- Haskell-Cafe - Haskell.org

List-to-outline
by Steve Schafer 15 Feb '06

15 Feb '06

I have some lists of integers; e.g., [0,1,2,2,3,3,3,2,1,2,3,3,1] Think of each integer value as representing the indentation level in a hierarchical outline: e.g., 0 1 2 2 3 3 3 2 1 2 3 3 1 I want to convert the list into a structure that better represents the hierarchy. So, I first define a datatype to represent each node of the new structure: data Node = Nd Int [Node] That is, a node consists of an Int representing the value of the node, followed by a list of its immediate child nodes. (In principle, I can deduce the value of a node simply from the nesting level, of course, but in the real problem I'm trying to solve, each node contains other information that I need to preserve as well.) Next, I define some functions to perform the transformation isChild :: Int -> Node -> Bool isChild i (Nd j _) = (j > i) isChild _ _ = False prepend :: Int -> [Node] -> [Node] prepend i [] = [Nd i []] prepend i ns = (Nd i f):s where (f,s) = span (isChild i) ns unflatten :: [Int] -> [Node] unflatten ns = foldr prepend [] ns Finally, I add some code to display the result in an aesthetically pleasing way: showsNodeTail :: [Node] -> String -> String showsNodeTail [] = showChar '}' showsNodeTail (n:ns) = showChar ' '.shows n.showsNodeTail ns showsNodeList :: [Node] -> String -> String showsNodeList [] = showString "" showsNodeList (n:ns) = showChar '{'.shows n.showsNodeTail ns showsNode :: Node -> String -> String showsNode (Nd i ns) = shows i.showsNodeList ns instance Show Node where showsPrec n = showsNode This all works just fine, and when I enter unflatten [0,1,2,2,3,3,3,2,1,2,3,3,1] I get [0{1{2 2{3 3 3} 2} 1{2{3 3}} 1}] as expected. The reason I'm posting this here is that I have a gnawing suspicion that the unflatten/prepend/isChild functions, and possibly the Node data type as well, are not the most elegant way to go about solving the problem, and that I'm missing another more obvious way to do it. Any suggestions? Thanks, Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

4 3

Overlapping instance problem
by Jeff.Harper＠handheld.com 14 Feb '06

14 Feb '06

Hi, I've posted a couple messages to the Haskell Cafe in the last few months. I'm new to Haskell. But, I've set out to implement my own vectors, matrices, complex numbers, etc. One goal I have, is to overload operators to work with my new types. The pursuit of this goal, has pushed me to learn a lot about the Haskell type system. When I get stuck from time-to-time, the kind folks on this list have pointed me in the right direction. I'm stuck now. One thing I want to avoid is adding new multiplication operators to handle multiplication of dissimilar types. For instance, I'd like to be able to have an expression like k * m where k is a Double and m is a Matrix. This doesn't work with the prelude's (*) operator because the prelude's (*) has signature: (*) :: (Num a) => a -> a -> a. To get around this, I wrote my own versions of a Multiply class that allows dissimilar types to be multiplied. You can see my Multiply class in the module at the end of this Email. At the bottom of the module, I've attempted to implement multiplication of the forms: scalar * matrix matrix * scalar matrix * matrix The problem is that when I try to do matrix * matrix at the interpreter, I get an error message from Glaskgow: *My_matrix> m1 * m2 <interactive>:1:3: Overlapping instances for Multiply (Matrix Double) (Matrix Double) (Matrix c) arising from use of `*' at <interactive>:1:3 Matching instances: My_matrix.hs:63:0: instance (Multiply a b c, Add c c c, Num a, Num b, Num c) => Multiply (Matrix a) (Matrix b) (Matrix c) My_matrix.hs:57:0: instance (Multiply a b c, Num a, Num b, Num c) => Multiply (Matrix a) b (Matrix c) My_matrix.hs:51:0: instance (Multiply a b c, Num a, Num b, Num c) => Multiply a (Matrix b) (Matrix c) In the definition of `it': it = m1 * m2 I don't understand how m1 * m2 can match the scalar multiplication instances. For instance, the scalar * matrix instance has signature: instance (Multiply a b c, Num a, Num b, Num c) => Multiply a (Matrix b) (Matrix c) where m1 in my expression would correspond to the 'a' type variable. But, 'a' is constrained to be a Num. However, I never made my Matrix type an instance of Num. Is there a work around for this? In my first implementation, I did not have the Num constraints in the matrix Multiply instances. I added the Num constraints specifically, to remove the ambiguity of the overlapping instance. Why didn't this work? Thanks, Jeff Harper >> Begining of code for My_matrix.hs ------------------------------------------ {-# OPTIONS -fglasgow-exts #-} module My_matrix where import qualified Prelude as P import Prelude hiding ( (*), (+), (-), negate) default ( ) class Add a b c | a b -> c where (+) :: a -> b -> c class Multiply a b c | a b -> c where (*) :: a -> b -> c class Coerce a b where coerce :: a -> b infixl 7 * infixl 6 + instance Coerce Float Float where { coerce x = x } instance Coerce Float Double where { coerce x = realToFrac x } instance Coerce Double Double where { coerce x = x } instance Add Float Float Float where { (+) x y = ( x) P.+ ( y) } instance Add Float Double Double where { (+) x y = (coerce x) P.+ ( y) } instance Add Double Float Double where { (+) x y = ( x) P.+ (coerce y) } instance Add Double Double Double where { (+) x y = ( x) P.+ ( y) } instance Multiply Float Float Float where { (*) x y = ( x) P.* ( y) } instance Multiply Float Double Double where { (*) x y = (coerce x) P.* ( y) } instance Multiply Double Float Double where { (*) x y = ( x) P.* (coerce y) } instance Multiply Double Double Double where { (*) x y = ( x) P.* ( y) } -- Matrices are stored in a list of list. For now, I can create a -- matrix of Float, or Double. Later, I'd like to extend this and -- make it possible to create a matrix of other number types. For -- instance, it might be possible to have a matrix of complex or -- imaginary numbers. data Matrix a = Matrix [[a]] deriving Show -- For simplicity, the instances below omit the implementation for (*). -- This instance of Multiply is for doing multiplication of the form -- k * m where k is a scalar and m is a matrix. instance (Multiply a b c, Num a, Num b, Num c) => Multiply a (Matrix b) (Matrix c) where (*) x y = Matrix [[]] -- This instance of Multiply is for doing multiplication of the form -- m * k where k is a scalar and m is a matrix. instance (Multiply a b c, Num a, Num b, Num c) => Multiply (Matrix a) b (Matrix c) where (*) x y = Matrix [[]] -- This instance of Multiply is for doing multiplication of the form -- m1 * m2 where m1 and m2 are both matrices instance (Multiply a b c, Add c c c, Num a, Num b, Num c) => Multiply (Matrix a) (Matrix b) (Matrix c) where (*) x y = Matrix [[]] -- Some test variables to use in the interpreter k = (3.0::Double) m1 = Matrix [[1.0::Double]] m2 = Matrix [[2.0::Double]]

