August 2014 - Beginners

Too much mapM, fmap and concat
by martin 14 Aug '14

14 Aug '14

Hello all, I never did much IO in haskell, but now I have to process some files. I catch myself adding a mapM here and an fmap there. While it is clear to me what these functions do, I wonder if there is a way to avoid the noise. Also I could not find a simple way to pair the lines of a file with its filename. I ended up writing a function "nameAndContent". Finally I am amazed by the many "concat"s I need. Maybe these things just lie in the nature of the problem ("process a number of files"). Otherwise any style suggestions would be much appreciated. import System.FilePath.Glob import Data.List filePattern="*.hs" fileDirectory = "." processFile :: (FilePath, [String]) -> [String] processFile (path, flines) = ["done"] main = do matchingFiles <- fmap (concat . fst) $ globDir [compile filePattern] fileDirectory flines <- mapM nameAndContent matchingFiles result <- mapM (return . processFile) flines mapM putStrLn $ concat result where nameAndContent :: FilePath -> IO (FilePath, [String]) nameAndContent fn = do content <- fmap lines $ readFile fn return (fn, content)

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Crash Course for C++ "Refugees"?
by Frank 13 Aug '14

13 Aug '14

Hi, I have recently been introduced to Haskell and find it quite fascinating. I am interested in incorporating it into a C++ project on which I am working, porting pieces of that program to Haskell when practical. Is Anyone able to point Me in the direction of a "crash course" or "quick start" guide for Anyone used to programming in C++ seeking to call Haskell code from the C++ portion and/or vice versa? I am especially interested in knowing how to pass information from, say, the C++ standard template library containers (e.g., vector, valarray, etc.) and into Haskell functions/methods/procedures/etc. While I might have missed such information when I performed a web search on the topic, I did not find any information to help. Any assistance in finding such material would be greatly appreciated. Sincerely, Frank D. Martinez -- P.S.: I prefer to be reached on BitMessage at BM-2D8txNiU7b84d2tgqvJQdgBog6A69oDAx6

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expressing constraints 101
by Dimitri DeFigueiredo 13 Aug '14

13 Aug '14

Hi All, I am working through an exercise in Chris Okasaki's book (#2.2). In the book, he is trying to implement a minimal interface for a Set. I wrote that simple interface in Haskell as: class Set s where empty :: s a -- returns an empty set of type Set of a insert :: a -> s a -> s a -- returns set with new element inserted member :: a -> s a -> Bool -- True if element is a member of the Set To implement that interface with the appropriately O(log n) insert and member functions he suggests the use of a Binary Search Tree, which I translated to Haskell as: data Tree a = Empty | MkTree (Tree a) a (Tree a) But for the tree to work, we also need the "a"s to be totally ordered. I.e. (Ord a) is a constraint. So, it makes sense to write: treeEmpty :: Tree a treeEmpty = Empty treeInsert :: Ord a => a -> Tree a -> Tree a treeInsert = undefined treeMember :: Ord a => a -> Tree a -> Bool treeMember = undefined Now, I would like to bind this implementation using Trees of an ordered type "a" to the set type class. So, I would like to write something like: instance Set Tree where empty = treeEmpty insert = treeInsert member = treeMember But that doesn't work. Using GHC 7.6.3, I get a: No instance for (Ord a) arising from a use of `treeInsert' Possible fix: add (Ord a) to the context of the type signature for insert :: a -> Tree a -> Tree a In the expression: treeInsert a In an equation for `insert': insert a = treeInsert a In the instance declaration for `Set Tree' Which makes sense, but I'm not sure how to express this constraint. So, what is the proper way to do this? Where have I gone wrong? Thanks! Dimitri

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State Monad vs. fold
by martin 13 Aug '14

13 Aug '14

Hello all, many times I see a problem and I say to myself: "there is some state". I then play around with the state monad and often I don't get anywhere. Then at some point I realizes, that all I need is a simple fold. I don't think I ever used the state monad outside of toy examples. Can someone give me some insights when the State Monad is beneficial and where a fold is the better choice.

