- Beginners - Haskell.org

how monomorphism restriction effects eta reduction
by Song Zhang 17 Jun '12

17 Jun '12

I think monomorphism restriction is for shared evaluation.like following function f xs = let len = genericLength xs in (len, len) from haskell wiki under monomorphism the type is Num t => [b] -> (t, t), if it no monomorphism restriction the type will be (Num t, Num t1) => [b] -> (t, t1) which lead to twice evaluation of len. However I didn't see how the restriction effects eta reduction, when define function like f = show, g = (==) or f = \x -> show x, g = \x -> \y -> (x==y). anyone can explain it to me? Thanks a lot.

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lambda notation vs function-argument notation in GHCi
by Morel Pisum 17 Jun '12

17 Jun '12

Let's define a function in GHCi: > Prelude> let f s n = and (map (==s) n) > Prelude> :t f > f :: Eq a => a -> [a] -> Bool This is fine. But when I define this function using lambda notation, I get this > Prelude> let f = \s n -> and (map (==s) n) > Prelude> :t f > f :: () -> [()] -> Bool which is really weird. Why does this happen?

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Simplify (normalize) symbolic polynom-like expression
by Daniel Hlynskyi 17 Jun '12

17 Jun '12

Hello. I am new to typefull programming, so I've got a question. I have a simple mathematical expression (addition, product and exponentiation only): > data Expr = I Int -- integer constant > | V -- symbolic variable > | Sum [Expr] > | Prod [Expr] > | Pow Expr Expr What I want is normalize\simplify this expression. Eg `Prod [Pow V (I 0), Pow V (I 1)] ` must be simplified to just `V`. What techniques should I use to write `normalize` function? Simplification rules are quite simple: > normalize (Sum [a]) = normalize a > normalize (Sum xs) | (I 0) `elem` xs = map nomalize . Sum $ filter (/= I 0) xs > | otherwise = map normalize xs > normalize (Prod xs) | (I 0) `elem` xs = I 0 > normalize (Prod xs) | (I 1) `elem` xs = map nomalize . Prod $ filter (/= I 1) xs > | otherwise = map normalize xs > normalize (Pow a (I 0)) = I 1 > normalize (Pow a (I 1)) = normalize a and so on. But rules like theese cannot simplify some expressions, for example, `Prod [Pow V (I 0), Pow V (I 1)] `.

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IO - getContents - putStrLn
by Robert Heumüller 16 Jun '12

16 Jun '12

Hi, this is probably an easy question, but i simply can't figure out, why this does not work: import Data.Char main = do contents <- getContents putStrLn $ show $ splitcomma contents splitcomma = split ',' split :: Char -> String -> [String] split _ "" = [""] split sp (c:cs) | c == sp = "": rest | otherwise = (c : head rest) : tail rest where rest = split sp cs The program compiles and runs without any problems. But there is absolutely no output, when f.eg. i type "hello, world" and hit return. Why would that be? Thank you very much

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about kind of (->)
by Song Zhang 16 Jun '12

16 Jun '12

a function has also a kind. According to haskell report 2010 4.1.2 it is * -> * -> *, which is easy to understand. However in ghci I type :k (->). the output is ?? -> ? -> *. I want to know what do ?? and ? mean. Thanks

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the "download" package fail to install
by Song Zhang 15 Jun '12

15 Jun '12

Hi There is a package for High-level file download based on URLs called "download", but failed to build on windows, If you don't know it, you can refer to hackage first. I compiled in C, but did not get useful massage, anyone can help me a little to fix that. cabal install download get a bad header file

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Difference between Char and Data.Char modules
by Ken Overton 14 Jun '12

14 Jun '12

I just started looking at the Happy packag; I get a compile error when attempting to build examples/glr/expr-eval: ExprData.hs:9:8: Could not find module `Char' It is a member of the hidden package `haskell98-2.0.0.1'. ExprData.hs:9 says “import Char” and if I change this to “import Data.Char” it compiles correctly (then I get a similar message for System, but when I understand Char I expect I will understand this too). Could someone kindly explain the difference between these two modules? From the look of the error, I’m guessing there’s a history lesson involved? Thanks, Ken ________________________________ This email and any attachments may contain information which is confidential and/or privileged. The information is intended exclusively for the addressee and the views expressed may not be official policy, but the personal views of the originator. If you are not the intended recipient, be aware that any disclosure, copying, distribution or use of the contents is prohibited. If you have received this email and any file transmitted with it in error, please notify the sender by telephone or return email immediately and delete the material from your computer. Internet communications are not secure and Lab49 is not responsible for their abuse by third parties, nor for any alteration or corruption in transmission, nor for any damage or loss caused by any virus or other defect. Lab49 accepts no liability or responsibility arising out of or in any way connected to this email.

