- Beginners - Haskell.org

Reader and ReaderT
by Grzegorz Balcerek 16 Oct '15

16 Oct '15

Hi, I have the following program: import Control.Monad.Reader first :: Reader [String] String first = do strings <- ask return $ if (null strings) then "empty" else head strings printFirst :: ReaderT [String] IO () printFirst = do strings <- ask let theFirstString = runReader first strings liftIO $ putStrLn theFirstString main = runReaderT printFirst ["first","second"] It compiles and works. However, in the printFirst function I am explicitly using ask and I am calling runReader. Can I somehow avoid doing that? The following version of the printFirst function does not compile. printFirst :: ReaderT [String] IO () printFirst = do theFirstString <- first liftIO $ putStrLn theFirstString Program2.hs:11:21: Couldn't match type `Data.Functor.Identity.Identity' with `IO' Expected type: ReaderT [String] IO String Actual type: Reader [String] String In a stmt of a 'do' block: theFirstString <- first In the expression: do { theFirstString <- first; liftIO $ putStrLn theFirstString } Can I somehow call first without using ask and runReader ? Regards Grzegorz Balcerek

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Can't install Haskell platform on Mac OS X El Capitan
by Daniel Hegyi 12 Oct '15

12 Oct '15

Hi, I’m trying to install Haskell on Mac I have OSX El Capitan so I downloaded from https://www.haskell.org/platform/ the Download (64 bit) (10.11). The installer runs, however, when I type into terminal ghci I get ghci -bash: ghci: command not found “which ghci” it just doesn’t return anything. Thanks, Daniel

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Reading a list of null-terminated filenames from stdin?
by Dan Stromberg 11 Oct '15

11 Oct '15

If I want to read a list of filenames, each terminated with a nul byte, from stdin (kind of like xargs -0), what's the best way to do that in Haskell? Or am I swimming against the current to do anything but newline-termination? Google's top hit is: https://downloads.haskell.org/~ghc/7.6.1/docs/html/libraries/bytestring-0.1… ...but I don't see anything about nul termination there. I checked the top ~5 hits, and didn't find much. I also checked https://www.haskell.org/hoogle/?hoogle=Lines The goal is to be able to operate on filenames that contain newlines, but it's not that end of the world if that isn't very practical. Thanks! -- Dan Stromberg

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ramin.honary@gmail.com has indicated you're a friend. Accept?
by ramin.honary＠gmail.com 10 Oct '15

10 Oct '15

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Learning Haskell — a new tutorial
by Manuel M T Chakravarty 08 Oct '15

08 Oct '15

For those getting started with Haskell, you might like to have a look at our new Haskell tutorial ”Learning Haskell”: http://learn.hfm.io/ It features a mix of text and screencasts and will be extended over time. There is a bit on the background at http://blog.haskellformac.com/blog/learning-haskell Enjoy! Manuel

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Adding Either around a List monad?
by Mario Lang 05 Oct '15

05 Oct '15

Hi. Consider this structure: vs :: Rational -> [Input] -> [[Output]] vs _ [] = return [] vs l (x:xs) = pms l x >>= \pm -> (pm :) <$> vs (l - dur pm) xs pms :: Rational -> Input -> [Output] pms l x = [x, x+1, x+2, ...] -- Just an example, not real code. -- in reality, l is used to determine -- the result of pms. This is basically traverse, but with a state (l) added to it. So without the state, vs could be written as vs = traverse pms Now, I want to add Either e to this, like: vs :: Rational -> [Input] -> Either e [[Output]] pms :: Rational -> Input -> Either e [Output] However, I have no idea how to implement vs. Interestingly, adding Either e to vs without changing the code lets it compile, but it gives me the wrong result: vs :: Rational -> [Input] -> Either e [[Output]] vs _ [] = return [] vs l (x:xs) = pms l x >>= \pm -> (pm :) <$> vs (l - pm) xs Since I am in the Either monad now, >>= does not do non-determinism, it simply unwraps the Either from pms. I have to admit, I dont fully understand why this compiles, and what exactly it does wrong. I only see from testing that the results can't be right. On IRC, Gurkenglas suggested to use the State monad, like this: vs :: Rational -> [Input] -> Either e [[Output]] vs l = `evalStateT l` . mapM v where v x = do l <- get pm <- lift $ pms l x put (l - dur pm) return pm This compiles, but also yields unexpected results. I have invested several hours now trying to add Either around this algorithm, so that I can emit hard failures. I am sort of frustrated and out of ideas. Somehow, I can't figure out what these transformations actually change in behaviour. I am being told, by quite experienced Haskell programmers, that this is supposed to be correct, but my testing tells me otherwise. So before I just give up on this, could someone please have a look and let me know if I have missed something obvious? -- CYa, ⡍⠁⠗⠊⠕

