- Beginners - Haskell.org

Lifting (Either SqlError a) in a monad stack
by Bryan Vicknair 26 Aug '13

26 Aug '13

I'm at the beginning of the monad transformers journey, and this is stumping me. I've read "Monad Transformers Step by Step" which made me confident enough to play with simple stacks, but I'm a bit lost in the following example from work. Please excuse any incorrect transformer terminology. In a WAI web app, I want to chain these two actions inside a monad stack: > safeConnect :: ConnectInfo -> IO (Either SqlError Connection) > safeInsert :: Thing -> Connection -> IO (Either SqlError Id) Here is an example where I would like 'result' to be (Right Id) only if the DB connection *and* the insert were successful, (Left SqlError) otherwise. > add :: Request -> ResourceT IO Response > add _ = do > result <- liftIO $ safeConnect devConnInfo >>= safeInsert thing > case result of > (Left e) -> return $ dbErr e > (Right _) -> return $ postRedirect > where thing = exampleThing The compiler tells me (edited for brevity): Expected type: Either SqlError Connection -> GHC.Types.IO a0 Actual type: Connection -> GHC.Types.IO (Either SqlError Id) In the return type of a call of `safeInsert' In the second argument of `(>>=)', namely `safeInsert thing' How can I chain safeConnect and safeInsert using the (Either SqlError a) monad inside WAI's (ResourceT IO Response) monad stack?

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fonction in a typeclass that does not mention the type variable
by TP 21 Aug '13

21 Aug '13

Hi, I have a simple test case containing a typeclass Foo with one type variable a. The goal is to write once and for all a function symbolToInfixLevel function that combines two other functions defined in the typeclass: ----------------------- class Foo a where symbolToConstructor :: String -> ( a -> a -> a ) infixLevel :: a -> Int symbolToInfixLevel :: String -> Int symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined undefined ----------------------- This yields an error because there is no "a" in the type signature for symbolToInfixLevel: $ runghc test_typeclass_without_typevariable.hs test_typeclass_without_typevariable.hs:1:1: The class method `symbolToInfixLevel' mentions none of the type variables of the class Foo a When checking the class method: symbolToInfixLevel :: String -> Int In the class declaration for `Foo' Now, if I define symbolToInfixLevel out of the typeclass: ----------------------- class Foo a where symbolToConstructor :: String -> ( a -> a -> a ) infixLevel :: a -> Int symbolToInfixLevel :: String -> Int symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined undefined ----------------------- Now, I obtain: $ runghc test_typeclass_without_typevariable.hs test_typeclass_without_typevariable.hs:7:24: No instance for (Foo a0) arising from a use of `infixLevel' In the expression: infixLevel In the expression: infixLevel $ (symbolToConstructor s) undefined undefined In an equation for `symbolToInfixLevel': symbolToInfixLevel s = infixLevel $ (symbolToConstructor s) undefined undefined How to get rid from this situation? Thanks in advance, TP

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cabal package haskore-vintage fails to build
by Dimitri Hendriks 21 Aug '13

21 Aug '13

Hi all, I'm new to this list, and know very little about haskell. I am trying to install the package haskore-vintage on mac osx version 10.6.8, using cabal, but this fails; see the log below. I have ghc version 7.6.3, and cabal-install version 1.16.0.2. ------------------------------------------------------------------------------ $ cabal install haskore-vintage Resolving dependencies... Configuring haskore-vintage-0.1... Building haskore-vintage-0.1... Preprocessing library haskore-vintage-0.1... [ 1 of 16] Compiling Haskore.Monads ( src/Haskore/Monads.hs, dist/build/Haskore/Monads.o ) [ 2 of 16] Compiling Haskore.Utils ( src/Haskore/Utils.hs, dist/build/Haskore/Utils.o ) src/Haskore/Utils.hs:87:23: Not in scope: `catch' src/Haskore/Utils.hs:93:23: Not in scope: `catch' Failed to install haskore-vintage-0.1 cabal: Error: some packages failed to install: haskore-vintage-0.1 failed during the building phase. The exception was: ExitFailure 1 $ ------------------------------------------------------------------------------ Does anyone know how to resolve this? Related problems I found: https://code.google.com/p/leksah/issues/detail?id=272 (no solution given). And here: https://github.com/haskell/cabal/issues/1137 it appears they think it is a combination of Cabal 1.10 / cabal-install 0.10 and GHC 7.6 the causes the problem with "catch", but I use cabal 1.16. Many thanks in advance, Dimitri

