- Beginners - Haskell.org

Re: [Haskell-beginners] Parsec problem
by Daniel Fischer 03 Feb '13

03 Feb '13

On Saturday 02 February 2013, 15:00:52, Kees Bleijenberg wrote: > module Main(main) where > > import Text.ParserCombinators.Parsec > > parseToNewLine = do > line <- manyTill anyChar newline > return line > > keyValue = do > fieldName <- many letter > spaces > char '=' > spaces > fieldValue <- parseToNewLine > return (fieldName,fieldValue) > > main = parseTest keyValue "key=\n" > > I don’t understand why te code above doesn’t parse to (“key”,””) Because the newline is already consumed by the `spaces`. So parseToNewLine gets an empty string as input, and fails on that. > parseToNewLine “\n” parses to “" > > parseTest keyValue “a=b\n” works fine and parses to (“a”,”b”) The input of parseToNewLine must contain a newline, or it fails. In the last example, the `spaces` after `char '='` stops at reaching the 'b', so the newline remains. In the first (problematic) example, all the remaining input after the '=' consists of whitespace.

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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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Re: [Haskell-beginners] Parsec problem
by Andres Löh 02 Feb '13

02 Feb '13

Hi. > keyValue = do > fieldName <- many letter > spaces > char '=' > spaces > fieldValue <- parseToNewLine > return (fieldName,fieldValue) > > main = parseTest keyValue "key=\n" > > I don’t understand why te code above doesn’t parse to (“key”,””) The problem is that the \n is already consumed by spaces. The subsequent parseToNewLine fails. Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com

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Parsec problem
by Kees Bleijenberg 02 Feb '13

02 Feb '13

module Main(main) where import Text.ParserCombinators.Parsec parseToNewLine = do line <- manyTill anyChar newline return line keyValue = do fieldName <- many letter spaces char '=' spaces fieldValue <- parseToNewLine return (fieldName,fieldValue) main = parseTest keyValue "key=\n" I don't understand why te code above doesn't parse to ("key","") parseToNewLine "\n" parses to "" parseTest keyValue "a=b\n" works fine and parses to ("a","b") Kees

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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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cabal-debian
by Ryan 01 Feb '13

01 Feb '13

Hello, I'm trying to compile a cabal package to debian with cabal-debian and having some trouble. git clone git://github.com/wavewave/hoodle.git cd hoodle/ cabal-debian --debianize --ghc --maintainer Ryan then I get a the following error: cabal-debian: cabal-debian: No bundled package list for ghc Version {versionBranch = [7,4,2], versionTags = []} shouldn't cabal-debian be pulling the package list from the hoodle.cabal file? Thanks, Ryan

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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 30 Jan '13

30 Jan '13

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

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