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HXT and missing or null values
by Ian Knopke 14 Jun '12

14 Jun '12

I got it. For anyone else running into the problem from my earlier email, the way to get a null string is as follows: Replace this line: second <- text <<< atTag "second" -< x with this: second <- ((text `orElse` (constA "")) <<< atTag "second" -< x Fixed program: data Thing = Thing {first :: String, second :: String} getThings = atTag "thing" >>> proc x -> do first <- text <<< atTag "first" -< x second <- ((text `orElse` (constA "")) <<< atTag "second" -< x returnA -< Thing {first = first,second = second} atTag tag = deep (isElem >>> hasName tag) text = getChildren >>> getText parseThings str = runX (readString [withValidate no] str >>> getThings)

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cabal trouble(s!)
by Gregory Guthrie 13 Jun '12

13 Jun '12

I have lots of problems with cabal - not sure why or how to correct them. ghc-pkg check reports tons of issues, but all seem to be haddock-html, which doesn't sound too serious, or haddock-interfaces. Warning: haddock-html: C:\Users\guthrie\AppData\Roaming\cabal\doc\wxdirect-0.90.0.1\html doesn't exist or isn't a directory ... Cabal install errors, For example; C:\Users\guthrie>cabal install gloss-examples Resolving dependencies... Configuring gloss-raster-1.7.4.4... Building gloss-raster-1.7.4.4... Preprocessing library gloss-raster-1.7.4.4... [1 of 2] Compiling Graphics.Gloss.Raster.Array ( Graphics\Gloss\Raster\Array.hs, dist\build\Graphics\Gloss\Raster\Array.o ) Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM ghc.exe: could not execute: opt What is LLVM, and do/how I need to install it? and another (of many) - perhaps unrelated: C:\Users\guthrie\AppData\Roaming\cabal\bin>cabal install wxc Resolving dependencies... <command line>: cannot satisfy -package Cabal-1.10.2.0: Cabal-1.10.2.0-db589dd5d526d3111ac2fde0f9ab986c is unusable due to missing or recursive dependencies: array-0.3.0.2-8e9cd0144e87fa9cc86cc9031631c4f3 base-4.3.1.0-f520cd232cc386 346843c4a12b63f44b containers-0.4.0.0-18deac99a132f04751d862b77aab136e directory -1.1.0.0-3a2367d72569467a8af8a231656ff1b8 filepath-1.2.0.0-f132e9f7703da4e20a47f f2b9acf1ea1 old-time-1.0.0.6-445ce39cbcebd38069c25c0f383b728d pretty-1.0.1.2-abc 7c632374e50e1c1927987c2651f0f process-1.0.1.5-b3dded8e54a2e13d22af410bdcfafff4 (use -v for more information) cabal: Error: some packages failed to install: wxc-0.90.0.3 failed during the configure step. The exception was: ExitFailure 1 Trying to resolve them: C:\Users\guthrie\AppData\Roaming\cabal\bin>cabal install base Resolving dependencies... cabal: internal error: impossible I've had several others, not sure if they are package specific issues, or some larger issue with cabal and libraries. So I tried to fix them from a suggestion in SO; delete the ~/ghc & ~/cabal files and restart cabal, by a "cabal install cabal-install". It gives: C:\Users\guthrie>cabal install cabal-install Resolving dependencies... In order, the following would be installed: deepseq-1.3.0.0 (reinstall) changes: array-0.4.0.0 -> 0.3.0.3 containers-0.4.2.1 (reinstall) changes: array-0.4.0.0 -> 0.3.0.3 old-time-1.0.0.7 (new version) directory-1.1.0.2 (reinstall) changes: filepath-1.3.0.0 -> 1.2.0.1, old-time-1.1.0.0 -> 1.0.0.7 ... cabal: The following packages are likely to be broken by the reinstalls: time-1.4 wxdirect-0.90.0.1 random-1.0.1.1 haskell-platform-2012.2.0.0 QuickCheck-2.4.2 ... ghc-7.4.1 ... Cabal-1.14.0 process-1.1.0.1 haskell98-1.1.0.1 haskell-src-1.0.1.4 haskell-platform-2011.2.0.1 haskell-src-1.0.1.4 haskell-platform-2011.3.0.0 ghc-7.0.4 Use --force-reinstalls if you want to install anyway. Lots of things there that I don't want to break, but not sure how to proceed. Even trying a -force-reinstall just gives more errors and failures... Running ghc 7.4.1, and Haskell platform 2012.2.0.0, cabal reports: cabal-install version 0.14.0, using version 1.14.0 of the Cabal library.

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HXT and missing or null values
by Ian Knopke 13 Jun '12

13 Jun '12

I have an xml file I'm trying to parse using HXT. This is the code (for some reason the list seems to be rejecting the XML example): data Thing = Thing {first :: String, second :: String} getThings = atTag "thing" >>> proc x -> do first <- text <<< atTag "first" -< x second <- text <<< atTag "second" -< x returnA -< Thing {first = first,second = second} atTag tag = deep (isElem >>> hasName tag) text = getChildren >>> getText parseThings str = runX (readString [withValidate no] str >>> getThings) The problem is that sometimes one of the tags is empty. I'd like the code to return something like Thing {first="val",second=""} but instead the entire entry is skipped. Can someone explain to me what I need to do to get a null or empty string value inserted instead? Ian

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