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File download using wreq.
by Mike Houghton 05 Oct '15

05 Oct '15

Hi, Please can someone explain how,using the wreq package, I can download and save a binary file? Say the file is at the end of http://x.y.z/files/myfile.jpg and it is a jpeg and no authentication is needed. I just want to 1. Check that the URL is syntactically correct - flag and error if not 2. If the URL is syntactically ok then download the file using GET. 3. Check that the response code is 200 and if so save the file 3a. if the response code is not 200 then delegate to an error handling function or some simple idiomatic way of error handling. Thanks once again. Mike

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Parsing keywords
by Mike Houghton 04 Oct '15

04 Oct '15

Hi, What is the idiomatic way of parsing keywords, in a case agnostic fashion, for a ‘mini-language’? For example I’m creating some structured text… project{ env{ } setup{ } run{ } } that has ‘project’, ‘env’, ‘setup’ and ‘run’ as keywords and I want to parse in such a way that ‘PROJECT’, ‘ProJect’ etc are all recognised as the keyword ‘project’ similarly for ‘env’ etc. I’m using monadic rather than applicative parsing. Many Thanks Mike

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Haskell package maturity
by Andrew Bernard 30 Sep '15

30 Sep '15

Since starting to look on Hackage for packages for such vital things as queues and algorithms, I am surprised to see very low numbers of downloads for packages that seem to me to be vitally important. For example, queuelike has only been downloaded 1617 times since being uploaded in 2009. Similar very low numbers seem to apply for many packages. Another example is cubicspline with only 485 downloads. My question is, are the numbers on Hackage correct, and if so, do they indicate hardly anybody uses them, or indeed Haskell? I am starting to wonder. I also notice version numbers are very low, often less than one and most often around 0.1 or so. This is either a display of extreme modesty on the part of Haskell library code developers (in fact, often found in open source communities), or an indication of lack of maturity of the code. Overall I am puzzled about this. I am trying to establish what packages to use in my coding and there seems to be little indication of what to choose, and how to assess code maturity. What am I missing? Andrew

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State and Maybe and monad transformers
by Andrew Bernard 30 Sep '15

30 Sep '15

Greetings All, I am writing code using a BankersDequeue from Data.Dequeue. I’d like to wrap the push and pop operations in a state monad for the standard reason, to avoid passing the dequeue around between function calls, and regard it as state instead, which it is. Wishing to avoid throwing a runtime error if the queue is empty (for the general case of queues) I have written a reduction of the concept using a crude stack to experiment, which uses State and Maybe. I notice that there are very few examples of using State and Maybe together. My questions are: Is this a faulty design pattern? Should this be done with monad transformers? Are there examples to be found of using State with Maybe as a monad transformer combination? Why is this pattern relatively rare, it seems? Is this program on the right track? Andrew — snip module Main where import Control.Monad.State type Stack = [Int] popIt :: Stack -> (Maybe Int, Stack) popIt [] = (Nothing, []) popIt (x:xs) = (Just x, xs) pop :: State Stack (Maybe Int) pop = state popIt push :: Int -> State Stack (Maybe ()) push a = state $ \xs -> (Just (), a:xs) main = do let a = evalState (do push 3 push 2 push 1 pop pop a <- pop return a ) [] print a — snip

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