March 2015 - Beginners

abstracting parametrized record types
by Alia 29 Apr '15

29 Apr '15

Hi Folks, Here's a problem that's frustrating me: I have defined a parametrized record type which looks like this: data Job a = Job { jobID :: String , jobName :: String , jobTable :: String , jobSource :: IO [a] , jobProcessors :: [a -> a] } where a is a data model type that maps on to a database table, for example: data Person = Person { name :: String , age :: Int } deriving (Show) data Car = Car { brand :: String , value :: Double , year :: Int } deriving (Show) I would like to define a higher-level record type which contains, for configuration purposes, a certain set of jobs for execution. Let's say we call it a JobSet and which could possibly look like this: data JobSet = JobSet { jobsetID :: String , jobsetName :: String , jobs :: [Job] <-- yes I know this is not legal } Is there a legal haskell way to achieve the above objective without having to do something like this which hardcodes the model type into the jobset schema? data JobSet = JobSet { jobsetID :: String , jobsetName :: String , personJobs :: [Job Person] , carJobs :: [Job Car] } Many thanks for any enlightenment on this front. Alia

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Are Arrows good for simulation purposes
by martin 15 Apr '15

15 Apr '15

Hello all, little do I know about arrows, but the "stream processor" metaphor suggests, that they can be used to simulate a network of things, services or data. Is this correct? I came across the following thought: When I simulate a billard game with a DES, I would compute collisions of balls with each other and with the banks, creatign a set of events, of which only the earliest will be considered to compute the next state. This is pure DES and does not seem to be a good candidate for arrows. In this case I'd be more interested in composing collision detectors than in stream processing. OTOH, when I move parcels around and process then in various stages, then a "stream processor" would make perfect sense to me. In that case, would I abandon the DES paradigm entirely (including the notion of an event queue) and just model the network and let it run?

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Why does QuickCheck insist on this class constraint?
by martin 10 Apr '15

10 Apr '15

Hello all, Ghc complainend about Could not deduce (Arbitrary a1) arising from a use of ‘arbitrary’ Could not deduce (Eq a1) arising from a use of ‘arbitrary’ in the following code .. data CList a = CList [Change a] deriving Show instance (Arbitrary c, Eq c) => Arbitrary (CList c) where arbitrary = do ts <- orderedList :: Gen[Time] vs <- listOf arbitrary :: Arbitrary c => Gen [c] return $ CList $ zipWith (\t v -> Chg t v) (nub ts) vs instance (Arbitrary a, Eq a) => Arbitrary (Temporal a) where arbitrary = do d <- arbitrary (CList cs) <- arbitrary :: (Arbitrary a, Eq a) => Gen (CList a) -- <=== return (Temporal d cs) ..if I leave out the class constraints in the line maked with "<===". Why is that so, i.e. why isn't the constraint four lines up (in the instance declaration) sufficient?

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Mutable collection of gui objects (wxHaskell)
by Jeffrey Brown 01 Apr '15

01 Apr '15

Hi, On Github, Jeremy O'Donoghue provides wx example code. From his collection I am trying to modify Layout.hs [1] to permit a variable number, rather than a fixed number, of text entry boxes. I shrank the program, mostly by removing features, until it was this: main = start $ do f <- frame [text := "Layout test"] p <- panel f [] xinput <- textEntry p [text := "100"] yinput <- textEntry p [text := "100"] myVar <- varCreate [xinput,yinput] set f [ layout := container p $ margin 10 $ column 5 [boxed "coordinates" (grid 5 5 [[hfill $ widget xinput], [hfill $ widget yinput]] -- replacing ) ] ] return () I want to replace the line marked "replacing". Rather than hard-coding the number of text entry boxes (2), I want it to deal with a mutable collection of them, in myVar. I tried this: [ fmap (\e -> hfill $ widget e) $ varGet myVar ] and got this error: Layout.hs:23:11: Couldn't match type `IO' with `[]' Expected type: [Layout] Actual type: IO Layout In the expression: fmap (\ e -> hfill $ widget e) $ varGet myVar In the third argument of `grid', namely `[fmap (\ e -> hfill $ widget e) $ varGet myVar]' In the second argument of `boxed', namely `(grid 5 5 [fmap (\ e -> hfill $ widget e) $ varGet myVar])' So then I tried something I thought would be equivalent: [[ (hfill $ widget e) | e <- (varGet myVar)]] and got a different error: Layout.hs:23:39: Couldn't match expected type `[w0]' with actual type `IO [TextCtrl ()]' In the return type of a call of `varGet' In the expression: (varGet myVar) In a stmt of a list comprehension: e <- (varGet myVar) I kind of understand the problem is that when I varGet myVar, I end up with type IO Layout, rather than type Layout, but I don't know what to do about it. Thanks, Jeff [1] https://github.com/jodonoghue/wxHaskell/blob/master/samples/wx/Layout.hs