3 2

Small syntax question
by Maurício 14 Feb '06

14 Feb '06

Hi, This is valid code: module Main where main = do let a = 3 return () Why isn't this one? module Main where main = do { let a = 3; return (); }; Thanks for your help, Maurício

5 4

Sorting Integers
by JimpsEd 13 Feb '06

13 Feb '06

I have 3 integers, a b c that I want to pass to a function, and I want the function to return the 3 integers sorted, from largest to smallest - any idea how to do this? Thanks; Jamie -- View this message in context: http://www.nabble.com/Sorting-Integers-t1113340.html#a2908719 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

3 4

How to deal with a translator???
by Huong Nguyen 13 Feb '06

13 Feb '06

Hello all, Now I am trying on a simple translator module. This module needs to translate an input string( this string represent a formula) as follows: (x^2 - 5x + 4=0) /\ (x^3 - 5>0) -> x>3 And I want the output string represent a formula like: (x^2 - 5x + 4=0) /\ (x^3 - 5>0) IMPLIES x>3, i.e, the translator attempt to translate (->) into IMPLIES. If you have ay suggesstions for that kind of problems, please share with me. I appreciate for that. Thanks all for your time. Nguyen,

2 1

round function
by Chatzianastassiou Achilleas 13 Feb '06

13 Feb '06

Hi all, I am trying to implement a function that finds the significant figure of a number to a specified point i.e. 2.55555 3 = 2.556. I have implemented something like: sig :: (RealFrac a, Integral b) => a -> Int -> a sig x y = round y However this doesn't work. Moreover at the Prelude when i type round 2.3333 2 i get an error. Any suggestions? Thanks, Skag55 _________________________________________________________________ Dont just search. Find. Check out the new MSN Search! http://search.msn.click-url.com/go/onm00200636ave/direct/01/

3 9

Why is $ right associative instead of left associative?
by Brian Hulley 12 Feb '06

12 Feb '06

Hi - In the Haskell98 report section 4.4.2 $ is specified as being right associative. This means that f $ a0 a1 $ b0 b1 would parse as f (a0 a1 (b0 b1)) which seems rather strange to me. Surely it would be much more useful if $ were defined as left associative so that it could be used to separate the args to f? Does anyone know why this strange associativity was chosen? Thanks, Brian. (The reason I'm asking is that I'm working on the syntax of a language similar to Haskell but which uses layout to allow expressions like: f #$ -- can be followed by an explicit block or layout block a0 a1 b0 b1 which is sugar for (f $ a0 a1) $ b0 b1 ie f (a0 a1) (b0 b1) ) and I was surprised to discover that the parentheses are needed for the most obvious reading)

16 40

How to check for memory leak.
by zbo 12 Feb '06

12 Feb '06

I was using the code from http://www.haskell.org/hawiki/HaskellNewbie_2fTcpIpAndNetworking It seems to be losing a few Kbytes every call (i use 'top' on linux to check the RES field value). I dont know whether this is a memory leak or just the GC not collecting yet. I was just wondering What is the general method to check this. BTW, I have tried using -prof and -P, but not really able to see anything suspicious. Cheers, ed

2 1

building ghc on win with gcc of cygwin, without cygwin1.dll (-mno-cygwin flag) ?
by Marc Weber 12 Feb '06

12 Feb '06

Hi.. I've found the nice documentation about building ghc on haskell.org/ghc -> documentation. There is one chapter (#1) about C compilers and environments to use: Either MSYS or cywin and gcc of MinGW because gcc of cygwin will link to cygwin1.dll by default which may change and therefore can brake your apps. Googling around I've found that there exists the -mno-cygwin flag which you can use to not include this lib.. So would it might be possible to get a ghc build not using cygwin.dll with just cygwin ? I haven't read anything about that in those docs. Or does this flag simply include cygwin.dll within your apps (statically linked) ? #1 :http://www.haskell.org/ghc/docs/latest/html/building/platforms.html#ghc-cygwin Marc

1 0

Digrams
by Dominic Steinitz 11 Feb '06

11 Feb '06

I've quickly put this together to measure frequencies of pairs of letters (e.g. 1st and 2nd) in words. It works fine on a small test data sets but I have a feeling that it will perform poorly as it spends a lot of time updating a 26*26 array. Before I throw a dictionary at it, does anyone have any suggestions? Thanks, Dominic.

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