- Beginners - Haskell.org

Creating modules
by Dananji Liyanage 10 May '15

10 May '15

Hi All, I'm following this lecture[1] series and stuck at the module creation section. Even though I understand the idea of the module I can't figure out the exact syntax to module creation. Do we have to compile the Haskell file containing the module declaration? For an example: Do I have to compile this; module metricCalc(.....) where ... in a file metricCalc.hs [1] http://shuklan.com/haskell/index.html -- Regards, Dananji Liyanage

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Re: [Haskell-beginners] Implementation question about directed acyclic graph
by Doug McIlroy 09 May '15

09 May '15

Your suggested this representation > type Connections = M.Map Int a [Int] > > data Graph a = Empty > | Gragh { nodes :: M.Map Int (Node a), nextID :: Int, > connections :: Connections } Making connections by name (in this case an Int) entails table lookup at every step of any graph traversal. That is avoided by this simple representation: data Graph a = Graph a [Graph a] perhaps elaborated to allow the empty case data Graph a = Graph a [Graph a] | Empty If you need node identifiers, e.g. to recognize repeat visits to a node, they can be incorporated in data type a, or abstracted into another field data Eq id => Graph id a = Graph id a [Graph id a] Doug McIlroy

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Import issue
by Shishir Srivastava 08 May '15

08 May '15

Hi, I've imported the State monad module and used in the 'pop' function that will pop the head out of the list but am getting error in GHCi. ----- import Control.Monad.State let pop = State $ \(x:xs) -> (x,xs) <interactive>:73:11: Not in scope: data constructor ‘State’ Perhaps you meant ‘StateT’ (imported from Control.Monad.State) ----- Please can anyone point out what the issue is here? Thanks, Shishir

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Implementation question about directed acyclic graph
by Christian Sperandio 08 May '15

08 May '15

Hi, I’m learning the Bayesian networks and I want to write my own implementation. So, I started implementing t a DAG in Haskell but I’ve got some questions about the best way to do it in a functional mind. I’m thinking about a DAG implementation avoids duplicate information. So, I imagined this implementation: data Node a = Node { event :: a, proved :: Bool, nodeID :: Int } type Connections = M.Map Int [Int] data Graph a = Empty | Gragh { nodes :: M.Map Int (Node a), nextID :: Int, connections :: Connections } My idea is to yield an ID for each node and use this ID for connections. Thus, I can have the same events connected many times without duplicate the event data. And when I have to change a node state, I don’t need to search all occurrences in the graph. Am I on the right way ? Chris

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Uses of record syntax
by Dananji Liyanage 08 May '15

08 May '15

Hi All, I'm Dananji, a new comer to this language and functional programming as well. Can anyone please help me understand what is record syntax and its use? -- Regards, Dananji Liyanage

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Re: [Haskell-beginners] Missing C libraries on windows
by Cameron P-B 07 May '15

07 May '15

Ok, thanks everyone, It all seems to be working now. Setting the environmental variables as suggested and generally following the steps here [https://wiki.haskell.org/SDL/Windows] appears to have been all that was needed; as well as a bit of trial and error as to what was meant by the development libraries.

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Missing C libraries on windows
by Cameron P-B 07 May '15

07 May '15

I'm not sure if this is the right place for this, but I've repeatedly run into issues when installing packages through cabal. It complains about missing C libraries and then explains that I can solve the problem by "installing the system package that provides this library". This has never worked for me and so I assume I'm not doing it correctly. For example, I am currently trying to install ftgl. Using --extra-include-dirs and --extra-lib-dirs to point at the lib and include of the c source folder of ftgl doesn't seem to work. Neither did putting the dll's in the path. The tutorial linked from the wiki:[ https://noamlewis.wordpress.com/2012/12/16/cabal-install-ftgl-on-windows-an… ] also doesn't work for me. And so I was wondering if anyone has had more recent success with that library and more generally if there were any suggestions as to what I might be doing wrong.

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unsubscribe
by Oldman, John 07 May '15

07 May '15

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Unix-style command line arguments and file input?
by Vale Cofer-Shabica 05 May '15

05 May '15

Hello all, Punchline first: What's the "best practice" way of doing unix-style command line argument and file input processing in Haskell? Background: I'm using Haskell to write programs that act like well-behaved, pipe-friendly unix tools. i.e., the following are all equivalent: % ./prog file % ./prog < file % cat file | ./prog Thus far, I've done this by directly inspecting the first element of System.Environment.getArgs, which has been fine thus far. I'd also like to be able to take simple command line arguments (boolean flags and numeric parameters) and the above doesn't adapt well to that case. I'd like to do this in the idiomatic, "standard" way (a la getopt() in C). Browsing through the wiki page on command line argument parsers [1] gave me a bewildering array of options. I'm not really sure where to start, though I remember reading a blanket endorsement of optparse-applicative somewhere. Any pointers or examples that address my use-case would be much appreciated. -vale [1]: https://wiki.haskell.org/Command_line_option_parsers

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Re: [Haskell-beginners] Data type
by Shishir Srivastava 05 May '15

05 May '15

Hi Brandon, Thanks for your response. My example was just a bad turnout which I conjured up using tutorials and playing with it. I was going to follow up my question with the possible practical use of why and where someone would use such a construct to wrap a function inside a new data-type. For all that matters I could have used 'length' function directly to get the same output. I appreciate that you already have given the practical example but anything more basic for beginners to highlight the usage would be helpful. Thanks, Shishir On Tue, May 5, 2015 at 4:28 PM, Shishir Srivastava < shishir.srivastava(a)gmail.com> wrote: > Thanks Matthew - that was crisp ! > > Cheers, > Shishir >

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