April 2011 - Beginners

Tuples .vs Either Vs datatypes
by Mike Meyer 07 Apr '11

07 Apr '11

One of the exercises in Real World Haskell is to write a glob matcher without translating it to regular expressions. This turns out to be a lot more subtle than it looks (character classes like [!]-}] tend to break things). In doing this, I wrote a matchCharClass function for handling character classes. The initial version had type: matchCharClass :: Pattern -> Char -> (Bool, Pattern) I.e. - it returned rest of pattern after the char class as well as a match succeed/fail indicator. Upon reflection, I realized that only one of the two was ever used, so rewrote it to be: matchCharClass :: Pattern -> Char -> Either Bool Pattern This made the calling code a little larger - taking a apart tuples is a bit easier than taking apart Either's - but simplified the values being generated. I then realized that the code only used one of the Bool values: if it was True, then Pattern got used instead. So I rewrote it a third time, giving: data CharClass = Fail | Pattern String matchCharClass :: Pattern -> Char -> CharClass This only required minor changes to the code, but made it easy to add "Error String" to the CharClass datatype later. That version can be seen at http://pastebin.com/eyre8795 (as always, critiques welcome). I'd like to hear what more experienced haskell programmers have to say about those three ways of returning multiple values. Thanks, <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/consulting.html Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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Dependence issue
by Alexey G 06 Apr '11

06 Apr '11

Hello. I have some trouble with HDBC-mysql package ( http://hackage.haskell.org/package/HDBC-mysql) It depends on base (<4), but in GHC 7 - base 4.3.1. I download a HDBC-mysql package and change dependence in .cabal file to base (<5). It works fine. But I want to build another package that depend on HDBC-mysql and cabal check dependencies from the hackage, where is HDBC-mysql depend on base (<4). What I should do?

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Happstack dependency problems
by Tom Murphy 06 Apr '11

06 Apr '11

Hi all, I'm having build troubles with Happstack. I'll let the error message speak for itself: " $ export PATH=~/.cabal/bin:$PATH $ cabal update $ cabal install happstack Resolving dependencies... cabal: cannot configure unix-2.4.2.0. It requires base >=4.2 && <4.4 For the dependency on base >=4.2 && <4.4 there are these packages: base-4.2.0.0, base-4.2.0.1, base-4.2.0.2, base-4.3.0.0 and base-4.3.1.0. However none of them are available. base-4.2.0.0 was excluded because of the top level dependency base -any base-4.2.0.1 was excluded because of the top level dependency base -any base-4.2.0.2 was excluded because of the top level dependency base -any base-4.3.0.0 was excluded because of the top level dependency base -any base-4.3.1.0 was excluded because of the top level dependency base -any " Thank you for any help! Tom

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Cabal question: Include CHANGES and TODO files?
by Patrick LeBoutillier 06 Apr '11

06 Apr '11

Hi , Is there anyway to tell cabal to include extra files in the tarball, i.e. TODO and CHANGES? I like to maintain these files along with my projects and I'd like them to be in the tar dist. Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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happstack versus yesod
by Ramy Abdel-Azim 06 Apr '11

06 Apr '11

I already posted about this a little bit but i think i should let the old thread get back to it's regularly scheduled conversaion. on to my question: So I know there are a million easier ways to createa blog (wordpress, django, drupal, etc. etc.) but which would you recommend I use to build a beginner website that is simliar to a blog: happstack or yesod? _Ramy

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Re: [Haskell-beginners] Haskell Package for Mac issues
by Andrew n marshall 05 Apr '11

05 Apr '11

Homebrew installed ghc 6.12.3, but not cabal, and does not seem to work for me at all. Downloaded the 32bit Haskell Platform, and now things seem to work. 64bit build seems to be the root cause. On 4/5/11 9:48 AM, Benedict Eastaugh wrote: > On 5 April 2011 16:46, Andrew n marshall<andrew.n.marshall(a)gmail.com> wrote: >> I've been using the Haskell package for the last week to learn Haskell, but >> I've found several issues that make it feel like abandonware (which I know >> its not). >> >> I'm running OS X 10.6.7 with XCode 4 and installed the Haskell Platform >> 2011.2.0.0-x86_64.pkg. >> (I just realized this is labeled an experiment build. I hope that isn't the >> root of my problems. Tell me if it is.) > Have you considered installing the Haskell Platform and GHC with > Homebrew instead? I haven't installed XCode 4 yet, but Homebrew does > make installing new versions of GHC and the Platform pretty painless. > > http://mxcl.github.com/homebrew/

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Haddock question
by Patrick LeBoutillier 05 Apr '11

05 Apr '11

Hi, I have a module that contains (among other stuff) the following code: module Music.Diatonic.Note ( Note( C,D,E,F,G,A,B ), Nte, noteMap, ($#), ) where -- | Use these constructors to create 'Note's. To alter them, use the 'flat' or 'sharp' functions. data Note = C | D | E | F | G | A | B | Note Accidental Note deriving (Eq) -- | Many musical objects have a note at their core (scales, chords, ...). The 'Nte' class allows these objects -- to make use of all the note-manipulating functions. class Nte a where -- | Applies a 'Note' manipulating function to an instance of the 'Nte' class. noteMap :: (Note -> Note) -> a -> a -- | Operator for 'noteMap'. ($#) :: (Note -> Note) -> a -> a ($#) = noteMap When I run haddock on it it gives the following warnings that I don't really understand: Warning: Music.Diatonic.Note: noteMap is exported separately but will be documented under Nte. Consider exporting it together with its parent(s) for code clarity. Warning: Music.Diatonic.Note: $# is exported separately but will be documented under Nte. Consider exporting it together with its parent(s) for code clarity. Warning: Music.Diatonic.Note: notes is exported separately but will be documented under Nts. Consider exporting it together with its parent(s) for code clarity. What exactly does this mean? I am exporting Type class stuff inapproriately? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Average of numeric list
by Nadav Chernin 05 Apr '11

05 Apr '11

Hello, I tried to write function mean - average of numeric list. mean::(Fractional a)=>[a]->a mean a = (realToFrac (sum a)) / (realToFrac (length a)) But error occures: Could not deduce (Real a) from the context (Fractional b) arising from a use of `realToFrac' To correct this function, i rewrite this function: mean::(Real a, Fractional a)=>[a]->b mean a = (realToFrac (sum a)) / (realToFrac (length a)) Is there most simple way to write this function? Thanks, Nadav

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llvm on Mac
by Alba Marchisio 05 Apr '11

05 Apr '11

Hi All. I want to build my code using -fllvm option in GHC. (ghc --make main.hs -fllvm -o lexer) Doing so I've got the warning: Warning: -fllvm is incompatible with -fPIC and -dynamic on this platform; ignoring -fllvm My system is MacOS X 10.6.7 with Xcode 4 and I've installed GHC 7.0.3 (64 bit) Any idea ? Alba.

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Word suggestion package?
by Tom Murphy 04 Apr '11

04 Apr '11

Hi, Can anyone point me towards a package for a UI element, which gives options for word completion? In other words, I type "Ha" into a text box, and it gives me choices to fill the text box with either "Haskore," "Haskell," or "Hayoo." Hoogle has a function "suggestions," which returns a list of suggestions, and Leksah has a tool within its code editor with functionality similar to what I'm looking for, but I'm looking for a package which has this as a UI element. Thanks for your time! Tom

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