- Beginners - Haskell.org

Is this an appropriate use of the writer monad?
by Alex Rozenshteyn 11 Mar '11

11 Mar '11

I need to (for class) write a minimax implementation. I'm currently doing this by functionally defining the entire tree and later pruning it down to the size I want. My assignment requires that I output the number of nodes I evaluated. I am considering wrapping the tree generation with the Writer monad (I'm not yet certain how I'd do this). Does this sound reasonable to you haskellers who know more than I? Would anyone suggest a better way? -- Alex R

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Which haskell array implementation to use? AKA what are the pros and cons of each
by Rohit Garg 10 Mar '11

10 Mar '11

I am reposting the question I posted here http://stackoverflow.com/questions/5196394/which-haskell-array-implementati… I am somewhat surprised at the answers. It could be that I didn't pose it very well, but I was surprised nonetheless to know that the present array library in Platform is considered horrendous, the suggested version (Data.Vector) can't parallelize operations over boxed arrays and the repa one (which actually does boxed arrays) doesn't work with any stable build as of today. I only need -> very easy parallelization -> map/filter type operations -> support for matrices Any help would be much appreciated. -- Rohit Garg http://rpg-314.blogspot.com/

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Beginners issue with 'Why Haskell matters' example code
by Michael Anckaert 10 Mar '11

10 Mar '11

Hey everyone I'm new on the list here and just started out learning Haskell and functional programming. I have over 12 years experience programming in various languages, from C to Python. I started out with the Why Haskell matters paper ( http://haskell.org/haskellwiki/Why_Haskell_Matters) and ran into a problem with the code below. It's discussed in the paper but when I save it to test.hs and load it into ghci, I get the following output: ------------------------ [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:6:22: parse error on input `=' Failed, modules loaded: none. ------------------------ Could someone give a pointer on where my error lies? qsort [] = [] qsort (x:xs) = qsort less ++ [x] ++ qsort more where less = filter (<x) xs more = filter (>=x) xs -- Kind regards Michael Anckaert <michael.anckaert(a)sinax.be> http://www.sinax.be

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Help!!
by Jessica Baker 10 Mar '11

10 Mar '11

3.1 Step 1 Create a Haskell script le called crypt.hs and enter the code for he Caesar cipher from Section 5.5 of the text book (Hutton). Make sure you understand how all the functions work. 3.2 Step 2 Transform your script le (from Step 1) into a Haskell program that reads text from the standard input, encrypts the text using the Caesar cipher, and writes the result to the standard output. Your program should take a command-line argument that species the shift factor to be used. So, for example: cat poem.txt | ./crypt 3 Eqvdqjxlqlqj wkh vnb Hrz khdylob lw glhv Iqwr wkh Whvw dzdb Pdvw wrxfk dqg vljkw dqg vrxqg Nr orqjhu wr eh irxqg Hrz krshohvv xqghujurxqg Fdoov wkh uhpruvhixo gdb If the command-line argument is 0 (zero) then the program should crack the code and output the deciphered text. So, the following command pipeline should echo the original message: cat poem.txt | ./crypt 3 | ./crypt 0 Ensanguining the sky How heavily it dies Into the West away Past touch and sight and sound No longer to be found How hopeless underground Falls the remorseful day

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Nested data types and bisimilarity
by dan portin 08 Mar '11

08 Mar '11

I asked this question in another location, but perhaps this is a better palce. It's unclear to me how conditions for a relation to be a bisimulation on nested data types can be generated. I understand, by analogy with streams and binary trees, how to define such conditions for regular types. However, consider the types T a = T a [T a] N a = N (T [N a]) and a relation R over then. What conditions must be fulfilled for R to a bisimulation? In particular, how is the nesting of a list (or tree) dealt with?

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HDBC - outside a transaction
by Petr Novotnik 08 Mar '11

08 Mar '11

Hello all, while working through `Real World Haskell' Chapter 21 I hit the following problem: Using sqlite3, enabling foreign keys with "PRAGMA foreign_keys = ON;" must be done outside a transaction. However, HDBC doesn't seem to provide a possibility to submit such a statement to the database outside of a transaction. The only workaround I've found looks like this: > openDatabase :: String -> IO Connection > openDatabase name = do > conn <- connectSqlite3 name > run conn "COMMIT;" [] -- manually close HDBC's underlyingTX > run conn "PRAGMA foreign_keys = ON;" [] -- execute outside of TX > run conn "BEGIN;" [] -- open a new TX for HDBC again > prepareDatabase conn > return conn Am I missing something in HDBC's API or is this the only way to do it? Many thanks in advance, Pete.

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Monads
by Patrick Lynch 07 Mar '11

07 Mar '11

In Learn You a Haskell for Great Good!, the author Miran Lipovaca indicates that to understand Monads you need to know Functors... So, I noticed the following: class Functor f where fmap::(a->b)->f a->f b instance Functor [] where fmap = map map::(a->b)->[a]->[b] if [] is substituted for f in the class definition of Functor the following is obtained class Functor [] where fmap::(a->b)->[]a->[]b my questions are: 1. is this substitution ok? 2. is []a = [a]? 3. is []b = [b]? if 2. and 3. are valid then the following is obtained: fmap::(a->b)->[a]->[b] which is the same as map type and therefore: fmap = map. QED. Can you please answer questions 2 and 3 above? Thank you

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GHC compilation without using module Main
by Andrew Pennebaker 06 Mar '11

06 Mar '11

Is there a way to compile a Haskell script with a different module name than Main? #!/usr/bin/env runhaskell module ScriptedMain where meaningOfLife :: Int meaningOfLife = 42 main :: IO () main = putStrLn $ "Main: The meaning of life is " ++ show meaningOfLife $ ghc -o scriptedmain scriptedmain.hs Undefined symbols: "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrtsmain.a(Main.o) (maybe you meant: _ZCMain_main_closure$non_lazy_ptr) "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrtsmain.a(Main.o) (maybe you meant: ___stginit_ZCMain$non_lazy_ptr) ld: symbol(s) not found collect2: ld returned 1 exit status Cheers, Andrew Pennebaker www.yellosoft.us

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cabal multi-library setup
by Cristian Petrescu Prahova 06 Mar '11

06 Mar '11

Hi, I have a small project consisting of a library and an application. I cabalized both. However, whenever I tweak the library I cabal install it, then move to the app directory, then reconfigure, then rebuild. This is annoying repetitive shell typing. Is there a way to configure my setup such that cabal build app automatically rebuilds (and reinstalls?) the library dependency? I'd like something like: # builds the library and runs the tests cabal make lib # builds the app and the library dependency, then runs the app cabal make app Thank you, Cristian

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Windows 7 time function
by Sayth Renshaw 05 Mar '11

05 Mar '11

I was completing the 5 step quickstart guide http://www.haskell.org/haskellwiki/Haskell_in_5_steps When running the program though haskell appears to be not able to access time on windows 7. Would this be normal? C:\Users\RenshawFamily\Documents\Scripts\Haskell>time ./A +RTS -N2 The system cannot accept the time entered. Enter the new time: C:\Users\RenshawFamily\Documents\Scripts\Haskell>time A.exe +RTS -N2 The system cannot accept the time entered. Enter the new time: 14.13 A required privilege is not held by the client.

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