October 2012 - Beginners

"downcasting"
by Emmanuel Touzery 29 Mar '13

29 Mar '13

Hello, using the Text.Json library (and I'm reading I should be using Aeson instead... I'll migrate to it later), I'm a bit frustrated. At one point I'm getting a JSValue object, which could be anything, including a JSObject. But in this case I know I'm looking at a JSObject. I don't mind if Haskell crashes if that's not the case (for now), either it's a JSObject or the JSON I'm parsing is invalid. I hoped to do that conversion using pattern matching by giving non-exhaustive pattern match, considering only cases where the JSValue is in fact a JSONObject but somehow I can't make it... getObject :: JSValue -> JSObject JSValue getObject x@(JSObject _) = x --> doesn't compile the function returns a JSValue, I somehow wanted to point out to the compiler that if it gets there it's in fact a JSObject. getObject :: JSValue -> JSObject JSValue getObject (JSObject x) = JSObject x -> same as with the other attempt.. the compiler is saying the function is returning a JSValue: Couldn't match expected type `JSObject JSValue' with actual type `JSValue' In the return type of a call of `JSObject' In the expression: JSObject x In an equation for `getObject': getObject (JSObject x) = JSObject x I think I'm missing an obvious pattern in Haskell here, probably something I've already used elsewhere too, but somehow I don't make it. Otherwise answers saying that if I need to make such a "cast" then I'm using that library the wrong way are welcome (together to maybe a link to a tutorial explaining it the right way), but by curiosity I would still be interested in how to actually do that "cast" in this case. Thank you! Emmanuel

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Prolog
by Christopher Howard 07 Nov '12

07 Nov '12

Hi. I still have a lot to learn in Haskell (who doesn't?) but I was thinking about learning Prolog on the side. I was wondering what you thought about the language. I was interested in it primarily because I understood it was a purely declarative language and that everything is defined in the programs as relations. However, at the Wikipedia page it states that Prolog is not actually purely declarative, referencing an article which points out that it is necessary to think about Prolog programs procedurally because of the way that Prolog resolves queries. Is this not as big a deal as it sounds like? Or is there some other similar language I should learn instead? (One of the "successors" of Prolog?) I'm fairly open minded, so long as all the tools are FOSS. Though of course, a language with better community support, documentation, etc. is preferable. -- frigidcode.com indicium.us

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Pattern guard inside function
by Nathan Hüsken 06 Nov '12

06 Nov '12

Hey I have a function which originaly locked like this: collInfo :: Rect -> Rect -> Maybe CollInfo collInfo (Rect x1 y1 w1 h1) (Rect x2 y2 w2 h2) | x1 + w1 < x2 = Nothing | x1 > x2 + w2 = Nothing | y1 + h1 < y2 = Nothing | y1 > y2 + h2 = Nothing | otherwiese = Just $ makeCollInfo r1 r2 Now I want to refactor it to take Maybe input values collInfo' :: Maybe Rect -> Maybe Rect -> Maybe CollInfo collInfo' mr1 mr2 = r1 <- mr1 r2 <- mr2 collInfo r1 r2 This is fine, but I would like to write it using only one function. Is there any possibility I can remove collInfo and merge its body into collInfo' and still somehow use the guards (and not a bunch of ifs)? Thanks, Nathan

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cascade of if statements
by Emmanuel Touzery 05 Nov '12

05 Nov '12

Hello, I am wondering if this code can be made a bit nicer? I am checking if a file exists... and then if it exists I'm checking whether it's older than two hours from now. It is a bit annoying to have that second block in the else (i could make a second function but i think it all belongs in this function). This structure makes me think about the Maybe monad, where it would automatically exit after it would find out that the file doesn't exists, instead of having a cascade of if statements inside one another. But this function is already in the IO monad (maybe I'm thinking about this in the wrong way though). Any idea for an improvement? -- don't re-fetch if I fetched in the last 2 hours needToFetchAgain :: FilePath -> IO Bool needToFetchAgain export_filename = do exists <- doesFileExist export_filename if (not exists) then return False else do modif <- getModificationTime export_filename curTime <- getClockTime let diff = diffClockTimes curTime modif return $ tdSec diff >= (3600*2) -- 2 hours Thank you! Emmanuel

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best way to proceed with package install
by Michael Litchard 02 Nov '12

02 Nov '12

I am trying to install updated version of yesod-platform, and got this output from "cabal install yesod-platform". Could someone please advise on the most efficient way to proceed? SHA-1.5.1 (new version) asn1-data-0.7.1 (new version) base-unicode-symbols-0.2.2.4 (new version) base64-bytestring-1.0.0.0 (new version) blaze-markup-0.5.1.1 (new version) blaze-html-0.5.1.0 (new version) cipher-aes-0.1.2 (new package) data-default-0.5.0 (new version) cookie-0.4.0.1 (new version) date-cache-0.3.0 (new package) file-embed-0.0.4.6 (new version) hashable-1.1.2.5 (new version) case-insensitive-0.4.0.3 (new version) hspec-expectations-0.3.0.2 (new package) hspec-1.3.0.1 (new version) ... many more, snipping ... my concern was the next part of the message cabal: The following packages are likely to be broken by the reinstalls: yesod-platform-1.0.5 tls-extra-0.4.6 http-conduit-1.4.1.10 yesod-auth-1.0.2.1 yesod-1.0.1.6 authenticate-1.2.1.1 certificate-1.2.3 tls-0.9.5 yesod-core-1.0.1.2 yesod-static-1.0.0.3 yesod-persistent-1.0.0.1 yesod-form-1.0.0.4 yesod-json-1.0.0.1 yesod-default-1.0.1.1 skein-0.1.0.7 clientsession-0.7.5 pureMD5-2.1.0.3 wai-app-static-1.2.0.4 pwstore-fast-2.2 cprng-aes-0.2.3 crypto-conduit-0.3.2 yaml-0.7.0.3 persistent-template-0.9.0.2 persistent-sqlite-0.9.0.2 persistent-0.9.0.4 yesod-test-0.2.1 Use --force-reinstalls if you want to install anyway.

