November 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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implicit or shorthand class instances?
by Christopher Howard 30 Nov '12

30 Nov '12

There is a lot of code in my project like so: code: -------- data SomeThing = Something { center :: ... velocity :: ... ... } instance Locatable SomeThing where center = Something.center instance Moving SomeThing where velocity = Something.velocity instance ... -------- Is there any special extension or syntax trick that would allow me to reduce the amount of this repetitious code? Someway to implicitly state that the functions in the data type are equal to the functions in the class instance, rather than having to spell it out? -- frigidcode.com

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division / multiplication efficiency
by Christopher Howard 28 Nov '12

28 Nov '12

Say I have functions like so: code: -------- circleRadians = 2 * pi radius = (/ circleRadians) -------- My question: each time I call radius, will a division operation be performed? Is there anything to be gained, following the old adage that multiplication is more efficient than division in hardware, from something like: code: -------- radius = (* (1 / circleRadians)) -------- ...imagining that the compiler (or runtime graph reduction mechanism) will transform (1 / circleRadians) into a single number (s) only once, and use (* s) in all applications of radius? -- frigidcode.com

4 3

types, parentheses, application, composition
by Christopher Howard 26 Nov '12

26 Nov '12

Could someone explain more precisely to me what is the significance of parentheses in the type of an expression? Are they just some kind of syntactic sugar, that simplifies into a type without them, or are they more like parentheses in algebra, or...? In particular, I'm having difficulty understanding the results of composition or application that involves parentheses... I think I've got it figured out and then I find another strange expression that doesn't have the type I expect. Here is an example using the (self-made) function "sqr": code: -------- > :t sqr sqr :: Double -> Double > :t (.) (.) :: (b -> c) -> (a -> b) -> a -> c > :t (. sqr) (. sqr) :: (Double -> c) -> Double -> c > :t ((. sqr) .) ((. sqr) .) :: (a -> Double -> c) -> a -> Double -> c -------- Everything makes sense up until the last expression, "((. sqr) .)". Where did the "(a -> Double -> c)" come from? What's going on here? -- frigidcode.com

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type signature error in a where clause
by Mark Wallace 25 Nov '12

25 Nov '12

I'm writing a merge sort function, but I get type error under such implementation: mergesort :: (a -> a -> Ordering) -> [a] -> [a] mergesort cmp xs = mergeAll (map (\x -> [x]) xs) where mergeAll :: [[a]] -> [a] mergeAll [x] = x mergeAll xs = mergeAll (mergePairs xs) mergePairs :: [[a]] -> [[a]] mergePairs (a:b:xs) = merge a b : mergePairs xs mergePairs xs = xs merge :: [a] -> [a] -> [a] merge as@(a:as') bs@(b:bs') | cmp a b == GT = b : merge as bs' | otherwise = a : merge as' bs merge [] bs = bs merge as [] = as And ghc says: Couldn't match type `a1' with `a' `a1' is a rigid type variable bound by the type signature for merge :: [a1] -> [a1] -> [a1] at /home/ice/Study/Haskell/tutorials/99Questions/21to30.hs:135:7 `a' is a rigid type variable bound by the type signature for mergesort :: (a -> a -> Ordering) -> [a] -> [a] at /home/ice/Study/Haskell/tutorials/99Questions/21to30.hs:124:1 In the first argument of `cmp', namely `a' In the first argument of `(==)', namely `cmp a b' In the expression: cmp a b == GT But if I comment all type signatures, ghc works fine on it. I would really appreciate it if you can point out what causes this question? And how to fix it without changing the structure of the program (i.e. not adding function `cmp' as a parameter of `merge' etc.). Thx.

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Can't install json-.0.4.4 - memory allocation failed
by Dan Tenenbaum 24 Nov '12

24 Nov '12

Hi all, Please tell me where I should post this if this is not the appropriate list. I tried to install the json-0.4.4 package on Mac OS 10.5.8, and got this error: $ sudo cabal install "json-0.4.4" Building json-0.4.4... [1 of 7] Compiling Text.JSON.Types ( Text/JSON/Types.hs, dist/build/Text/JSON/Types.o ) [2 of 7] Compiling Text.JSON.Pretty ( Text/JSON/Pretty.hs, dist/build/Text/JSON/Pretty.o ) [3 of 7] Compiling Text.JSON.ReadP ( Text/JSON/ReadP.hs, dist/build/Text/JSON/ReadP.o ) Text/JSON/ReadP.hs:104:21: Warning: A do-notation statement discarded a result of type b. Suppress this warning by saying "_ <- n", or by using the flag -fno-warn-unused-do-bind [4 of 7] Compiling Text.JSON.Parsec ( Text/JSON/Parsec.hs, dist/build/Text/JSON/Parsec.o ) ghc: memory allocation failed (requested 2097152 bytes) cabal: Error: some packages failed to install: json-0.4.4 failed during the building phase. The exception was: ExitFailure 1 This is on a machine with 16GB of RAM. I don't think http://www.haskell.org/cabal/FAQ.html#cabal-goes-into-an-infinite-loop--run… applies because I'm using higher versions than mentioned there: $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 $ cabal --version cabal-install version 0.8.2 using version 1.8.0.6 of the Cabal library I know Mac OS 10.5.8 is old, but unfortunately ugrading this particular machine is not an option. Hope someone can help. Thanks, Dan

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Using StateT with List monad
by Nathan Hüsken 24 Nov '12

24 Nov '12

Hey, I have an example function in the list monad. twoSucc :: Int -> [Int] twoSucc i = [i+1, i+2] Now I want write something similar with the StateT monad transformer. twoSucc :: StateT Int [] () twoSucc = do i <- get put (i+1) -- how do I put [i+1,i+2] here? As mentioned in the comment, when doing runStateT twoSucc n it outputs [((),n+1)] (which makes sense). How do I write it that it behaves similar to the original twoSucc and outputs [((),n+1),((),n+2)]? Thanks! Nathan

4 3

Is all Haskell syntax pointfree compatible?
by Christopher Howard 24 Nov '12

24 Nov '12

After reading the Haskell wiki article about pointfree style I naturally started playing around with with converting my regular code to pointfree. However, I immediately got stuck on the specific cases of 1) do syntax and 2) record syntax. For example: code: -------- playMusic rt = do [source] <- genObjectNames 1 buffer source $= getSound rt "music.wav" sourceGain source $= 0.4 loopingMode source $= Looping play [source] -------- And another (contrived) example: code: -------- data A = A { u :: Int , v :: Double , w :: String , ... } f a b = a { v = b } -------- -- frigidcode.com

2 1

Orphan instances
by Emmanuel Surleau 22 Nov '12

22 Nov '12

Hi there, I have a design issue, which is causing me to generate orphan instances. I'm writing a small program which persists data to a database. In order to be able to switch between different backends, I have a Store typeclass in one module, which is exposed to the frontend, and which has instances in each backend module. But obviously, the backend instances are therefore orphan. Is there a better solution which would let me keep the concrete implementation separate from the interface? Thanks, Emm

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On Windows defragment (Defraggler'ize, etc) the hard drive and turn off your A/V before installing cabal-install :)
by KC 19 Nov '12

19 Nov '12

:) Casey -- -- Regards, KC

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