April 2013 - Beginners

Editor choices
by Jeff Lasslett 23 Apr '13

23 Apr '13

Greetings, So I find myself playing around with XMonad source code. I'm a long time Vim user and I've made a tags file to help me navigate the source code. I've got decent syntax highlighting. What I lack is insight into the libs. I'd like the editor/IDE to tell me the type of a thing, or at least the module it is defined in. Is this a sensible question for haskell? I just don't want to have to hunt for where lib functions are defined (tags take me around the xmonad sources with drama). So what editors/IDEs to people use for haskell to help with this? Or do you all just keep everything in your massive brains? :-) Cheers, Jeff

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Re: [Haskell-beginners] HLint: use void instead of return()
by harry 22 Apr '13

22 Apr '13

So when is void better? Kim-Ee Yeoh <ky3 <at> atamo.com> writes: > In corner cases like this, it really looks like six of one, half dozen of the other. > In other words, feel free to ignore hlint when you need to. The heuristics aren't tuned to be 100% perfect.

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HLint: use void instead of return()
by harry 22 Apr '13

22 Apr '13

I have some monadiccode of the form: foo >>= something >>= bar >> return () HLint is telling me to change this to: void (foo >>= something >>= bar) Why is the second form considered better? I actually find the original to be clearer.

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Monad instances and type synonyms
by Christopher Howard 16 Apr '13

16 Apr '13

I am playing around with some trivial code (for learning purposes) I wanted to take code: -------- -- SaleVariables a concrete type defined early -- `Adjustment' represents adjustment in a price calculation -- Allows functions of type (a -> Adjustment a) to be composed -- with an appropriate composition function type Adjustment a = SaleVariables -> a -------- And put it into code: -------- instance Monad Adjustment where (>>=) = ... return = ... -------- If I try this, I get code: -------- Type synonym `Adjustment' should have 1 argument, but has been given none In the instance declaration for `Monad Adjustment' -------- But if I give an argument, then it doesn't compile either (it becomes a "*" kind). And I didn't want to make the type with a regular "data" declaration either, because then I have to give it a constructor, which doesn't fit with what I want the type to do. -- frigidcode.com

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Down in Data.Ord vs GHC.Exts
by Matthew O'Connor 16 Apr '13

16 Apr '13

While attempting to do some sorting, I ran across Down. The docs: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Ord.h… it's in Data.Ord, but GHCi 7.4.1 complains the Down data constructor isn't in scope, but if I import GHC.Exts ( http://hackage.haskell.org/packages/archive/base/latest/doc/html/GHC-Exts.h…) I get the constructor I expect. Are the docs wrong? Or is there something I'm not doing right to get Down exported from Data.Ord? Or other things to check? Thanks, Matthew

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Re: [Haskell-beginners] help with lists and tuples
by Miro Karpis 15 Apr '13

15 Apr '13

millions of thanks for all the info. m. On Mon, Apr 15, 2013 at 1:21 AM, Paul Higham <polygame(a)me.com> wrote: > Wouldn't it do just to have > > nActivationSum = map nActivation > > or am I missing your intent here? > > Note that on the left-hand side of the last equation inputs would have to > have type (Float, Float) and weights would have to have type > [(Float,Float)], but on the right-hand side both inputs and weights have to > be Float. The result of > nActivation (inputs, weights) is Float, but the result of > nActivationSum (inputs:weights) should be [(Float,Float)], hence the error > message. > > :: paul > > On 2013-04-14, at 16:04 , Miro Karpis wrote: > > Please can you help me with this? What I want is to have on input a list > of tuple (size 2) and on output list of floats. So now I'm returning only > float instead of a list. But unfortunately I can't figure out how to > implement the list...... > > > sigmoid :: Float -> Float > sigmoid x = 1.0 / (1 + exp (-x)) > > nActivation :: (Float,Float) -> Float > nActivation (x,y) = sigmoid(x*y) > > nActivationSum :: [(Float, Float)] -> [Float] > nActivationSum [] = [] > nActivationSum (inputs:weights) = *nActivation* (inputs, weights) > > > > Couldn't match expected type `[Float]' with actual type `Float' > In the return type of a call of `nActivation' > In the expression: nActivation (inputs, weights) > In an equation for `nActivationSum': > nActivationSum (inputs : weights) = nActivation (inputs, weights) > > > thanks, > m. > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > >

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help with lists and tuples
by Miro Karpis 14 Apr '13

14 Apr '13

Please can you help me with this? What I want is to have on input a list of tuple (size 2) and on output list of floats. So now I'm returning only float instead of a list. But unfortunately I can't figure out how to implement the list...... sigmoid :: Float -> Float sigmoid x = 1.0 / (1 + exp (-x)) nActivation :: (Float,Float) -> Float nActivation (x,y) = sigmoid(x*y) nActivationSum :: [(Float, Float)] -> [Float] nActivationSum [] = [] nActivationSum (inputs:weights) = *nActivation* (inputs, weights) Couldn't match expected type `[Float]' with actual type `Float' In the return type of a call of `nActivation' In the expression: nActivation (inputs, weights) In an equation for `nActivationSum': nActivationSum (inputs : weights) = nActivation (inputs, weights) thanks, m.

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Will Haskell Platform 2012.4.0.0 uninstall cleanly for the installation of 2013.2.0.0
by KC 12 Apr '13

12 Apr '13

:) -- -- Regards, KC

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Warsaw Haskell Group
by Przemyslaw Kaminski 12 Apr '13

12 Apr '13

Hello! We are organizing a group of Haskell fans from Warsaw, Poland. So far we have set up a community on G+ (https://plus.google.com/u/0/communities/103183708602453146804) a mailing list is on the way. We would like to meet regularly someplace in the capital (we still decide where), and hack on Haskell together as much as we can :) Should there be more people involved, we could arrange regular lectures. Hope to see you there! Warsaw Haskell Group

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FAQ: When to ask on Haskell-Beginners, when on Haskell-Cafe (was haskell testing framework - missing htfpp)
by Kim-Ee Yeoh 12 Apr '13

12 Apr '13

On Thu, Apr 11, 2013 at 10:50 PM, Chaddaï Fouché <chaddai.fouche(a)gmail.com> wrote: > More probably you should rather have asked this on Haskell-Cafe, it's not > really a beginner question. htfpp sounds like a pre processor The short answer is: if your question falls within the scope of LYAH, then it belongs here. If it's package specific (wxhaskell, htfpp), you're probably better off on the bigger list. Understandably, there's a huge gap between the two. The rule of thumb I'd use (and this is certainly not without controversy) is: when in doubt use Cafe. -- Kim-Ee

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