February 2012 - Beginners

cmath Install troubles
by Tom Murphy 20 Apr '12

20 Apr '12

Any advice? If you can't even install a package with no dependencies except base, I think there's trouble: $ cabal install cmath Resolving dependencies... Configuring cmath-0.3... Preprocessing library cmath-0.3... Building cmath-0.3... [1 of 1] Compiling Foreign.C.Math.Double ( dist/build/Foreign/C/Math/Double.hs, dist/build/Foreign/C/Math/Double.o ) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2sB_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: (Each undeclared identifier is reported only once /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: for each function it appears in.) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2t0_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2896:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2uH_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3088:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2v6_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3134:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vv_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3180:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vU_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3226:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wj_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3272:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wI_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3318:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2Kh_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4856:0: error: ‘significand’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2KG_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4902:0: error: ‘significand’ undeclared (first use in this function) cabal: Error: some packages failed to install: cmath-0.3 failed during the building phase. The exception was: ExitFailure 1 Thanks for any help! amindfv / Tom

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Haskell wants the type, but I only know the class.
by Amy de Buitléir 26 Mar '12

26 Mar '12

I'm trying to read something from a file, do something with it, and then write it out again. I don't know, or care about, the type of the object I'm reading, but I do know its class. Here's my code: ---------- import Data.Binary ( Binary, encode, decode ) import Data.ByteString.Lazy as B ( readFile, writeFile ) import Codec.Compression.GZip ( compress, decompress ) class ( Binary a, Show a, Eq a ) => Thing a where doSomething :: a -> IO a readThing :: ( Thing a ) => FilePath -> IO a readThing f = return . decode . decompress =<< B.readFile f writeThing :: ( Thing a ) => FilePath -> a -> IO () writeThing f = B.writeFile f . compress . encode main = do a <- readThing "file1.txt" a' <- doSomething a writeThing "file2.txt" a' ---------- And here's the compiler error I get. Amy.hs:18:3: Ambiguous type variable `a0' in the constraint: (Thing a0) arising from a use of `writeThing' Probable fix: add a type signature that fixes these type variable(s) In the expression: writeThing "file2.txt" a' In the expression: do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } Is there a way to fix my code? I tried adding a type signature, e.g., a <- readThing "file1.txt" :: (Thing t) => IO t a' <- doSomething a :: (Thing t) => IO t but then it says... Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: Thing t => IO t The following variables have types that mention t0 a :: t0 (bound at Amy.hs:16:3) In a stmt of a 'do' expression: a' <- doSomething a :: Thing t => IO t In the expression: do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' }

5 16

A question about an infinite type
by Costello, Roger L. 22 Mar '12

22 Mar '12

Hi Folks, Here is an interesting phenomena: let f = (\x y -> x y x) let p = (\x y -> 1) Now let's evaluate f p 1 f p 1 = (\x y -> x y x) p 1 -- by replacing f with its definition = p 1 p -- by substituting x with p and y with 1 = (\x y -> 1) 1 p -- by replacing the first p with its definition = 1 -- the function returns 1 regardless of its arguments However, Haskell will not proceed with that evaluation because it will first determine the type signature of f and judge it to be an infinite type. Conversely, if f is omitted and this expression p 1 p is evaluated then the result 1 is generated. Why does f p 1 (which evaluates to p 1 p) fail whereas p 1 p succeeds? What lesson should I learn from this example? /Roger

2 1

gui pkgs
by Andy 18 Mar '12

18 Mar '12

Hi I'm trying to create a calculator in Haskell, something like the one in the enclosed image. To program the gui , I thought i might use a toolkit such as wxHaskell to create the buttons and frame etc.. I followed all the instructions on the wxHaskell homepage and everything seemed to install properly. Next I tried to use it with ghci, but to no avail. It seems you have to register wxHaskell with ghci by using a command like <ghci -package wx> but that crashes . Every tutorial I read didn't work. Does anyone have a clear set of instructions,that does work? Also, is there a better toolkit for doing guis than wxHaskell? I tried Glade, but it's so bug-ridden I couldn't get anything done. I need help.

