September 2010 - Beginners

Graham Scan exercise from Chapter 3 RWH -Spoilers. Don't read if you want to do this exercise.
by Michael Litchard 07 Sep '10

07 Sep '10

Below is my solution for the Graham Scan Algorithm. I tried to limit myself to information in the first three chapters while completing this problem. Now, I want to remove the explicit recursion and use more idiomatic Haskell. I'd like to get some advice on which functions/modules would be helpful in this. <code> data Direction = DStraight | DLeft | DRight deriving (Eq,Show) type PointXY = (Double,Double) calcTurn :: PointXY -> PointXY -> PointXY -> Direction calcTurn a b c | crossProduct == 0 = DStraight | crossProduct > 0 = DLeft | otherwise = DRight where crossProduct = ((fst b - fst a) * (snd c - snd a)) - ((snd b - snd a) * (fst c - fst a)) calcDirectionList :: [PointXY] -> [Direction] calcDirectionList (x:y:z:zs) = (calcTurn x y z) : (calcDirectionList (y:z:zs)) calcDirectionList _ = [] sortListByY :: [PointXY] -> [PointXY] sortListByY [] = [] sortListByY [a] = [a] sortListByY (a:as) = insert (sortListByY as) where insert [] = [a] insert (b:bs) | snd a <= snd b = a : b : bs | otherwise = b : insert bs sortListByCoTangent :: [PointXY] -> [PointXY] sortListByCoTangent [] = [] sortListByCoTangent [a] = [a] sortListByCoTangent (a:as) = a : insert (sortListByCoTangent as) where insert :: [PointXY] -> [PointXY] insert [b] = [b] insert (b:c:cs) | (myCoTan a b) >= (myCoTan a c) = b : insert (c:cs) | otherwise = c : insert (b:cs) where myCoTan :: PointXY -> PointXY -> Double myCoTan p1 p2 = (fst p2 - fst p1) / (snd p2 - snd p1) createHull :: [PointXY] -> [PointXY] createHull (a:b:c:cs) = a : filterPoint (createHull (b:c:cs)) where filterPoint :: [PointXY] -> [PointXY] filterPoint (x:y:z:zs) | calcTurn x y z == (DLeft) = [x] ++ [y] ++ [z] ++ filterPoint (zs) | otherwise = [x] ++ [z] ++ filterPoint (zs) filterPoint [x] = a:[x] filterPoint _ = [] createHull _ = [] main :: IO () main = do let points = [(5.0,0.0),(5.0,6.0),(3.0,-4.2),(0.0,6.0)] print $ createHull points

4 3

Convert bits to bytes
by Alec Benzer 06 Sep '10

06 Sep '10

What's the quickest/the best/a way of converting a series of bits into bytes? Ie, converting a [Bool] to a [Word8]. So if I had replicate 8 True ++ replicate 8 False :: [Bool], it would spit out [255,0] :: [Word8].

4 7

Real world example of Typeclasses > Interfaces
by Alec Benzer 06 Sep '10

06 Sep '10

Very new to Haskell, but I was just mulling this over; when I first learned about typeclasses, I saw the apparent benefit of being able to assert that a parameter to a function belongs to two typeclasses instead of just one. Ie, in Java, for example, you can ensure that a parameter to a function belongs to a particular interface by doing something like: int doSomething(Comparable c) { ... } But there's no real way of ensuring an argument implements two interfaces, whereas in Haskell you can make sure that an argument belongs to two (or however many) typeclasses. doSomething :: (Ord a, Eq a, OtherTypeClass a) => a -> Int ... But as I thought about this, I couldn't seem to think of a practical case where this would be useful. The basic typeclasses in Haskell seem to follow a hierarchy. Ie, anything that belongs to Ord belongs to Eq, anything that belongs to Enum belongs to Ord, anything that belongs to Num belongs to Enum. I suppose there may be certain cases involving Functors where there would be a point to specifying multiple typeclasses (eg, an Int would not work with (Functor a, Ord a)), but I guess I'm still somewhat skeptical about how often such a check would be necessary in practical applications. So I guess I'm just wondering if someone could give me an example of a practical case of the typeclass system allowing you to do a check like this that would not have been possible with interfaces. Or is there another benefit to typeclasses over interfaces that I've missed?

