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  • Haskell: Constrain function on type Double to only work with Integers

    - by thurn
    Suppose I'm writing a function that takes a list of integers and returns only those integers in the list that are less than 5.2. I might do something like this: belowThreshold = filter (< 5.2) Easy enough, right? But now I want to constrain this function to only work with input lists of type [Int] for design reasons of my own. This seems like a reasonable request. Alas, no. A declaration that constraints the types as so: belowThreshold :: [Integer] -> [Integer] belowThreshold = filter (< 5.2) Causes a type error. So what's the story here? Why does doing filter (< 5.2) seem to convert my input list into Doubles? How can I make a version of this function that only accepts integer lists and only returns integer lists? Why does the type system hate me?

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  • Haskell: Problems with overloading: Interpreter can´t tell which + to use

    - by Ben
    Hi, I want to make functions Double - Double an instance of the Num typeclass. I want to define the sum of two functions as sum of their images. So I wrote instance Num Function where f + g = (\ x - (f x) + (g x)) Here the compiler complains he can´t tell whether I´m using Prelude.+ or Module.+ in the lambda expression. So I imported Prelude qualified as P and wrote instance Num Function where f + g = (\ x - (f x) P.+ (g x)) This compiles just fine, but when I try to add two functions in GHCi the interpreter complains again he can´t tell whether I´m using Prelude.+ or Module.+. Is there any way I can solve this problem?

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  • Haskell Add Function Return to List Until Certain Length

    - by kienjakenobi
    I want to write a function which takes a list and constructs a subset of that list of a certain length based on the output of a function. If I were simply interested in the first 50 elements of the sorted list xs, then I would use fst (splitAt 50 (sort xs)). However, the problem is that elements in my list rely on other elements in the same list. If I choose element p, then I MUST also choose elements q and r, even if they are not in the first 50 elements of my list. I am using a function finderFunc which takes an element a from the list xs and returns a list with the element a and all of its required elements. finderFunc works fine. Now, the challenge is to write a function which builds a list whose total length is 50 based on multiple outputs of finderFunc. Here is my attempt at this: finish :: [a] -> [a] -> [a] --This is the base case, which adds nothing to the final list finish [] fs = [] --The function is recursive, so the fs variable is necessary so that finish -- can forward the incomplete list to itself. finish ps fs -- If the final list fs is too small, add elements to it | length fs < 50 && length (fs ++ newrs) <= 50 = fs ++ finish newps newrs -- If the length is met, then add nothing to the list and quit | length fs >= 50 = finish [] fs -- These guard statements are currently lacking, not the main problem | otherwise = finish [] fs where --Sort the candidate list sortedps = sort ps --(finderFunc a) returns a list of type [a] containing a and all the -- elements which are required to go with it. This is the interesting -- bit. rs is also a subset of the candidate list ps. rs = finderFunc (head sortedps) --Remove those elements which are already in the final list, because -- there can be overlap newrs = filter (`notElem` fs) rs --Remove the elements we will add to the list from the new list -- of candidates newps = filter (`notElem` rs) ps I realize that the above if statements will, in some cases, not give me a list of exactly 50 elements. This is not the main problem, right now. The problem is that my function finish does not work at all as I would expect it to. Not only does it produce duplicate elements in the output list, but it sometimes goes far above the total number of elements I want to have in the list. The way this is written, I usually call it with an empty list, such as: finish xs [], so that the list it builds on starts as an empty list.

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  • Linking/Combining Type Classes in Haskell

    - by thegravian
    Say I have two type classes defined as follows that are identical in function but different in names: class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a class PhantomMonad p where pbind :: p a -> (a -> p b) -> p b preturn :: a -> p b Is there a way to tie these two classes together so something that is an instance of PhantomMonad will automatically be an instance of Monad, or will instances for each class have to be explicitly written? Any insight would be most appreciated, thanks!

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  • Point-free in Haskell

    - by wwrob
    I have this code that I want to make point-free; (\k t -> chr $ a + flip mod 26 (ord k + ord t -2*a)) How do I do that? Also are there some general rules for point free style other than "think about this amd come up with something"?

