]. One idea is i should wipe ~/.ghc and install all the libraries individually. Could some one please suggest if there is another idea to resolve this issue. Currently i have ghc-6.12.3 and installing ghc-7.0 will resolve the issue ? Thank you Mukesh Tiwari .

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