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Netwire loop
by Nathan Hüsken 01 Nov '12

01 Nov '12

Hi, When testing this netwire sample program {-# LANGUAGE Arrows #-} import Control.Wire mainWire :: WireP () Double mainWire = proc i -> do rec value <- integral_ 0.0 -< oldValue oldValue <- delay 1.0 -< value returnA -< value main = do wireLoop mainWire wireLoop :: WireP () Double -> IO () wireLoop w' = do let (mx, w) = stepWireP w' 1.0 () case mx of Left ex -> putStrLn $ "Inhibited:" ++ (show ex) Right x -> putStrLn $ "Produced:" ++ (show x) wireLoop w I get this output: NetwireTest: <<loop>> But there should be not infinite loop, should there? Regards, Nathan

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firering event in netwire
by Nathan Hüsken 29 Oct '12

29 Oct '12

Hey, I have a few question about the usage of netwire: How do I fire events? For example my main wire should take keyboard events as input and output some state (which is than somehow outputed). I see that there is an event type, which has the some type as the identiy wire. I am just not sure how to use it? Or asked differently, how is the "keyDown" event defined, that was talked of in an earlier mail? Thanks! Nathan

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Re: [Haskell-beginners] applicativeParsec.hs in Real World Haskell not compiling any more
by Daniel Fischer 25 Oct '12

25 Oct '12

On Mittwoch, 24. Oktober 2012, 19:40:02, Kees Bleijenberg wrote: > A year ago I made a program. I took code from ApplicativeParsec.hs (capt. 16 > Real World Haskell). > The program compiled and ran flawless. Now (after a year) I've upgraded to > GHC version 7.0.4 (Windows) and I want to change something. But the Hum, the current version is 7.6.1, you should consider installing that, 7.0.4 is already rather oldish. > unchanged version doesn't compile any more. I get warning: > ApplicativeParsec.hs:17:10: > Illegal instance declaration for `Applicative (GenParser s a)' > (All instance types must be of the form (T t1 ... tn) > where T is not a synonym. > Use -XTypeSynonymInstances if you want to disable this.) > In the instance declaration for `Applicative (GenParser s a)' > > ApplicativeParsec.hs:22:10: > Illegal instance declaration for `Alternative ....(same advice as above) That's unfortunate, it should better have reported the below immediately. But it's hard to make the compiler always give the best error message, so sometimes it gives a not-best. > > So I added TypeSynonymInstances. Then I got: > ApplicativeParsec.hs:22:10: > Overlapping instances for Applicative (GenParser s a) > arising from the superclasses of an instance declaration > Matching instances: > instance Applicative (Text.Parsec.Prim.ParsecT s u m) > -- Defined in Text.Parsec.Prim > instance Applicative (GenParser s a) > -- Defined at ApplicativeParsec.hs:17:10-36 > In the instance declaration for `Alternative (GenParser s a)' > > Then I added the OverlappingInstances directive. Same error and no > compilation. > > I wonder, is this the new GHC version? And what can I do about it? It's a new parsec version, that now provides Applicative and Alternative instances out-of-the-box. Just remove (delete or comment out) the instance declarations from your code. You might get a few further compilation errors due to the changes in parsec, in that case, either the error messages help you fix it, or you can ask a new question.

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applicativeParsec.hs in Real World Haskell not compiling any more
by Kees Bleijenberg 24 Oct '12

24 Oct '12

A year ago I made a program. I took code from ApplicativeParsec.hs (capt. 16 Real World Haskell). The program compiled and ran flawless. Now (after a year) I've upgraded to GHC version 7.0.4 (Windows) and I want to change something. But the unchanged version doesn't compile any more. I get warning: ApplicativeParsec.hs:17:10: Illegal instance declaration for `Applicative (GenParser s a)' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `Applicative (GenParser s a)' ApplicativeParsec.hs:22:10: Illegal instance declaration for `Alternative ....(same advice as above) So I added TypeSynonymInstances. Then I got: ApplicativeParsec.hs:22:10: Overlapping instances for Applicative (GenParser s a) arising from the superclasses of an instance declaration Matching instances: instance Applicative (Text.Parsec.Prim.ParsecT s u m) -- Defined in Text.Parsec.Prim instance Applicative (GenParser s a) -- Defined at ApplicativeParsec.hs:17:10-36 In the instance declaration for `Alternative (GenParser s a)' Then I added the OverlappingInstances directive. Same error and no compilation. I wonder, is this the new GHC version? And what can I do about it? Kees

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How to divide a pair of Num values?
by Costello, Roger L. 24 Oct '12

24 Oct '12

Hi Folks, Here is a function that takes a pair of Integral values and divides them: divide_v1 :: Integral a => (a, a) -> a divide_v1 (m, n) = (m + n) `div` 2 I invoke the function with a pair of Integral values and it works as expected: divide_v1 (1, 3) Great. That's perfect if my numbers are always Integrals. Here is a function that takes a pair of Fractional values and divides them: divide_v2 :: Fractional a => (a, a) -> a divide_v2 (m, n) = (m + n) / 2 I invoke the function with a pair of Fractional values and it works as expected: divide_v2 (1.0, 3.0) Great. That's perfect if my numbers are always Fractionals. I would like a function that works regardless of whether the numbers are Integrals or Fractionals: divide_v3 :: Num a => (a, a) -> a divide_v3 (m, n) = (m + n) ___ 2 What operator do I use for ___? Thanks. /Roger

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