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  • Haskell search an element on a List

    - by user1887556
    I want a function that changes (1 to 0) on a list, when number of 1's isn't even. I have done these functions: 1) Sees if the lines in a list are even or not: parityLine :: [[Int]] -> [Bool] parityLine [] =[] parityLine (x:xs) |sum(x) `mod` 2 == 0 = True:(parityLine(xs)) |otherwise = False:(parityLine(xs)) 2) Sum the corresponding elements on a list of lists: sumPositions :: [[Int]] -> [Int] sumPositions [] = [] sumPositions (x:xs) = foldl (zipWith (+)) (repeat 0) (x:xs) 3) Sees if the columns in a list are even or not: parityColumn :: [Int] -> [Bool] parityColumn [] = [] parityColumn (x:xs) |head(x:xs) `mod` 2 == 0 = True:parityColumn(xs) |otherwise = False:parityColumn(xs) 4) Does the operation or with two boolean lists: bol :: [Bool] -> [Bool] -> [[Bool]] bol [] _ = [] bol (x:xs) (y:ys)= (map (||x) (y:ys)):(bol xs (y:ys)) 5) Correct List: correct :: [[Int]] -> [[Bool]] correct [] = [] correct (x:xs)=(bol(parityLine (x:xs))(parityColumn(sumPositions(x:xs)))) So what I want is to alter the function correct to [[Int]]-[[Int]] that does this: My Int list(x:xs) With my correct function applied [[0,0,1,1], [[True,True,False,True], [1,0,1,1], [True,True,True,True], [0,1,0,1], [True,True,True,True] [1,1,1,1]] [True,True,True,True]] Now I can see that in the first line third column, False, so I have to correct that number 1 to have a number of 1's even. Result I want that function correct does: [[0,0,0,1], [1,0,1,1], [0,1,0,1], [1,1,1,1]] Thanks.

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  • best way to write a-> ..->[a] recursive functions in haskell

    - by Roman A. Taycher
    So I keep having this small problem where I have something like func :: a -> b -> [a] -- or basically any a-> ...-> [a] where ... is any types -> func x y = func' [x] y -- as long as they are used to generate a list of [a] from x func' :: [a] -> b -> [a] func = undefined --situation dependant generates a list from each element and returns it as one long list should I keep it like this? should I use func' hidden by a where? should I only use the [a] - b - [a] version and leave the responsibility of passing [variable] to the callee? I might well need to compose these functions and might want to mess around with the order so I'm leaning towards option 3. What do you think?

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  • Haskell lists difference

    - by user559354
    I'm trying make a lists difference. Found directly prelude operator \\\\ that makes lists difference. But errors Not in scope: '\\\\' occurs. Here is my simple from command line interpreter: Prelude> ([1,2,3] ++ [5,6]) -- works like expected [1,2,3,4,5,6] prelude> ([1,2,3] \\\\ [1,2]) -- erros occurs <interactive>:1:11: Not in scope: "\\\\" Thanks for explanation where I make a mistake.

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  • find if list 1 is a sequence of list 2 in haskell

    - by Isaak Wahb
    im trying to check if a given list is a subsequence of another list: here are example of lists which gives true: subseq "" "w" subseq "w" "w" subseq "ab" "cab" subseq "cb" "cab" subseq "aa" "xaxa" not (subseq "aa" "xax") not (subseq "ab" "ba") i just come to this but in some cases it gives a wrong result subseq :: Eq a => [a] -> [a] -> Bool subseq [] [] = True subseq [] ys = True subseq xs [] = False subseq (x:xs) (y:ys) = x == y || subseq xs ( 1 `drop` ys )

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  • haskell. words into binary numbers

    - by Katja
    I need to convert words into binary numbers. With a bit help of yours I got this blCo::String -> Integer blCo x = num2bin(lett2num x) blCo (x:xs) | x:xs = num2bin(lett2num x):blCo xs num2lett :: Int -> Char num2lett n | (n <= ord 'A') && (n <= ord 'Z') = chr(ord 'A'+ n - 1) | (n <= ord 'a') && (n <= ord 'Z') = chr(ord 'A'+ n - 1) num2bin :: Integer -> String num2bin n | n >= 0 = concatMap show (reverse ( n2b n)) | otherwise = error "num2bin: negative number" where n2b 0 = [] n2b n = n `mod` 2 : n2b (n `div` 2) he tells me a mistake.I dont undertsand it mistake: Couldn't match expected type `Char' against inferred type `String' In the first argument of `lett2num', namely `x' In the first argument of `num2bin', namely `(lett2num x)' In the expression: num2bin (lett2num x)