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How Haskell Fits Into an Operating System / API Environment
by Philippe Sismondi 20 Aug '13

20 Aug '13

I am interested in tracking down discussions on how Haskell fits into an operating system environment. That is probably not a very clear question, so let me explain. One of the great obstacles I have faced in using two of my favourite languages (Common Lisp and Haskell) for anything practical is that I am constantly frustrated by their relationship with the host OS, or perhaps more accurately with the APIs provided by the OS. For example, I tend to program mostly on OS X. I have a reasonable knowledge of Cocoa, and can get quite a lot done with Objective-C. So, if I want to use Haskell on OS X, I can run off and learn about Haskell's FFI, or some hand-rolled widget package, etc. But here is the problem: Cocoa et al provide an entire ecosystem that includes data structures, error handling, and so on. So, when trying to use Haskell with a GUI, or perhaps to do some audio processing, or other things that are provided by the OS X APIs, I feel that I am layering two quite incompatible worlds together: Haskell and its host environment. I ran into the same dilemma with Common Lisp. Clozure Common Lisp provides an excellent Cocoa binding. But using it feels (to me) like communications between alien races. The OS APIs want to do things the Objective-C way, and that is (clearly) not the Haskell or Lisp way. One soon notices that books on Haskell pay virtually no attention to this issue. At least, the ones I have don't. Maybe what I am really experiencing is that OS X just "wants" to be programmed in its native language. I had something of a better feeling about using F# on Windows, because that architecture does leverage the native stuff much better. The upshot is that I remain skeptical that Haskell is actually a practical language for many kinds of development. Can anyone point me to discussions on this? I may be having a dimwitted interval here, but I don't know what to search to find this. - P -

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Type inference in ST monad?
by Aurimas 19 Aug '13

19 Aug '13

I have the following code which does not compile due to explicit type annotation (ST s Double). Error message says usual thing about "s" type variables. ---------------------------------------------------------------------------------------------------------- import Control.Monad.ST import System.Random.MWC (initialize, uniformR, Gen) import Control.Monad.Loops (whileM) import Data.Vector (singleton) import Data.Word(Word32) main :: IO () main = do print $ runSimulation 1 runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- uniformR (-1.0, 1.0) gen :: ST s Double -- does not compile due to this if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10) gen if r2 > 5 then return r2 else return 0) --------------------------------------------------------------------------------------------------------- if I rewrite runSimulation like this (below), everything is OK. --------------------------------------------------------------------------------------------------------- runSimulation :: Word32 -> [Int] runSimulation seed = runST $ do gen <- initialize (singleton seed) whileM (do r1 <- tempFun gen if r1 > 0.0 then return True else return False) (do r2 <- uniformR (0, 10) gen if r2 > 5 then return r2 else return 0) where tempFun :: Gen s -> ST s Double -- this line automatically provides required type annotation tempFun g = uniformR (-1.0, 1.0) g --------------------------------------------------------------------------------------------------------- Ca somebody explain what's wrong with the first version? Best Regards, Aurimas

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Re: [Haskell-beginners] Compiler can't deduce Bool as instance of ToField
by Bryan Vicknair 15 Aug '13

