December 2012 - Beginners

How does this function append the log to the beginning of the list?
by jugree＠lavabit.com 24 Dec '12

24 Dec '12

Hello. Could you explain this example(0)? Could you show its step by step execution? gcd' :: Int -> Int -> Writer (DiffList String) Int gcd' a b | b == 0 = do tell (toDiffList ["Finished with " ++ show a]) return a | otherwise = do result <- gcd' b (a `mod` b) tell (toDiffList [show a ++ " mod " ++ show b ++ " = " ++ show (a `mod` b)]) return result Why does the above append the log to the beginning of the list? What value will result have in the following? result <- gcd' 2 (3 `mod` 2) (0) http://learnyouahaskell.com/for-a-few-monads-more#writer

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process with mutable state in haskell
by Alexander _ 24 Dec '12

24 Dec '12

Hello, Can i run process with some mutable state and communicate with it in haskell? Something like gen_server in erlang. Thank you.

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programming novice :: chess engine interest - request for resource suggestions.
by B F 23 Dec '12

23 Dec '12

Dear All, So far, I have experienced nothing but a warm reception to the Haskell community. I am new to programming (a dabbler in Python), a mathematics grad. student and a chess enthusiast. As a personal goal, I would like to learn Haskell by exploring ideas in chess evaluation development from the statistical and pattern similarity side (rather than simply max ply search). As I will be learning principles of (functional) programming via Haskell - in the context of chess related concepts- I would like to request some Haskell resources that are explicative enough to suit the novice, while at the same time tailored to learning concepts related to pattern analysis, array methods, and matrix manipulation. Many Many Thanks - in advance- Diabellical

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Simple database application with gtk
by Emanuel Koczwara 22 Dec '12

22 Dec '12

Hi, I'm looking for an example of database application using gtk. Here: http://stackoverflow.com/questions/13949405/how-to-switch-from-qt4-mvc-to-g… user ntd pointed on libgda, but it looks like it's not covered by Gtk2Hs. Emanuel

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Build HTTP package error
by Alexander _ 22 Dec '12

22 Dec '12

Hello, I try to install HTTP package. I make: cabal install HTTP and got error: [ 1 of 18] Compiling Network.HTTP.Base64 ( Network/HTTP/Base64.hs, dist/build/Network/HTTP/Base64.o ) [ 2 of 18] Compiling Network.HTTP.MD5Aux ( Network/HTTP/MD5Aux.hs, dist/build/Network/HTTP/MD5Aux.o ) [ 3 of 18] Compiling Paths_HTTP ( dist/build/autogen/Paths_HTTP.hs, dist/build/Paths_HTTP.o ) dist/build/autogen/Paths_HTTP.hs:21:13: Not in scope: `catch' dist/build/autogen/Paths_HTTP.hs:22:13: Not in scope: `catch' dist/build/autogen/Paths_HTTP.hs:23:14: Not in scope: `catch' dist/build/autogen/Paths_HTTP.hs:24:17: Not in scope: `catch' cabal: Error: some packages failed to install: HTTP-4000.2.6 failed during the building phase. The exception was: ExitFailure 1 How can i install it correctly? Thank you.

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Type checking to prevent data injection attacks?
by Mike Meyer 21 Dec '12

21 Dec '12

While thinking about my next blog posting, I realized that it was a technique similar to what Perl does with the concept of "tainted" strings. If you're not familiar with it (and I'm probably getting the details wrong, not having written any perl in decades), a string that comes from an external source is considered "tainted". String constants in the program source are not. There's a builtin that can be used to say that declares a tainted string as not being tainted (presumably, the programmer has examined it somehow to determine this). Any string operations that return strings return tainted strings if any of the strings involved were tainted. Finally, functions that are subject to data injection attacks don't work on tainted strings. It seems like the Haskell type system ought to be able to support this. I can see a couple of approaches that might work (a "tainted" monad for use on the different string types or an Either-ish datatype) that might work, but don't have the chops to decide which approach might be better - or maybe that the answer is that it depends on the goal. I did some googling for this, but didn't turn up anything that seemed promising. Lots of stuff on type checking untrusted values, but no flagging strings as untrusted. Which leaves the questions... Is this actually a sane idea? Is there already a Haskell package that does this? Possibly part of a web framework? A package for another language, or a paper discussing doing this? Thoughts on other approaches than the two I mentioned? Thoughts on the best way do to do this? Thanks, <mike

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problem installing network on windows
by Emmanuel Touzery 21 Dec '12

21 Dec '12

this is using GHC 7.4.1 and using the cabal which came with the latest haskell platform. I wiped clean my$HOME/cabal and $HOME/ghc, $HOME being %AppData%. In $HOME/cabal/configI set those options: library-profiling: True user-install: True Any idea? emmanuel ... [8 of 8] Compiling Network ( Network.hs, dist\build\Network.p_o ) Network.hs:291:13: Warning: Pattern match(es) are overlapped In a case alternative: _ -> ... Registering network-2.4.0.1... Installing library in C:\Users\emmanuelto\AppData\Roaming\cabal\network-2.4.0.1\ghc-7.6.1 Registering network-2.4.0.1... cabal: Error: some packages failed to install: network-2.4.0.1 failed while downloading the package. The exception was: DeleteFile "C:\\Users\\EMMANU~1\\AppData\\Local\\Temp\\network-2.4.0.1-2768\\network-2.4.0. 1\\conf5576.lnk": permission denied (Access is denied.)

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A Key Part of Functional Programming
by Costello, Roger L. 21 Dec '12

21 Dec '12

Hi Folks, Richard Bird write on page 91 of his book, "Introduction to Functional Programming using Haskell": The datatype of lists is the workhorse of functional programming. Notice that he does not say that lists are the workhorse of "Haskell." No. He makes a much broader statement. Lists are the workhorse of Haskell and Lisp and Miranda and all the functional languages. Lists are a key part of functional languages. On page 95 he defines the list concatenation operator: (++) :: [a] -> [a] -> [a] [] ++ ys = ys (x:xs) ++ ys = x : (xs ++ ys) Then on page 96 he states: Once one has come to grips with the definition of ++, one has understood a good deal about how lists work in functional languages. Again notice that he does not say that by coming to grips with the definition of ++ one will have a good understanding of how lists work in "Haskell." No. He makes a much broader statement. One will have a good understanding of how lists work in all functional languages. Although the syntax may change, this definition of concatenation is for all functional languages. Wow! That is so cool. /Roger

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Install bullet-0.2.2 failure in OS X 10.8.2
by Trung Quang Nguyen 20 Dec '12

20 Dec '12

Hi all, Any body can help me with installing bullet in OS X 10.8.2. I'm trying to install bullet using these commands: brew install bullet cabal update cabal install bullet And I got this Registering vect-0.4.7... Installed vect-0.4.7 Downloading bullet-0.2.2... Configuring bullet-0.2.2... cabal: The program c2hs is required but it could not be found. Failed to install bullet-0.2.2 Updating documentation index /Users/trungnguyen/Library/Haskell/doc/index.html cabal: Error: some packages failed to install: bullet-0.2.2 failed during the configure step. The exception was: ExitFailure 1 Thank you in advance. Kind regards, Trung

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About monad
by Trung Quang Nguyen 20 Dec '12

20 Dec '12

Hi all, I saw this 1. instance Monad Maybe where 2. return x = Just x 3. Nothing >>= f = Nothing 4. Just x >>= f = f x 5. fail _ = Nothing I am wondering about the implementation of function (>>=). Why don't it be *Just x >>= f = Just (f x)*? Any body knows about this? --Trung

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