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  • Haskell Input & Passing Values to Functions

    - by Pradeep
    putStrLn "Enter the Artist Name" art <- getLine putStrLn "Enter the Number of CD's" num <- getLine let test= buyItem currentStockBase art num printListIO (showcurrentList test) the values i have to pass for buyItem is buyItem currentStockBase "Akon" 20 but i want to send "Akon" to art and for 20 i want to send num it gives me this error ERROR file:.\Project2.hs:126 - Type error in application *** Expression : buyItem currentStockBase art num *** Term : num *** Type : [Char] *** Does not match : Int please help me

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  • How do you solve this Haskell problem?

    - by Linda Cohen
    I want to define a function replicate to replicate a list of numbers by its value using only list comprehension, for example: replicate [5,1,3,2,8,1,2] output: [5,5,5,5,5,1,3,3,3,2,2,8,8,8,8,8,8,8,8,1,2,2] I know this would be easy to use the 'replicate' built in function but only list comprehension is allow, how can I do this? THANKS!

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  • Haskell type signature with multiple type somethings (predicates?, for example Eq a =>)

    - by Andrew
    I'm not sure if type predicates is the right term, in fact I've never learned the word for this, so an edit to correct would be helpful - I'm referring to when you give the tipe of function f :: a -> b and you want to say a is a Eq and you say f :: Eq a => a -> b, the name for Eq a => - this is the thing i called a type predicate. My question, though, is how to have multiple of these, so if A is an Eq and B is a Num, I could say either f :: Eq a => a -> b or f :: Num b => a -> b. So, how can I have Eq a => and Num b => at the same time? f :: Eq a => Num b => a -> b, f :: Eq a -> Num b => a -> b, and f :: Eq a, Num b => a -> b all didn't do what I wanted.

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  • Simple haskell splitlist

    - by js7354
    I have the following function which takes a list and returns two sublists split at a given element n. However, I only need to split it in half, with odd length lists having a larger first sublist splitlist :: [a] -> Int -> ([a],[a]) splitlist [] = ([],[]) splitlist l@(x : xs) n | n > 0 = (x : ys, zs) | otherwise = (l, []) where (ys,zs) = splitlist xs (n - 1) I know I need to change the signature to [a] - ([a],[a]), but where in the code should I put something like length(xs) so that I don't break recursion? Thank you.

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  • Haskell: list of elements with class restriction

    - by user1760586
    here's my question: this works perfectly: type Asdf = [Integer] type ListOfAsdf = [Asdf] Now I want to do the same but with the Integral class restriction: type Asdf2 a = (Integral a) => [a] type ListOfAsdf2 = (Integral a) => [Asdf2 a] I got this error: Illegal polymorphic or qualified type: Asdf2 a Perhaps you intended to use -XImpredicativeTypes In the type synonym declaration for `ListOfAsdf2' I have tried a lot of things but I am still not able to create a type with a class restriction as described above. Thanks in advance!!! =) Dak

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  • Nested/Sub data types in haskell

    - by Tom Carstens
    So what would be nice is if you could do something like the following (not necessarily with this format, just the general idea): data Minor = MinorA | MinorB data Major = Minor | MajorB isMinor :: Major -> Bool isMinor Minor = True isMinor _ = False So isMinor MinorA would report True (instead of an error.) At the moment you might do something like: data Major = MinorA | MinorB | MajorB isMinor :: Major -> Bool isMinor MinorA = True isMinor MinorB = True isMinor _ = False It's not terrible or anything, but it doesn't expand nicely (as in if Minor when up to MinorZ this would be terribly clunky). To avoid that problem you can wrap Minor: data Minor = MinorA | MinorB data Major = MajorA Minor | MajorB isMinor :: Major -> Bool isMinor (MajorA _) = True isMinor _ = False But now you have to make sure to wrap your Minors to use them as a Major... again not terrible; just doesn't really express the semantics I'd like very well (i.e. Major can be any Minor or MajorB). The first (legal) example is "Major can be MinorA..." but doesn't have any knowledge of Minor and the second is "Major can be MajorA that takes a Minor..." p.s. No, this isn't really about anything concrete.

