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best performance with foldl
by Roland Leißa 06 Oct '13

06 Oct '13

Hi all, for a benchmark I'd like to write code like this: foldl (+) [0 100000000] how do I get the best performance out of it? However, It is important for me to rely on a higher-order function. Is it possible to be en par with the C version? int64_t result = 0; for (int64_t i = 0; i <= 100000000; ++i) result += i; Many thanks in advance for helping out, Roland

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ghci and module with FFI code
by Nathan Hüsken 04 Oct '13

04 Oct '13

Hey, I wrote a little package which gives bindings to a C++ library (to the random forest from vigra). Works fine! But when I try to load a module using it in ghci, and run an function (which is unrelated to the FFI bindings) I get: /lib/hsVigraRF-0.1.0.0/ghc-7.6.2/HShsVigraRF-0.1.0.0.o: unknown symbol `_ZGVZN5vigra10rf_defaultEvE6result' _ZGVZN5vigra10rf_defaultEvE6result is a symbol from vigra. So why do I get this in ghci and not when compiling/running? What can I do about this? Thanks! Nathan

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auto set objects imported
by Nathan Hüsken 04 Oct '13

04 Oct '13

Hey, It seems to be good practice, to explicitly say, which objects one wants to import in an import statement: import SomeModule (someFunc) while this is good documentation, it can be cumbersome to maintain. So I was wondering: Is there some tool, that automaticly finds out which functions I use from which module and modifies my import statements? That would be most usefull! Regards, Nathan

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Defaulting the following constraint ....
by Alan Buxton 03 Oct '13

03 Oct '13

Hi I have something like this - purpose is that I need to drop redundant .0 in a whole number - so to show 1.2345 as 1.2345 and to show 1.0 as 1 module Test where niceShow x = if (isInt x) then show (floor x) else show x isInt x = x == fromInteger (floor x) But the hlint in my vim plugin keeps warning me that test.hs|3 col 38 warning| Defaulting the following constraint(s) to type `Integer' || (Integral a0) arising from a use of `floor' at /tmp/test.hs:3:38-42 || (Show a0) arising from a use of `show' at /tmp/test.hs:3:32-35 || In the first argument of `show', namely `(floor x)' || In the expression: show (floor x) || In the expression: if (isInt x) then show (floor x) else show x What does this mean? And what would I need to do in order to prevent this warning?

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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 02 Oct '13

02 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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