January 2013 - Beginners

"downcasting"
by Emmanuel Touzery 29 Mar '13

29 Mar '13

Hello, using the Text.Json library (and I'm reading I should be using Aeson instead... I'll migrate to it later), I'm a bit frustrated. At one point I'm getting a JSValue object, which could be anything, including a JSObject. But in this case I know I'm looking at a JSObject. I don't mind if Haskell crashes if that's not the case (for now), either it's a JSObject or the JSON I'm parsing is invalid. I hoped to do that conversion using pattern matching by giving non-exhaustive pattern match, considering only cases where the JSValue is in fact a JSONObject but somehow I can't make it... getObject :: JSValue -> JSObject JSValue getObject x@(JSObject _) = x --> doesn't compile the function returns a JSValue, I somehow wanted to point out to the compiler that if it gets there it's in fact a JSObject. getObject :: JSValue -> JSObject JSValue getObject (JSObject x) = JSObject x -> same as with the other attempt.. the compiler is saying the function is returning a JSValue: Couldn't match expected type `JSObject JSValue' with actual type `JSValue' In the return type of a call of `JSObject' In the expression: JSObject x In an equation for `getObject': getObject (JSObject x) = JSObject x I think I'm missing an obvious pattern in Haskell here, probably something I've already used elsewhere too, but somehow I don't make it. Otherwise answers saying that if I need to make such a "cast" then I'm using that library the wrong way are welcome (together to maybe a link to a tutorial explaining it the right way), but by curiosity I would still be interested in how to actually do that "cast" in this case. Thank you! Emmanuel

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How to unnest "do"
by Martin Drautzburg 09 Feb '13

09 Feb '13

Hello all, in the code snippet below, is there a way to factor out the second "do"? import System (getArgs) main :: IO () main = do args <- getArgs case args of [fname] -> do fstr <- readFile fname let nWords = length . words $ fstr nLines = length . lines $ fstr nChars = length fstr putStrLn . unwords $ [ show nLines , show nWords , show nChars , fname] _ ->putStrLn "usage: wc fname"

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Literate Haskell - capturing output
by Martin Drautzburg 03 Feb '13

03 Feb '13

Hello all, I am using literate haskell quite a lot (otherwise I don't understand my own code). This works fine for the code as such. But when I give an example usage, I run code snippets in ghci and copy&paste into the main document/program, which turns them into "text" (and not code). When I make changes to the program these examples tend to no longer reflect the actual program. Is there a way to automatically run examples and include them in the .lhs file, preferably with the haskell prompt and everything? -- Martin

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Using Parsec to parse Haskell Language
by Sean Cormican 01 Feb '13

01 Feb '13

I'm working on a project in which I am attempting to write a parser for Haskell within Haskell itself using Parsec. The parser itself only deals with a subset of the features of the full Haskell implementation but I'm having difficulty figuring out how to implement the grammar with precedence of operations for example boolean expression involving && over || and also 'if then else" statements in relation to other operations. I'm attempting to do this with the Text.Parsec.Expr module more specifically BuildExpressionParser combinator using table and term as used in a number of tutorials I have seen which deal with arithmetic expression. Is this the correct course of action? Or will it fail to meet my needs. Will it be necessary to create an AST for the grammar by creating new data types as outlined in the following tutorial? http://www.haskell.org/haskellwiki/Parsing_a_simple_imperative_language Also what's the difference between the modules: Text.Parsec.Expr Text.ParserCombinators.Parsec.Expr Thanks in advance, Seán

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How to interact with a process
by Martin Drautzburg 01 Feb '13

01 Feb '13

Hello all, I know how to play a Midi file. I also know how to read the keyboard. But I don't know how to bring the two together such that I can start and stop playing from the keyboard. It is not required (just nice to have) that the console remains responsive while the midi file plays. But it is essential that I can stop the performance. Is this difficult? -- Martin

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Type classes vs Java interfaces
by Mateusz Kowalczyk 01 Feb '13

01 Feb '13

Greetings, I often wonder how one would explain type classes to someone coming from an environment such as Java. Whenever I think about type classes, I seem to think of them as Java interfaces. Bah, even the bottom of [1] states > Haskell classes are roughly similar to a Java interface. Like an interface declaration, a Haskell class declaration defines a protocol for using an object rather than defining an object itself. Is there more to this `roughly similar' statement? Syntax is an obvious difference but beyond that, I can't think of anything I can do with a Haskell type class that I wouldn't be able to do with similar amount of effort with a Java interface, except for the fact that the interface would look absolutely disgusting syntax wise. Any insight appreciated. [1] - http://www.haskell.org/tutorial/classes.html

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Organizing cmd line flow
by Bryan Vicknair 30 Jan '13

30 Jan '13

I have an executable, which gets a file path from the command line, and passes it to this function:: createDb :: FilePath -> IO () createDb fpath = do fileExists <- doesFileExist fpath if fileExists then putStrLn "File already exists" else do parDirExists <- parentDirExists fpath if parDirExists then do con <- openCon (Config fpath) create con closeCon con putStrLn $ "created db @ " ++ fpath else putStrLn "parent dir doesn't exist" 2 checks: File exists? Parent dir exist? But already, the code is quite nested. How can I refactor this so that a few if expressions that check an IO action doesn't result in very deep nesting of the code? Is there a pattern here that I can use? I read somewhere about wrapping a common pattern like this into a Monad such that it will somehow signal to the main function that we can't proceed, but being an absolute beginner coming from Python, I would need some help with that. Perhaps exceptions are what I'm looking for since I am working with IO, but that is what I would do in Python, so I instinctively assume it's done differently in Haskell :) In Python, I might write something like this:: def createDb(fpath): if doesFileExists(fpath): raise FileExistsError(fpath) if not parDirExists(fpath): return ParentDirNoExistsError(fpath) con = openCon(Config fpath) create(con) closeCon(con) Is there any way to get closer to the following? I think this is much clearer, but perhaps I'm missing a much larger point in how this should be done in Haskell:: createDb fpath = do checkFileExists fpath checkParentDirExists fpath con <- openCon (Config fpath) create con closeCon con

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Looking for a 20/min day study partner
by Heath Matlock 29 Jan '13

29 Jan '13

Would anyone be willing to spend 20 minutes each learning Haskell and the concepts surrounding it?

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Get resource from number of possible sources
by Libor Wagner 29 Jan '13

29 Jan '13

Hi, say I have a number of functions tryGetA, tryGetA', tryGetA'' … with the type tryGetA :: IO (Maybe A), what I want is to get A, trying these function one by one until a get Just a. The first shot code I came up with is using if: getA :: IO A getA = do a <- tryGetA if isJust a then return $ fromJust a else do a' <- tryGetA' if isJust a then return $ fromJust a' … there would be some default value at the end, this looks really ugly so I have tried another shot: firstJust :: a -> [Maybe a] -> a firstJust a [] = a firstJust a (x:xs) = if isJust x then fromJust x else firstJust a xs getA :: IO A getA = do r <- sequence [tryGetA, tryGetA', tryGetA'', …] return $ firstJust "OK" r which is a lot better, but I will appreciate any comment or suggestion. Thanks, Libor

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reorganizing lists
by Bryce Verdier 28 Jan '13

28 Jan '13

Hi All, At the moment I have a list of lists. The inner list is a coordinate, like (x,y) but is [x,y] instead. What I would like to do is group all the x's into one list, and the y's into another. I know I can do this with calling 2 maps on the container, but I would also like to do this in one iteration. Does anyone have any ideas? Thanks in advance, Bryce

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