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  • tfs 2010 RC Agile Process template update New Task progress report

    Maybe my next post will just be about why I am so excited and impressed with the out of the box templates.  But, for this first blog with my new focus, I thought I would just walk through the process I went through to create a task progress report (to enhance the out of the box Agile template). So, I started with the MSF for Agile Development 5.0 RC template.  After reviewing the template, I came away pretty excited about many of the new reports.  I am especially excited about the reporting services reports.  The big advantage I see here is that these are querying the Warehouse directly instead of the Analysis Services Cube which means that they are much closer to real-time which I find very important for reports like Burndown and task status.  One report that I focused on right away was the User Story Progress Report.  An overview is shown below: This report is very useful, but a lot of our internal managers really prefer to manage at the task level and either dont have stories in TFS or would like to view this type of report for tasks in addition to the User Stories.  So, what did I do? Step 1: Download the Agile Template In VS 2010 RC, open Process Template Manager from Team->Team Project Collection Settings.  Download the MSF for Agile Development template to your local file system.  A project template is a folder of xml files.  There is a ProcessTemplate.xml in the root and then a bunch of directories for things like Work Item Definitions and Queries, Reports, Shared Documents and Source Control Settings.  Step 2: Copy the folder My plan here is to make a new template with all of my modifications.  You can also just enhance update the MSF template.  However, I think it is cleaner when you start making modifications to make your own template.  So, copy the folder and name it with your new template name. Step 3: Change Template Name Open ProcessTemplate.xml and change the <name> of the template. Step 4: Copy the rdl of the Report you want to use a starting point In my case, I copied Stories Progress.rdl and named the file Task Progress Breakdown.rdl.  I reviewed the requirements for the new report with some of the users here and came up with this plan.  Should show tasks and be expandable to show subtasks.  Should add Assigned To and Estimated Finish Date as 2 extra columns. Step 5: Walkthrough the existing report to understand how it works The main thing that I do here is try to get the sql to run in SQL Management Studio.  So, I can walkthrough the process of building up the data for the report. After analyzing this particular report I found a couple of very useful things.  One, this report is already built to display subtasks if I just flip the IncludeTasks flag to 1.  So, if you are using Stories and have tasks assigned to each story.  This might give you everything you want.  For my purposes, I did make that change to the Stories Progress report as I find it to be a more useful report to be able to see the tasks that comprise each story.  But, I still wanted a task only version with the additional fields. Step 6: Update the report definition I tend to work on rdl in visual studio directly as xml.  Especially when I am just altering an existing report, I find it easier than trying to deal with the BI Studio designer.  For my report I made the following changes. Updated Fields Removed Stack Rank and Replaced with Priority since we dont use Stack Rank Added FinishDate and AssignedTo Changed the root deliverable SQL to pull @tasks instead of @deliverablecategory and added a join CurrentWorkItemView for FinishDate and Assigned to SELECT cwi.[System_Id] AS ID FROM [CurrentWorkItemView] cwi             WHERE cwi.[System_WorkItemType] IN (@Task)             AND cwi.[ProjectNodeGUID] = @ProjectGuid SELECT lh.SourceWorkItemID AS ID FROM FactWorkItemLinkHistory lh             INNER JOIN [CurrentWorkItemView] cwi ON lh.TargetWorkItemID = cwi.[System_Id]             WHERE lh.WorkItemLinkTypeSK = @ParentWorkItemLinkTypeSK                 AND lh.RemovedDate = CONVERT(DATETIME, '9999', 126)                 AND lh.TeamProjectCollectionSK = @TeamProjectCollectionSK                 AND cwi.[System_WorkItemType] NOT IN (@DeliverableCategory) Added AssignedTo and FinishDate columns to the @Rollups table Added two columns to the table used for column headers <Tablix Name="ProgressTable">         <TablixBody>           <TablixColumns>             <TablixColumn>               <Width>2.7625in</Width>             </TablixColumn>             <TablixColumn>               <Width>0.5125in</Width>             </TablixColumn>             <TablixColumn>               <Width>3.4625in</Width>             </TablixColumn>             <TablixColumn>               <Width>0.7625in</Width>             </TablixColumn>             <TablixColumn>               <Width>1.25in</Width>             </TablixColumn>             <TablixColumn>               <Width>1.25in</Width>             </TablixColumn>           </TablixColumns> Added Cells for the two new headers Added Cells to the data table to include the two new values (Assigned to & Finish Date) Changed a bunch of widths that would change the format of the report to display landscape and have room for the two additional columns Set the Value of the IncludeTasks Parameter to 1 <ReportParameter Name="IncludeTasks">       <DataType>Integer</DataType>       <DefaultValue>         <Values>           <Value>=1</Value>         </Values>       </DefaultValue>       <Prompt>IncludeTasks</Prompt>       <Hidden>true</Hidden>     </ReportParameter> Change a few descriptions on how the report should be used This is the resulting report I have attached the final rdl. Step 7: Update ReportTasks.xml Last step before the template is ready for use is to update the reportTasks.xml file in the reports folder.  This file defines the reports that are available in the template.           <report name="Task Progress Breakdown" filename="Reports\Task Progress Breakdown.rdl" folder="Project Management" cacheExpiration="30">             <parameters>               <parameter name="ExplicitProject" value="" />             </parameters>             <datasources>               <reference name="/Tfs2010ReportDS" dsname="TfsReportDS" />             </datasources>           </report> Step 8: Upload the template Open the process Template Manager just like Step 1.  And upload the new template. Thats it.  One other note, if you want to add this report to existing team project you will have to go into reportmanager (the reporting services portal) and upload the rdl to that projects directory.Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • Expression Engine Template Exporting