4 7

Motivation to Learn Haskell
by Lorenzo Isella 06 Sep '10

06 Sep '10

Dear All, It is my first post to this list and please do not take it as an attempt to start any flamewar. From time to time, I try to find the motivation to learn at least the fundamentals of another programming language. I normally use R and Python on a daily basis (but I am not that much into OO programming) and have a good knowledge of Fortran and a rather superficial one of C. Beside learning a new language as a sort of mind expanding exercise, I try to figure out how and if it can save me some time in my work and how it measures up against other languages. These days I tend to rely on R for data analysis and visualization whereas I use Python (in particular Numpy+SciPy) for number crunching (it is very convenient to use scipy/numpy to solve ODE's, manipulate arrays and so on). Now, I wonder what benefit I would gain from learning Haskell since I mainly write codes for numerical simulations/data analysis. I know Haskell is gaining momentum e.g. in the financial environment (I happened to see Haskell knowledge as a specification in some quant jobs) hence it must be more than suitable for numerical work and, by the little I have understood so far, it allows one to write code really resembling mathematical expressions (I was impressed by guards and curried functions). However, it also looks to me (correct me if I am mistaken) that Haskell is a far cry from the wealth of standard and contributed scientific modules you have in Python or R and thanks to which you do not re-implement the wheel yourself. Any thoughts/suggestions are really appreciated. Cheers Lorenzo

6 5

glib heterogenous hash table
by John Obbele 05 Sep '10

05 Sep '10

Hi everyone ! To exercise my Haskell skills, I'm currently trying to bind libxfconf-0 with Haskell. A small preview can be found on patch-tag[0]. [0]: https://patch-tag.com/r/obbele/xfconf/home Since XFCE projects rely heavily on glib, I am borrowing (lots of) code from the gtk2hs package(s). I am still having issues with gobject casts and foreign pointer finalizers, but lets focus on GHashTable for now. For those who are not familiar with glib (neither I am), GHashTable[1] are hashes with homogenous key types but values of arbitrary types. In xfconf, keys are all cstring but values can be any type of gpointers (gchar *, gint*, gdouble*, widget*, struct* ?). [1]: http://library.gnome.org/devel/glib/stable/glib-Hash-Tables.html I've tried to wrap the glib hash table in a haskell data type this way: > data GHashTable k v = GHashTable GHashTablePtr The GHashTablePtr is our Haskell foreign pointer, courtesy of C→HS. It is bound later with a g_hash_table_destroy finalizer in order to (automatically) manage memory . My real problems are: - I cannot simply map (GHashTable k v) to a (Data.Map.Map k v) - I don't know how to constraint k and v to be C pointers (Ptr CChar is ok but not Char or CChar). Does anyone have a solution or some pointers on it ? regards, /John