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Re: [Haskell-beginners] Beginners Digest, Vol 74, Issue 5
by Neeraj Rao 13 Aug '14

13 Aug '14

Why not add the Ord constraint to the function signatures in the class definition? class Set s where empty :: s a insert :: Ord a => a -> s a -> s a member :: Ord a => a -> s a -> Bool Seems to me like insert and member do need those constraints, irrespective of what type 's' is. Message: 1 > Date: Tue, 12 Aug 2014 21:14:09 -0600 > From: Dimitri DeFigueiredo <defigueiredo(a)ucdavis.edu> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] expressing constraints 101 > Message-ID: <53EAD801.30801(a)ucdavis.edu> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > Hi All, > I am working through an exercise in Chris Okasaki's book (#2.2). In the > book, he is trying to implement a minimal interface for a Set. I wrote > that simple interface in Haskell as: > class Set s where > empty :: s a -- returns an empty set of type Set of a > insert :: a -> s a -> s a -- returns set with new element inserted > member :: a -> s a -> Bool -- True if element is a member of the Set > To implement that interface with the appropriately O(log n) insert and > member functions he suggests the use of a Binary Search Tree, which I > translated to Haskell as: > data Tree a = Empty | MkTree (Tree a) a (Tree a) > But for the tree to work, we also need the "a"s to be totally ordered. > I.e. (Ord a) is a constraint. So, it makes sense to write: > treeEmpty :: Tree a > treeEmpty = Empty > treeInsert :: Ord a => a -> Tree a -> Tree a > treeInsert = undefined > treeMember :: Ord a => a -> Tree a -> Bool > treeMember = undefined > Now, I would like to bind this implementation using Trees of an ordered > type "a" to the set type class. So, I would like to write something like: instance Set Tree where empty = treeEmpty insert = treeInsert member = treeMember But that doesn't work. Using GHC 7.6.3, I get a: No instance for (Ord a) arising from a use of `treeInsert' Possible fix: add (Ord a) to the context of the type signature for insert :: a -> Tree a -> Tree a In the expression: treeInsert a In an equation for `insert': insert a = treeInsert a In the instance declaration for `Set Tree' Which makes sense, but I'm not sure how to express this constraint. So, what is the proper way to do this? Where have I gone wrong? Thanks! Dimitri

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Feedback on my first, small project (piece).
by Dominik Bollmann 06 Aug '14

06 Aug '14

Hello Haskeller's, I recently dived into Haskell and then wanted to practice it a bit! Therefore I wrote a small program that matches UNIX-style globs. The program behaves kind of like grep, just that it matches a glob and not a regular expression. And also, it offers only *very* rudimental functionality compared to grep. The code is available on github: https://github.com/bollmann/Globber When writing the program I tried to satisfy the specification as given at: http://www.scs.stanford.edu/14sp-cs240h/labs/lab1.html. This Lab material btw. also inspired me to try writing such a program :-). In order to improve my programming in Haskell, I would love to hear feedback from you guys on this first small project of mine. Any comments regarding style, used idioms, as well as general and specific code improvements are highly appreciated. Thanks! Cheers, Dominik.

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wxhaskell compile issue for HelloWorld demo
by Paraic OCeallaigh 02 Aug '14

02 Aug '14

Hi My first bite at installing wxhaskell on Mac OS Mavericks and I am almost there after jumping thru a few hoops. I am on wxhaskell 3.0 and installed the haskell platform using homebrew. I had to edit the Setup.hs to indicate I was on 3.0 not 2.9 as one of the hurdles to getting it even install. So this is what I get try to run HelloWorld.hs: make HelloWorld ghc -package wx -o HelloWorld HelloWorld.hs Linking HelloWorld ... ld: warning: ignoring file /System/Library/Frameworks//QuickTime.framework/QuickTime, file was built for i386 which is not the architecture being linked (x86_64): /System/Library/Frameworks//QuickTime.framework/QuickTime Undefined symbols for architecture x86_64: "_CPSSignalAppReady", referenced from: _s7vsd_info in libHSwxcore-0.90.1.1.a(GHCiSupport.o) ld: symbol(s) not found for architecture x86_64 collect2: error: ld returned 1 exit status make: *** [HelloWorld] Error 1 I did a search under /usr/local/Cellar for the QuickTime and found it in ./wxmac/3.0.0/lib/wx/config/osx_cocoa-unicode-3.0 and I removed the inclusion of QuickTime Framework, but it made no difference. Any help appreciated at this stage as I am feel I am so close to getting this to work. Paraic

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