October 2013 - Beginners

confusing type signature with sections
by Patrick Redmond 03 Oct '13

03 Oct '13

I wrote code to shift and scale the range [-1, 1] to [0, 1] and mapped it on a list. Prelude> map (\n -> ((n + 1) / 2)) [-1, 0, 1] [0.0,0.5,1.0] I thought using sections would better express the simplicity of this operation. Prelude> :t (+ 1) (+ 1) :: Num a => a -> a Prelude> :t (/ 2) (/ 2) :: Fractional a => a -> a Given the types of those sections, I thought it sensible for me to compose them like this. Prelude> :t ((+ 1) / 2) ((+ 1) / 2) :: (Fractional (a -> a), Num a) => a -> a This didn't work of course, and its type baffles me. Though it ends with "Num a) => a -> a", it gives an error when given a number. I don't understand what "(Fractional (a -> a), ...) =>" really means. It seems like I've asked Haskell to perform "/" on the arguments "(+ 1)" and "2", but that ought to be disallowed by the type of "/". I eventually realized I should use the composition operator (below), but I'm still curious what I created with "((+ 1) / 2)". Prelude> :t (/ 2) . (+ 1) (/ 2) . (+ 1) :: Fractional c => c -> c Thanks! Patrick

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Num instnace for [Double]
by Nathan Hüsken 03 Oct '13

03 Oct '13

Hey, I am working a lot with timeseries (with instances at discrete moments) of doubles. So I use [Double]. Now I want to be able to do stuff like timeSeries3 = timeSeries1 + timeSeries2 So I was thinking, I create a newtype newtype TimeSeries a = TimeSeries [a] Now, can I somehow autoderive List, Monad, MonadPlus for TimeSeries? Also I would like to derive from Num. Most of the things can be done pointwise! What about fromInteger??? Should that just be a list with one element? Or does it simple not make sense? Thanks! Nathan

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My own State Monad and State Arrow
by Thiago Negri 03 Oct '13

03 Oct '13

I think I did understand Monads and Arrows. So I put it to the test: I tried to write my own State Monad and an State Arrow without looking at the real implementation. This is what I got: http://lpaste.net/93801 Please take a look on line 146 where I try to define a State Monad that is represented with a full function (state+result -to- state+result -- like my Arrow implementation). Things got hairy with the bind operator (line 153) and I just hit the keyboard until I hit the correct types and got it to compile. Yet, I can't fully understand my own bind implementation. Using the result "a" twice seems to be wrong, but I couldn't write a test to prove it.

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Creating arrays in Haskell
by Michal Kawalec 02 Oct '13

02 Oct '13

Hello, I want to use the FFT library in Haskell and for that I need to create an array. However, when I input array (1,100) [(i, i*i) | i <- [1..100]] in ghci, I get an error saying: No instance for (Enum e0) arising from the arithmetic sequence `1 .. 100' The type variable `e0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Enum Double -- Defined in `GHC.Float' instance Enum Float -- Defined in `GHC.Float' instance Integral a => Enum (GHC.Real.Ratio a) -- Defined in `GHC.Real' ...plus 15 others In the expression: [1 .. 100] In a stmt of a list comprehension: i <- [1 .. 100] In the second argument of `array', namely `[(i, i * i) | i <- [1 .. 100]]' Could you please tell me how to add the type signature? -- Michal

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Haskell book
by Michael Loegering 02 Oct '13

02 Oct '13

I am looking for a general Haskell book with syntax reference to self-teach. I have a computer science background, so technical and theoretical is fine. Something similar in size and scope as the Camel book is to perl would be ideal - covering basic language idioms, with a decent language reference, but by no means exhaustive. I have looked at Learn You a Haskell and Real World Haskell online, both of which were accessible but were difficult to follow beyond the basics. I'm not sure if it's the organization of the material or just the learning curve, so I'm open to both if these are hands-down the favorites. Thanks, -Mike

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Libzip for Haskell under Window
by Riaan 02 Oct '13

02 Oct '13

I am using the Haskell Libzip package [ http://hackage.haskell.org/package/LibZip] to unzip a password protected file. This works great in OSX but I now need to build the application using this package for Windows. I have not been able to figure out how to get libzip [ http://www.nih.at/libzip/index.html] installed under windows so that cabal can install the package. I don't seem to be able to find a compiled binary anywhere. Can anyone help? Thanks Riaan

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Why html in platform?
by q 01 Oct '13

01 Oct '13

HP includes the html package, but everything I've seen that produces html uses blaze-html. Is html included for some historical reason?

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scotty installation error
by Ovidiu D 01 Oct '13

01 Oct '13

I'm trying to write a small web application starting from here: http://adit.io/posts/2013-04-15-making-a-website-with-haskell.html Resolving dependencies... Configuring scotty-0.2.0... Building scotty-0.2.0... Preprocessing library scotty-0.2.0... Web/Scotty.hs:35:8: Could not find module `Control.Monad.Trans.Resource' It is a member of the hidden package `resourcet-0.4.8'. Perhaps you need to add `resourcet' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. Failed to install scotty-0.2.0 My build-depends clause is the following: build-depends: base ==4.5.* , wai ==1.3.* , warp ==1.3.* , http-types ==0.7.* , resourcet , scotty , text , bytestring , blaze-html , persistent , persistent-template , persistent-sqlite , persistent-postgresql , heroku , transformers ==0.3.0.0 , wai-middleware-static ==0.3.2 , wai-extra , time , monad-logger ==0.2.4 Any idea what is wrong with it? I also tried to use the dependencies list without any versions and I get the same error Thanks ovidiu

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