    - by Lark
    I have a project that was developed into a Expression Engine template. I used the Core version of EE to develop this template on my local machine. Now I need to export this template/weblog so it can be loaded onto the production server so the client can begin using EE to update their site. The issue I am running into is how to export the templates plus the weblogs that go along with the template. This is my first time using EE so if you know how to accomplish this please explain it in detail. Thank you in advance.

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  • C++: Declaration of template class member specialization (+ Doxygen bonus question!)

    - by Ziv
    When I specialize a (static) member function/constant in a template class, I'm confused as to where the declaration is meant to go. Here's an example of what I what to do - yoinked directly from IBM's reference on template specialization: template<class T> class X { public: static T v; static void f(T); }; template<class T> T X<T>::v = 0; template<class T> void X<T>::f(T arg) { v = arg; } template<> char* X<char*>::v = "Hello"; template<> void X<float>::f(float arg) { v = arg * 2; } int main() { X<char*> a, b; X<float> c; c.f(10); // X<float>::v now set to 20 } The question is, how do I divide this into header/cpp files? The generic implementation is obviously in the header, but what about the specialization? It can't go in the header file, because it's concrete, leading to multiple definition. But if it goes into the .cpp file, is code which calls X::f() aware of the specialization, or might it rely on the generic X::f()? So far I've got the specialization in the .cpp only, with no declaration in the header. I'm not having trouble compiling or even running my code (on gcc, don't remember the version at the moment), and it behaves as expected - recognizing the specialization. But A) I'm not sure this is correct, and I'd like to know what is, and B) my Doxygen documentation comes out wonky and very misleading (more on that in a moment). What seems most natural to me would be something like this, declaring the specialization in the header and defining it in the .cpp: ===XClass.hpp=== #ifndef XCLASS_HPP #define XCLASS_HPP template<class T> class X { public: static T v; static void f(T); }; template<class T> T X<T>::v = 0; template<class T> void X<T>::f(T arg) { v = arg; } /* declaration of specialized functions */ template<> char* X<char*>::v; template<> void X<float>::f(float arg); #endif ===XClass.cpp=== #include <XClass.hpp> /* concrete implementation of specialized functions */ template<> char* X<char*>::v = "Hello"; template<> void X<float>::f(float arg) { v = arg * 2; } ...but I have no idea if this is correct. The most immediate consequence of this issue, as I mentioned, is my Doxygen documentation, which doesn't seem to warm to the idea of member specialization, at least the way I'm defining it at the moment. It will always present only the first definition it finds of a function/constant, and I really need to be able to present the specializations as well. If I go so far as to re-declare the entire class, i.e. in the header: /* template declaration */ template<class T> class X { public: static T v; static void f(T); }; /* template member definition */ template<class T> T X<T>::v = 0; template<class T> void X<T>::f(T arg) { v = arg; } /* declaration of specialized CLASS (with definitions in .cpp) */ template<> class X<float> { public: static float v; static void f(float); }; then it will display the different variations of X as different classes (which is fine by me), but I don't know how to get the same effect when specializing only a few select members of the class. I don't know if this is a mistake of mine, or a limitation of Doxygen - any ideas? Thanks much, Ziv

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