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Is there an aeson-like XML serialization library?
by Thomas Koster 31 Mar '15

31 Mar '15

Hi list, I want to write a simple XML web service in Haskell that services a .NET WCF client, but am struggling with choosing the right XML library. "aeson" [1] has been a pleasure to use, so I am looking for an XML serialization library similar to aeson, where I can write ToElement/FromElement instances using simple applicative combinators, working with a high-level document model (i.e. not a parse tree), just like I have done in the past with ToJSON/FromJSON. Does such a library exist? I need the library to understand XML namespaces and the mandatory predefined entities, but I do not need any other extensions like XML Schemas, XPath or XSLT. Preferably, "xmlns" attributes should be handled specially and namespace prefixes resolved by the parser. "xmlns" attributes should be placed automatically by the renderer (I don't care what prefixes it chooses). I do not want to have to keep track of seen namespace prefixes while I am traversing the document, or to manually place "xmlns" attributes on elements for rendering. The "xml" [2] library is giving me grief at the moment with how unspecial its treatment of namespaces is. HXT appears to be beyond my skill level at the moment. There appear to be too many new things I would have to learn all at once before I could use it for this simple task. Cf. aeson, which I was able to use for practical applications the day I learned what an applicative functor was. [1] https://hackage.haskell.org/package/aeson [2] https://hackage.haskell.org/package/xml Thanks, -- Thomas Koster

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Arrow vs. function
by martin 31 Mar '15

31 Mar '15

Hello all I skimmed several Arrow tutorial and looked at the pretty diagrams and thought to myself "Hmm, these guys seem to take input and produce output". But that's what a function does. So what's the fundamental difference between an arrow and a fuction?

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Arithmetic expression parsing problem
by m00nlight 30 Mar '15

30 Mar '15

Dear Haskellers, I am new to programming in haskell, recently I came up with an task to write an simple arithemtic evaluator in haskell. I try to write it using Text.Parsec, it can handle binary operation and some unary operation, but will give information of parsing error in some case. The following is my code import Text.Parsec import Text.Parsec.Expr import Text.Parsec.Combinator import Data.Functor data Exp = Num Int | Add Exp Exp | Sub Exp Exp | Mul Exp Exp | Div Exp Exp | Pos Exp | Neg Exp expr = buildExpressionParser table factor table = [[op "*" (Mul) AssocRight, op "/" (Div) AssocRight] ,[op "+" (Add) AssocLeft, op "-" (Sub) AssocLeft] ,[prefix "-" (Neg), prefix "+" (Pos)]] where op s f assoc = Infix (f <$ string s) assoc prefix s f = Prefix (f <$ string s) factor = between (char '(') (char ')') expr <|> (Num . read <$> many1 digit) eval :: (Num a, Integral a) => Exp -> a eval e = case e of Num x -> fromIntegral x Pos a -> eval a Neg a -> negate $ eval a Add a b -> eval a + eval b Sub a b -> eval a - eval b Mul a b -> eval a * eval b Div a b -> eval a `div` eval b solution :: (Num a, Integral a) => String -> a solution = either (const (error "Did not parse")) eval . parse expr "" The following is some test, *Main> solution "-4/(2+3)" 0 *Main> solution "-4/(2-3)" 4 *Main> solution "-4/-2" *** Exception: Did not parse *Main> solution "16/-4" *** Exception: Did not parse *Main> solution "-16/4" -4 *Main> Can anyone teach me how to solve this? Thanks in advanced. --m00nlight

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failure $ cabal install wx
by Jeffrey Brown 29 Mar '15