2 1

A tree code
by bahadýr altan 06 Mar '12

06 Mar '12

Hello. There is a code below and I couldn't understand it myself. Could you help me with that please? I especially have no idea about "data Tree alpha = Empty | Node " part. data Tree alpha = Empty | Node ( alpha ,Tree alpha , Tree alpha ) x = Node (1,Node (2,Empty ,Empty),Node(3,Empty ,Empty)) y = Node(3,Empty ,Empty) Thanks in advance

3 2

Trying to compile Glade Gtk2Hs demo / cabal install glade problem
by Tobias 29 Feb '12

29 Feb '12

I am trying to compile a Glade Gtk2Hs demo and I get the same error as described at http://www.haskell.org/pipermail/haskell-cafe/2010-August/081881.html when I try install cabal glade: "setup.exe: The pkg-config package libglade-2.0 version >=2.0.0 is required but it could not be found." The next reply to the email ( http://www.haskell.org/pipermail/haskell-cafe/2010-August/081941.html ) is the interrogator himself again thanking some people who told him how to fix that / "libglade-2.0.pc needed the edit"- Can you tell me what I have to edit in this libglade-2.0.pc.file and where this file should be? Thanks in advance!

2 4

Temporary values with polymorphic types
by Amy de Buitléir 28 Feb '12

28 Feb '12

I'm trying to write a function that builds a temporary graph, performs some operation on the graph, and then returns the result of the operation. The graph isn't returned. Here's an example. (The real function is much more complicated.) ----- 8< ----- import Data.Graph.Inductive.Graph ( labNodes, mkGraph ) doSomething ∷ [a] -> [a] doSomething xs = map snd $ labNodes $ mkGraph xs' [] where xs' = zip [1..] xs ----- 8< ----- When I load this in GHCI, I get: amy3.hs:4:39: Ambiguous type variable `gr0' in the constraint: (Data.Graph.Inductive.Graph.Graph gr0) arising from a use of `mkGraph' Probable fix: add a type signature that fixes these type variable(s) In the second argument of `($)', namely `mkGraph xs' []' In the second argument of `($)', namely `labNodes $ mkGraph xs' []' In the expression: map snd $ labNodes $ mkGraph xs' [] Failed, modules loaded: none. >From this, I gather that I need to specify a type for the temporary graph "g"? I can arbitrarily pick an instance of the Graph class, but what can I put for the type parameter that it expects? It should be of the same type as the input array elements. This doesn't compile: ----- 8< ----- import Data.Graph.Inductive.Graph ( labNodes, mkGraph ) import qualified Data.Graph.Inductive.Tree as T ( Gr ) doSomething ∷ [a] -> [a] doSomething xs = map snd $ labNodes g where xs' = zip [1..] xs g = mkGraph xs' [] :: T.Gr a Int ----- 8< ----- amy3.hs:7:21: Couldn't match type `a' with `a2' `a' is a rigid type variable bound by the type signature for doSomething :: [a] -> [a] at amy3.hs:5:1 `a2' is a rigid type variable bound by an expression type signature: T.Gr a2 Int at amy3.hs:7:13 Expected type: [Data.Graph.Inductive.Graph.LNode a1] Actual type: [(Int, a)] In the first argument of `mkGraph', namely `xs'' In the expression: mkGraph xs' [] :: T.Gr a Int Failed, modules loaded: none. Thank you in advance for any advice.

2 1

Re: [Haskell-beginners] Read just one word from input
by Alexander Batischev 26 Feb '12

26 Feb '12

On Sun, Feb 26, 2012 at 12:31:06PM -0800, Tim Perry wrote: > Turn off buffering and call "words" What "words" do you mean? There's no suck function in System.IO. And if you're talking about Data.List's "words", meaning something like: getLine >>= words then it's not what I need - such a construction would consume whole line, while function I'm seeking should consume as few words as required. P.S. Excuse me for mistakenly replying directly to you, Tim. -- Regards, Alexander Batischev 4096R/0C8BFD03 CE6C 4307 9348 58E3 FD94 A00F 3569 61A2 0C8B FD03

2 1

Read just one word from input
by Alexander Batischev 26 Feb '12

26 Feb '12

Hello Haskellers, Is there really no way to read input word-by-word? E.g., if user types 1 2 3 4 5 function (let's call it getWord :: IO String) should first return "1", then "2", and so on. The main idea behind that is if user entered more values then required, they won't be lost but instead consumed by subsequent reads (calls to getWord). That's how scanf() in C and 'cin >>' in C++ behave, and I'm quite surprised that I can't do that in Haskell. -- Regards, Alexander Batischev 4096R/0C8BFD03 CE6C 4307 9348 58E3 FD94 A00F 3569 61A2 0C8B FD03

3 4

Example code on haskell.org
by Joshua Weaver 25 Feb '12

25 Feb '12

FYI - Most of the links at http://www.haskell.org/haskellwiki/Example_code return with a 403. -- _josh

2 1