2 1

Trouble in installing Packages
by Lorenzo Isella 05 Sep '10

05 Sep '10

Dear All, I am quite new to Haskell and have only very recently found out about cabal. I am interested in some haskell packages for scientific applications, but I am experiencing some problems in installing them on my machine (ubuntu 10.04, amd64 architecture). For instance, I pasted below the output of my attempts to install hmatrix-static. Any idea about how to fix this? Cheers Lorenzo $ cabal install hmatrix-static Resolving dependencies... Configuring array-0.2.0.0... Preprocessing library array-0.2.0.0... Building array-0.2.0.0... [ 1 of 10] Compiling Data.Array ( Data/Array.hs, dist/build/Data/Array.o ) Data/Array.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. Data/Array/IO.hs:1:15: Warning: -#include is deprecated: No longer has any effect Data/Array/IO/Internals.hs:1:15: Warning: -#include is deprecated: No longer has any effect [ 2 of 10] Compiling Data.Array.Base ( Data/Array/Base.hs, dist/build/Data/Array/Base.o ) Data/Array/Base.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. Data/Array/Base.hs:1177:8: Warning: Bindings containing unlifted types must use an outermost bang pattern: { W# e# = if initialValue then maxBound else 0 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `newArray': newArray (l, u) initialValue = ST $ \ s1# -> case safeRangeSize (l, u) of { (n@(I# n#)) -> ... } where W# e# = if initialValue then maxBound else 0 In the instance declaration for `MArray (STUArray s) Bool (ST s)' Data/Array/Base.hs:1489:8: Warning: Bindings containing unlifted types must use an outermost bang pattern: { I# last# = 8 * 8 - 1 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `bOOL_SCALE': bOOL_SCALE n# = (n# +# last#) `uncheckedIShiftRA#` 3# where I# last# = 8 * 8 - 1 Data/Array/Base.hs:1491:8: Warning: Bindings containing unlifted types must use an outermost bang pattern: { I# last# = 8 * 8 - 1 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `bOOL_WORD_SCALE': bOOL_WORD_SCALE n# = bOOL_INDEX (n# +# last#) where I# last# = 8 * 8 - 1 Data/Array/Base.hs:1492:37: Warning: Bindings containing unlifted types must use an outermost bang pattern: { I# scale# = 8 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `wORD_SCALE': wORD_SCALE n# = scale# *# n# where I# scale# = 8 Data/Array/Base.hs:1493:37: Warning: Bindings containing unlifted types must use an outermost bang pattern: { I# scale# = 8 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `dOUBLE_SCALE': dOUBLE_SCALE n# = scale# *# n# where I# scale# = 8 Data/Array/Base.hs:1494:37: Warning: Bindings containing unlifted types must use an outermost bang pattern: { I# scale# = 4 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `fLOAT_SCALE': fLOAT_SCALE n# = scale# *# n# where I# scale# = 4 Data/Array/Base.hs:1505:8: Warning: Bindings containing unlifted types must use an outermost bang pattern: { W# mask# = 8 * 8 - 1 } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `bOOL_BIT': bOOL_BIT n# = int2Word# 1# `uncheckedShiftL#` (word2Int# (int2Word# n# `and#` mask#)) where W# mask# = 8 * 8 - 1 Data/Array/Base.hs:1506:47: Warning: Bindings containing unlifted types must use an outermost bang pattern: { W# mb# = maxBound } *** This will be an error in GHC 6.14! Fix your code now! In the definition of `bOOL_NOT_BIT': bOOL_NOT_BIT n# = bOOL_BIT n# `xor#` mb# where W# mb# = maxBound [ 3 of 10] Compiling Data.Array.IArray ( Data/Array/IArray.hs, dist/build/Data/Array/IArray.o ) Data/Array/IArray.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. [ 4 of 10] Compiling Data.Array.MArray ( Data/Array/MArray.hs, dist/build/Data/Array/MArray.o ) Data/Array/MArray.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. [ 5 of 10] Compiling Data.Array.IO.Internals ( Data/Array/IO/Internals.hs, dist/build/Data/Array/IO/Internals.o ) Data/Array/IO/Internals.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. [ 6 of 10] Compiling Data.Array.IO ( Data/Array/IO.hs, dist/build/Data/Array/IO.o ) Data/Array/IO.hs:73:13: `haFD' is not a (visible) field of constructor `Handle__' Data/Array/IO.hs:73:22: `haBuffer' is not a (visible) field of constructor `Handle__' Data/Array/IO.hs:73:36: `haIsStream' is not a (visible) field of constructor `Handle__' Data/Array/IO.hs:74:5: Not in scope: data constructor `Buffer' Data/Array/IO.hs:74:13: `bufBuf' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:74:25: `bufWPtr' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:74:36: `bufRPtr' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:75:4: Not in scope: `bufferEmpty' Data/Array/IO.hs:82:24: `bufWPtr' is not a (visible) constructor field name Data/Array/IO.hs:82:35: `bufRPtr' is not a (visible) constructor field name Data/Array/IO.hs:86:24: `bufRPtr' is not a (visible) constructor field name Data/Array/IO.hs:95:27: Not in scope: type constructor or class `RawBuffer' Data/Array/IO.hs:101:10: Not in scope: `readRawBuffer' Data/Array/IO.hs:125:22: `haFD' is not a (visible) field of constructor `Handle__' Data/Array/IO.hs:125:31: `haBuffer' is not a (visible) field of constructor `Handle__' Data/Array/IO.hs:125:45: `haIsStream' is not a (visible) field of constructor `Handle__' Data/Array/IO.hs:127:18: Not in scope: data constructor `Buffer' Data/Array/IO.hs:127:26: `bufBuf' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:127:42: `bufWPtr' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:127:53: `bufSize' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:135:30: `bufWPtr' is not a (visible) constructor field name Data/Array/IO.hs:142:7: Not in scope: data constructor `Buffer' Data/Array/IO.hs:142:15: `bufBuf' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:142:27: `bufState' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:142:36: Not in scope: data constructor `WriteBuffer' Data/Array/IO.hs:143:8: `bufRPtr' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:143:19: `bufWPtr' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:143:34: `bufSize' is not a (visible) field of constructor `Buffer' Data/Array/IO.hs:151:22: Not in scope: type constructor or class `RawBuffer' Data/Array/IO.hs:151:43: Not in scope: type constructor or class `RawBuffer' Data/Array/IO.hs:153:22: Not in scope: type constructor or class `RawBuffer' Data/Array/IO.hs:153:35: Not in scope: type constructor or class `RawBuffer' cabal: Error: some packages failed to install: array-0.2.0.0 failed during the building phase. The exception was: ExitFailure 1 containers-0.3.0.0 depends on array-0.2.0.0 which failed to install. cpphs-1.11 depends on array-0.2.0.0 which failed to install. haskell-src-exts-1.2.0 depends on array-0.2.0.0 which failed to install. haskell-src-meta-0.0.6 depends on array-0.2.0.0 which failed to install. haskell98-1.0.1.1 depends on array-0.2.0.0 which failed to install. hmatrix-0.7.2.1 depends on array-0.2.0.0 which failed to install. hmatrix-static-0.3 depends on array-0.2.0.0 which failed to install. packedstring-0.1.0.1 depends on array-0.2.0.0 which failed to install. template-haskell-2.4.0.0 depends on array-0.2.0.0 which failed to install. tfp-0.2 depends on array-0.2.0.0 which failed to install.