29 Mar '15

Hi! I first sent this to Haskell-Cafe, then thought this would be a better destination. I switched OSs, from Ubuntu to OpenSuse 13.2, to fix some other problems. I since installed The Haskell Platform, and would like to install wxHaskell. The first time I tried that under OpenSuse, cabal install wx provided more output, installing a lot of things. Since then it has only given this: jeff@linux-ee07:~> cabal install wx Resolving dependencies... [1 of 1] Compiling Main ( /tmp/wxc-0.91.0.0-16672/wxc-0.91.0.0/Setup.hs, /tmp/wxc-0.91.0.0-16672/wxc-0.91.0.0/dist/setup/Main.o ) Linking /tmp/wxc-0.91.0.0-16672/wxc-0.91.0.0/dist/setup/setup ... Configuring wxc-0.91.0.0... Warning: No config found to match: /usr/bin/wx-config --version=2.9 --version-full in /usr/lib64/wx/config If you require this configuration, please install the desired library build. If this is part of an automated configuration test and no other errors occur, you may safely ignore it. You may use wx-config --list to see all configs available in the default prefix. readProcess failed: readProcess: wx-config "--version=2.9" "--version-full" (exit 1): failed Configuring wxc to build against wxWidgets 3.0.2.0 Building wxc /usr/bin/gcc -Wl,--hash-size=31 -Wl,--reduce-memory-overheads -Wl,--disable-new-dtags -Isrc/include -I/usr/include/wx-3.0 -I/usr/lib64/wx/include/gtk3-unicode-3.0 -D__WXGTK__ -DWXUSINGDLL -DwxDEBUG_LEVEL=0 -D_FILE_OFFSET_BITS=64 -DwxcREFUSE_MEDIACTRL -fPIC -c src/cpp/apppath.cpp -o dist/build/src/cpp/apppath.o gcc: error trying to exec 'cc1plus': execvp: No such file or directory Failed to install wxc-0.91.0.0 cabal: Error: some packages failed to install: wx-0.91.0.0 depends on wxc-0.91.0.0 which failed to install. wxc-0.91.0.0 failed during the building phase. The exception was: ExitFailure 1 wxcore-0.91.0.0 depends on wxc-0.91.0.0 which failed to install. jeff@linux-ee07:~> The file /usr/bin/wx-config exists. It may have gotten there today when I installed some wx packages via zypper (OpenSuse's equivalent to apt-get). I don't remember which those packages were, but in the wx-configg file appear these lines: this_version="3.0" [ -z "$output_option_release" ] || echo "3.0" [ -z "$output_option_version" ] || echo "3.0.2" [ -z "$output_option_version_full" ] || echo "3.0.2.0" so I believe the version of wx I have is 3.0.2.0. I'm using a Lenovo Thinkpad Edge 535 or 545. Thanks, Jeff

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parser frontend
by Maurizio Vitale 28 Mar '15

28 Mar '15

G'day, I'm trying to decide how to architect the frontend of a compiler I'm using as an excuse for learning Haskell. Suppose we have a language (SystemVerilog) that has the notion of multi-file compilation units. This means that specific subset of files form separate scopes for certain things. For instance if we represent with lists of lists compilation units: [[a,b,c], [d], [e,f]] a macro definition in file b, would affect the rest of the file and file c, but wouldn't have effect on files d,e or a. Furthermore, if a toplevel construct is split between two files, say c and d, the compiler should treat the original compilation unit specification as if it was: [[a,b,(c,d)][e,f]], where (c,d) means we're compiling the logical concatenation of c and d. If (c,d) is also incomplete, (c,d,e) should be tried and so on. Now in well designed systems, all files can actually be compiled in parallel and so I'd like to optimize for that case and recompile files (either because we need side effects from files before them or because they're incomplete. So I'm not to concerned if wasted work is done for the degenerate cases. Any suggestion on how to go about this? Le's assume that parsing a file is done with a function: p :: file -> Either Error (ast, [defs], [use]) where the [defs] are things that might affects files down the line and [use] are things that, if defined by some prior file, cause recompilation. I'm interested in both a sequential and parallel solution and it would be sweet if they where similar, with the proper abstractions. Thanks a lot for any insight/ideas, Maurizio

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Automatic Differentiation
by Animesh Saxena 28 Mar '15

28 Mar '15

I was implementing automatic differentiation in haskell and was able to code the calculation part, but I wanted to extend it to show the symbols instead of just the final value. Let me explain and copy/paste the code… 3 data ADif a = ADif a a deriving (Eq) 10 instance Floating x => Floating (ADif x) where 11 pi = ADif pi 0 12 exp (ADif x x') = ADif (exp x) (x' * exp x) 13 log (ADif x x') = ADif (log x) (x' / x) 14 sqrt (ADif x x') = ADif (sqrt x) (x' / (2 * sqrt x)) 15 sin (ADif x x') = ADif (sin x) (x' * cos x) 16 cos (ADif x x') = ADif (cos x) (x' * (- sin x)) ….And so on all the functions 27 instance Num x => Num (ADif x) where 28 ADif x x' + ADif y y' = ADif (x+y) (x'+y') 29 ADif x x' * ADif y y' = ADif (x*y) (y'*x + x'*y) 30 fromInteger x = fromInteger x let myfunction x = exp (log (sin x)) *Main> myfunction (ADif 2 1) -0.4161468365471424 I verified that this is the correct solution by hand! (& well mathematica too!) Anyway now I was hoping to print the actual symbols, so I was googling around for extending “Show” typeclass for floating and Num, kinda similar pattern. Is this the right approach? Or I need to rethink the problem? Basically my aim is to do something like mathematica where if I specify D[f[x],x] then I get the answer in symbols. -Animesh

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