15 Aug '13

> > Bryan Vicknair: > > postgresql-simple declares Bool as an instance of the ToField class. The > > compiler can't deduce that given this simple code however: > > > > import Database.PostgreSQL.Simple.ToField (ToField(..)) > > > > foo :: (ToField a) => a > > foo = True > > > > > > It fails with this error: > > > > Db.hs:64:7: > > Could not deduce (a ~ Bool) > > from the context (ToField a) > > bound by the type signature for foo :: ToField a => a > > at Db.hs:63:8-23 > > `a' is a rigid type variable bound by > > the type signature for foo :: ToField a => a at Db.hs:63:8 > > In the expression: True > > In an equation for `foo': foo = True > > Failed, modules loaded: none. > > > > What am I missing? > > > Oliver Charles > You have declared that foo is *any* type that has a ToField instance, > allowing the caller of foo to determine the type at their will. However, > your implementation of foo is more specific and requires a is actually Bool > and nothing else. > On 14 Aug 2013 10:24, "Bryan Vicknair" <bryanvick(a)gmail.com> wrote: I see. I was confused, because the following works: bar :: (Num a) => a bar = 5 I guess it is because the literal '5' could be an Int or a Float, and the type system doesn't know. When I saw that the following does not work...:: bar :: (Show a) => a bar = False ...it made a bit more sense. It seems that if a literal can be considered part of a typeclass in more than one way, you can declare that literal to be of that typeclass. However, if there is only one way for a literal to belong to a typeclass, then you can't. Are there other literals (besides (Num a) => [a]) that don't have a concrete type like literal numbers? If I understand correctly, -XOverloadedStrings turns all literal strings into IsString instead of [Char], so that seems to be another case. I don't have a need for this, but now I'm curious: is there a way to declare that all literal numbers in a module are of a certain concrete type? Something like -XAllNumbersAreInt8?

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Trouble with layout in wxHaskell
by Nathan Hüsken 15 Aug '13

15 Aug '13

Hey, I am trying to understand how do a layout in wxHaskell. I am trying to make a frame with 2 text entries. One single lined at the top and one multiline below it. Both should expand with the window size horizontal. When the window is expanded vertical, only the multiline text entry should expand. Her is what I wrote: main = start mainFrame mainFrame = do f <- frame [text := "BiVision"] buffer <- textEntry f [] mainArea <- textCtrl f [] set f [layout := expand $ column 2 [hstretch $ widget buffer, stretch $ widget mainArea]] This gives me a window with the desired controls. But they are not expanding. Why? I tried different combinations of expand and stretch (I do not really understand the difference?). But nothing works. I am guessing there is something simple I do not yet understand :). Thanks for any help! Nathan

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Heterogeneous Lists
by harry 14 Aug '13

14 Aug '13

Every OO language which supports generics allows a declaration such as List<Show> alist, where Show is an interface. Any type implementing Show can be put in alist, and any Show operation can be performed on the alist's members. No casts, wrappers, or other special types and plumbing are needed. Why isn't it possible to do this directly in Haskell?

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Compiler can't deduce Bool as instance of ToField
by Bryan Vicknair 14 Aug '13

14 Aug '13

postgresql-simple declares Bool as an instance of the ToField class. The compiler can't deduce that given this simple code however: import Database.PostgreSQL.Simple.ToField (ToField(..)) foo :: (ToField a) => a foo = True It fails with this error: Db.hs:64:7: Could not deduce (a ~ Bool) from the context (ToField a) bound by the type signature for foo :: ToField a => a at Db.hs:63:8-23 `a' is a rigid type variable bound by the type signature for foo :: ToField a => a at Db.hs:63:8 In the expression: True In an equation for `foo': foo = True Failed, modules loaded: none. What am I missing?

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Sorting Bank Accounts problem in Haskell
by Nadav Chernin 14 Aug '13

14 Aug '13

Hi all, I learn Haskell and i try to solve questions from SPOJ. Currently, i try to solve problem *Sorting Bank Accounts*<http://www.spoj.com/problems/SBANK/> Here is my code: *import qualified Data.Map as M* * * *extractTest []=[]* *extractTest (x:xs)=(take n xs):(extractTest (drop (n+1) xs)) where* * n=read x* * * *emp=M.empty* * * *collect m []=m* *collect m (x:xs)* * |M.member x m =collect (M.insert x (amount+1) m) xs* * |otherwise=collect (M.insert x 1 m) xs* * where* * Just amount=M.lookup x m* * * *getList m=map (\(x,k)->x++" "++(show k)) (M.toList m)* *f x=unlines$(getList (collect emp x))++[""]* *main=getLine>>=(\x->interact$unlines.map f.take (read x).extractTest.lines) * * * The problem is that i can't finish it during 7 sec. So i want to know is there more quick solution/method for this problem. Thanks, Nadav

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