Below is some code to use SDL with Haskell to draw a diagonal line. I get a CYAN line when the RGB clearly should be white. This is on Ubuntu. Am I doing something wrong? import qualified Graphics.UI.SDL as SDL import qualified Graphics.UI.SDL.Primitives as SDLP main = do SDL.init [SDL.InitEverything] SDL.setVideoMode 640 480 3...
I have this test: testReadFile = runTestTT $ TestLabel "InteractionDomain.readFileContentsToList" (TestList [ (TestLabel "testing infrastructure: read test file" (TestList [ TestCase (withTempFileContainingText (\fileHandle -> assertEqual "contents as expected" "The dog barks at midnight." (do co...
I need to manipulate expressions like 1 + sqrt(3) and do basic arithmetic like addition, subtraction, and division. I'd like the result to be in some sort of canonical form so that it can be used as a key in a map. Turning 1 + sqrt(3) into a float is not feasible due to roundoff problems. I used SymPy for this task in Python. Is there a...
These lines execute correctly: Prelude> 1 / (1 + 1) 0.5 Prelude> (/) 1 $ (+) 1 1 0.5 Prelude> (/) 1 $ 1 + 1 0.5 This one does not: Prelude> 1 / $ (+) 1 1 <interactive>:1:4: parse error on input `$' Why? ...
Hello everyone, I've designed a function to compute the mean of a list. Although it works fine, but I think it may not be the best solution due to it takes two functions rather than one. Is it possible to do this job done with only one recursive function ? calcMeanList (x:xs) = doCalcMeanList (x:xs) 0 0 doCalcMeanList (x:xs) sum length...
If I write foo :: (Num a) => a foo = 42 GHC happily accepts it, but if I write bar :: (Num a) => a bar = (42 :: Int) it tells me that the expected type a doesn't match the inferred type Int. I don't quite understand why, since Int is an instance of the class Num that a stands for. I'm running into this same situation while trying...
The blazehtml tutorial and other blog posts make it very clear how to get string literals to work. I've got that. But how do I get strings (bytestrings, Data.Text, etc. would work too.) in general into the attributes/content of elements. Blazehtml looks great, but it seems pretty useless without that. =P here's sample output and cod...
I was reading on perceptrons and trying to implement one in haskell. The algorithm seems to be working as far as I can test. I'm going to rewrite the code entirely at some point, but before doing so I thought of asking a few questions that have arosen while coding this. The neuron can be trained when returning the complete neuron. let n...
I have a Transaction monad that looks like: newtype Transaction t m a = .. my monad stack here .. t is a phantom type I use to make sure the transactions I chain up apply to the same backend store. My main loop's iteration uses a type like: Transaction DB IO (Widget (Transaction DB IO ())) The above type means: "A transaction that g...
I have SEVERE space restraints as far as linux goes, since I run linux off a 4GB flash drive. I know GHC is the preferred compiler for Haskell, but the GHC package is 280MB, which is way too big for me. Is there a smaller Haskell compiler for linux that works fine? ...
Hello everyone. I'm a haskell newbie and I couldn't find an answer to this question. Can we define types with conditions? For example, simple user-defined data type would be: data MyList = MyList [a] Can I somehow modify this code so MyList constructor can take only lists with even number of elements? Something like data MyList = ...
I am trying to take my Haskell project and split it apart into a library and a set of executables that depend on the library. When I try to compile now I get the following error: src/Main.hs:23:0: Bad interface file: /Users/<MyHomeDir>/.cabal/lib/Core-0.0.1/ghc-6.12.1/<MyModule>.hi mismatched interface file ways (wanted "", ...
I compiled my helloworld.hs and got a helloworld.o file, I tried ./helloworld, but it didn't work, so what is the right way to execute the helloworld? I am using cygwin, I just write down $ ghc --make helloworld.hs and I get helloworld.hi, helloworld.exe.manifest, helloworld.o files, I don't know what do I need to do next... ...
In the below code, fibseq represents a sequence of numbers from the fibonacci sequence. (from code to solve Project Euler #2) I have defined an infix function |> as (|>) x y = y x. This lets me do the following (like a unix pipeline): take 34 fibseq |> filter even |> filter (< 4000000) |> sum My question is, why does this work? I wou...
This is how the Cont monad is defined: newtype Cont r a = Cont { runCont :: (a -> r) -> r } instance Monad (Cont r) where return a = Cont ($ a) m >>= k = Cont $ \c -> runCont m $ \a -> runCont (k a) c Is there a tutorial somewhere that picks this mumbo-jumbo apart and explains how and why this works? Not looking for a monad ...
I have a function (exercise 10.11 in Thompson's The Craft of Functional Programming) which computes an approximation to the value of the definite integral of a function over a domain (a, b). It might not be the most elegant function, but I'm still a a beginner: import Data.Ratio (Rational, (%), denominator, numerator) type R = Rational ...
Hi, I'm trying to build a simple lexer/parser with Alex/Happy in Haskell, and I would like to keep some localisation information from the text file into my final AST. I managed to build a lexer using Alex that build a list of Tokens with localisation: data Token = Token AlexPosn Foo Bar lexer :: String -> [Token] in my Happy file, w...
how come that following piece gives 4? (\x -> (x-1) `mod` 5) 0 ...
I borrowed the example presented here http://www.haskell.org/alex/doc/html/introduction.html I am trying to make an interpreter for numerical expressions. (literals only, no variables) And I want to have newlines separate two different expressions. Unfortunaltely, when I do this $ignoredWhite = [\t\f\v\r] -- ignored whitespace ...
I'm having a hard time finding out how to make something change every time the user interacts with my program. It's hard to explain so here's an example (Haskell + wxhaskell): simulate :: Int -> Frame () -> IO () simulate qNr window = do fdata <- readFile "qarchive" case (split (listTake (split fdata '\n') qNr 0) '#') of (qst:a:b:c:...