March 2011 - Beginners

Anyone have problems with cabal and SDL?
by Tom Murphy 17 Jul '11

17 Jul '11

When I use Cabal to get any program which depends on the SDL library, this is the error I get. " checking for sdl-config... no checking for sdl11-config... no configure: error: *** SDL not found! Get SDL from www.libsdl.org. If you already installed it, check it's in the path. If problem remains, please send a mail to the address that appears in ./configure --version indicating your platform, the version of configure script and the problem. cabal: Error: some packages failed to install: Raincat-1.1.1.2 depends on SDL-0.6.2 which failed to install. SDL-0.6.2 failed during the configure step. The exception was: exit: ExitFailure 1 SDL-image-0.6.1 depends on SDL-0.6.2 which failed to install. SDL-mixer-0.6.1 depends on SDL-0.6.2 which failed to install. " I have SDL 1.2.14 (correctly) installed on my system. I'd really appreciate any help with this. Tom

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haskell for FPGAS
by David Blubaugh 10 May '11

10 May '11

To All, Has anyone developed HASKELL FOR FPGA development ?? Thank You, Daid Blubaugh

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dealing with chained functions
by Alec Benzer 21 Apr '11

21 Apr '11

Is there a generally accepted "better" way of dealing with the following: say I have three functions that all rely on output from each other. Ie, we get raw input, which one function processes. another function then requires the output from this function and produces some other output, which a third function then requires. would it be better to implement the functions like this: f1 :: a -> b f2 :: b -> c f3 :: c -> d final :: a -> d or like this: f1 :: a -> b f2 :: a -> c f3 :: a -> d Ie, in the latter set, f2 calls f1 on its input, and then proceeds to process it. f3 calls f2 on its input, etc. In the former case, final would just chain all three functions together itself (or I might just do it in main or somewhere else that's appropriate) Are one of these methods generally considered a better practice than the other, specifically in a functional context? My gut tells me that the former method looks more appropriate, but I'm not sure if it'd really be the accepted practice regardless of context, or if one might use either of these methods depending on the specifics of what they're doing. Or if it's just a matter of taste/opinion?

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Idiomatic way to avoid type class instance definitions for Int and Integer separately
by Amitava Shee 13 Apr '11

13 Apr '11

While reading "Learn You a Haskell for Great Good!" I came across the YesNo type class I tried a minimal version as below module Kind where class Yesno a where yesno :: a -> Bool instance Yesno Int where yesno 0 = False yesno _ = True I was surprised to get an error *Kind> :load kind.hs [1 of 1] Compiling Kind ( kind.hs, interpreted ) Ok, modules loaded: Kind. *Kind> yesno 10 <interactive>:1:6: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `10' at <interactive>:1:6-7 `Yesno t' arising from a use of `yesno' at <interactive>:1:0-7 Probable fix: add a type signature that fixes these type variable(s) Turns out 10 in this instance is an Integer and I have not defined Yesno over Integer Easy fix - just define an instance over Integer instance Yesno Integer where yesno 0 = False yesno _ = True My question - Is there a way to avoid this kind of boilerplate? What is the idiomatic way? Thanks & Regards, Amitava Shee

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Question about functors (or maybe not...)
by Patrick LeBoutillier 03 Apr '11

03 Apr '11

Hi, I'm writing a music library and have the following types and functions: data Note = C | D | E | F | G | A | B | Sharp Note | Flat Note raise C = Sharp C data Quality = Major | Minor data Chord = Chord Quality Note -- Note is the root data Scale = Scale Quality Note -- Note is the tonic What I would like to do is to be able to lift the raise function and apply it to Chords and Scales. Something like this: fmap raise (Chord Major C) = Chord Major (Sharp C) But these are not functors as it doesn't seem to make sense for them to be polymorphic. A chord can't really made of anything except Notes. Does anybody have any ideas? Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Critique my Code!
by blackcat＠pro-ns.net 02 Apr '11

02 Apr '11

Hi, I was hoping to get a code critique from some experienced Haskell people. I put my code up in this blog post: http://learnmeahaskell.blogspot.com/2011/03/playing-around-with-quickcheck.… . thanks Lee

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Another request for code critique...
by Mike Meyer 02 Apr '11

02 Apr '11

Shorter, but thing I've done that wasn't just an exercise from a haskell book. The problem statement can be found at http://kernelbob.wordpress.com/2011/03/20/same-five-digits/. My solution can be seen at http://pastebin.com/iW95q2ex. 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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Haskellers in St. Louis?
by aditya siram 01 Apr '11

01 Apr '11

Hi all, I'm looking to start a Haskell user group in St. Louis, MO. Anyone Haskellers or people interested in learning Haskell around these parts? -deech

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Does haskell have to have values?
by Mike Meyer 31 Mar '11

31 Mar '11

I'm working my way through Real World haskell, and so far have found the experience quite pleasant (though the harder exercises seem to require things net yet covered). Among the comments in the IO chapter was a discussion of whether or not some monad was or was not a function, which got me thinking. Values in haskell aren't evaluated until they're needed. They're implemented as thunks, meaning they're roughly zeroadic functions that will return the value when called. The syntax of the language seems to make treating values as zeroadic functions that return the value in question a reasonable interpretation as a degenerate case: (+) accepts two arguments and returns their sum. (+ 5) accepts one argument and returns that plus 5. (3 + 5) accepts zero arguments and returns 8. or (more pedantically): (+) accepts one argument and returns a function that accepts one argument and returns a zeroadic function that returns the value of the sum of the two arguments. (+ 5) accepts one argument and returns a zeroadic function that ... (3 + 5) a zeroadic function that returns 8 So the question is - is there any advantage or harm in this way of looking at 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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hsgsom lib
by Luca Ciciriello 31 Mar '11

31 Mar '11

Hi All. Im' interested in using the Haskell implementation of the data mining GSOM algorithm. I've searched on Google some samples or hints about how to use this lib, but the wiki page has been deleted and no other material seems to exist about the use of this lib. Has someone used this library or knows where I can found some example about the use of this lib? Thanks. Luca

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