2 1

Meaning of (Eq a)
by Rohit Garg 05 Sep '10

05 Sep '10

Hi, RWH: chapter 3 - in question 5, you have to write a function which determines if a list is a palindrome. Here is my solution isPalindrome :: (Eq a) => [a] -> Bool isPalindrome [] = False isPalindrome x = compareLists x (reverse x) where compareLists [x] [y] = x == y compareLists (x:xs) (y:ys) = if x == y then compareLists xs ys else False Although it works, my question is why ghci refuses to run it without the "(Eq a) => " being added to the type signature of the function. Presumably, it is to let ghc know that you can perform equlity tests on a. If so, then why does the sumList function below work without any type signature of any kind? I haven't told ghc that the input list elements can be added together. sumList [] = 0 sumList (x:xs) = x + sumList xs Thanks, -- Rohit Garg http://rpg-314.blogspot.com/

5 4

Re: [Haskell-beginners] Meaning of (Eq a)
by Hein Hundal 05 Sep '10

05 Sep '10

> Date: Sun, 5 Sep 2010 14:28:09 +0530 > From: Rohit Garg <rpg.314(a)gmail.com> > > RWH: chapter 3 - in question 5, you have to write a > function which determines if a list is a palindrome. > Here is my solution > > isPalindrome :: (Eq a) => [a] -> Bool > isPalindrome [] = False > isPalindrome x = compareLists x (reverse x) > where compareLists [x] [y] = x == y > compareLists (x:xs) (y:ys) = if x == y > then compareLists xs ys > else False > > Although it works, my question is why ghci refuses to > run it without the "(Eq a) => " being added to the type > signature of the function. Presumably, it is to let ghc > know that you can perform equality tests on a. If so, > then why does the sumList function below work without > any type signature of any kind? I haven't told ghc > that the input list elements can be added together. > > sumList [] = 0 > sumList (x:xs) = x + sumList xs When I compile isPalindrome with ghc 6.12.1, I do not get any errors even if I drop the type signature. If I drop the type signature and use ghci to get the type signature (":t isPalindrome"), then I get *Main> :t isPalindrome isPalindrome :: (Eq t) => [t] -> Bool So, it seems that ghc 6.12.1 correctly infers the types. Cheers, Hein PS: Haskell does equality of lists, so you can also use the definition isPalindrome v = v == reverse v

2 1

Pattern matching on binary trees
by Rohit Garg 05 Sep '10

05 Sep '10

Hi, I am working on exercises in RWH, chapter 3 and I have a a question about the pattern matching in the depth-of-tree question. The tree datatype is defined as data Tree a = Node a (Tree a) (Tree a) | Empty deriving (Show) And to pattern match on it, I need to write depthTree (Node x u v) = 1 + max (depthTree u) (depthTree v) My question is why do I need to prefix x with Node? Why can't it pattern match on depthTree (x u v)? More generally, where do I need to put data constructors in pattern matches? -- Rohit Garg http://rpg-314.blogspot.com/

2 2

Too lazy/Too strict
by Alex Rozenshteyn 03 Sep '10

03 Sep '10

I'm trying to take an html page whose last pre tag contains a hexdump and write the hex out as an executable. Link to truncated html example: http://pastebin.com/ENMDrsAT Link to my code: http://pastebin.com/XY0wgM6r I've got a memory leak somewhere, likely due to laziness, and I would appreciate help in finding it; this is the first one i've encountered, so I'm a bit clueless. -- Alex R

1 0