June 2005 - Haskell-Cafe

Annotating calculations
by Rene de Visser 16 Jun '05

16 Jun '05

Hello, I have a somewhat complicated calculation programmed in Haskell. This calculation is coded without using monads. I want to also produce a report describing the details of this calculation for each particular set of inputs. e.g. Number of hours worked = 100. Gross pay per hour = 50. Total gross = 100 * 50 = 500. etc. But about 20 times more complicated than the above. Naturally I need to write functions to produce the above description/report as it should be well presented. Only showing the important parts of the calculation in a sensible order. But I am wondering how to combine the generation of the report with the calculation together. I think if I add the report generating functions into the calculation functions, it will make them twice as messy, and they are already complicated enough. On the other hand replicating the calculation source code twice, once without reporting and once without seems bad. Any suggestions on how to handle this? Rene.

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Documentation of Data.Map
by Shiqi Cao 16 Jun '05

16 Jun '05

Hi, I just read the API of Data.Map and the source code of it. The documentation of "insertWithKey" is not precise. The following is copied from API. *insertWithKey* :: Ord <http://www.haskell.org/ghc/docs/6.4/html/libraries/base/Prelude.html#t%3AOrd> k => (k -> a -> a -> a) -> k -> a -> Map <http://www.haskell.org/ghc/docs/6.4/html/libraries/base/Data.Map.html#t%3AM…> k a -> Map <http://www.haskell.org/ghc/docs/6.4/html/libraries/base/Data.Map.html#t%3AM…> k a /O(log n)/. Insert with a combining function. This function take a function to solve collision, which can be considered as an operator. However the documentation does not specify the order of arguments to the operator. Because of that we can only supply operator which is communitive. But we all know that there must exist an ordering of arguments. I explored the source code of it as the following. -- | /O(log n)/. Insert with a combining function. insertWithKey :: Ord k => (k -> a -> a -> a) -> k -> a -> Map k a -> Map k a insertWithKey f kx x t = case t of Tip -> singleton kx x Bin sy ky y l r -> case compare kx ky of LT -> balance ky y (insertWithKey f kx x l) r GT -> balance ky y l (insertWithKey f kx x r) EQ -> Bin sy ky (f ky x y) l r By the last line the ordering is clear such that the inserted value goes the first. Shiqi Cao

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Trivial question... solve_qe
by Matej 'Yin' Gagyi 16 Jun '05

16 Jun '05

Hi all! I'm new to Haskell... absolutely new. I need some practice, but little problem seems to me like unresolvable... Is there some tutorial of "Exercise, set of resolutions" pairs type? My english is very bat, so there is it in a picture: Exercise 1: hello world Hint: use 'putStrLn' Resolution 1.1 Resolution 1.2 Exercise 2: write functions 'max_num', 'solve_quad_equation', 'reverse_ary' Hint: (Explanation of some technics) Res... Excercise 3: Use do-notation to reverse word in a line read from stdin Hint: Use Exercise 2... (Explain how 'do' works and what to do in common error sitations) So, in a word: I need a tutorial for dummies Now, my current problem is: my program for solving quadratic equation won't compile... import System solve_qe :: Maybe (Double, Double, Double) -> Maybe (Double, Double) solve_qe Nothing = Nothing solve_qe (a, b, c) | d >= 0 = Just ((-b + (sqrt d)) / (2 * a), (-b - (sqrt d)) / (2 * a)) | otherwise = Nothing where d = b^2 - 4*a*c evalArgs :: [String] -> Maybe (Double, Double, Double) evalArgs (a:b:c:[]) = (read a, read b, read c) evalArgs _ = Nothing main = do args <- getArgs print (solve_qe (evalArgs args))

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(**) over integers
by Maurício 15 Jun '05

15 Jun '05

Hi, I'm trying to do this: let f n = mod (n**5) 12 but I see that I can't because ** only operates on floats. Is there an equivalent operator that works on integers? How should I do that? Thanks, Maurício

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RE: [Haskell-cafe] Garbage collection and finalizer thread priority
by Simon Marlow 14 Jun '05

14 Jun '05

On 13 June 2005 11:30, Gracjan Polak wrote: > My space problems continued... :) > > I have some ForeignPtr's, with finalizers (finalizerFree only). And I > have lazyly produced list of those (yes, there is some unsafePerformIO > magic behind, but I think I got this right). The problem is I get > out-of-memory condition because elements get produced faster than > those consumed are garbage collected. > > Example: > > list = ... > mapM_ writeOut list > > writeOut :: Ptr Word8 > writeOut dat = do > hPutBuh handle dat 1024 > -- Control.Concurrent.threadDelay 1000 > > Uncommenting this line allows gc thread to run finalizers, memory gets > freed, everything runs smoothly... > > As far as I know finalizers are run in separate thread. How do I > increase priority of this thread so it runs faster? I presume you're running GHC. There's no way to increase the priority of a thread - GHC's scheduler doesn't have a concept of priorities. I would look into whether you can use mallocForeignPtr: this is much faster than using newForeignPtr with finalizerFree, because the memory is garbage collected directly and there's no need for a finalizer. Cheers, Simon

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Generic types
by Adde 14 Jun '05

14 Jun '05

I'm writing a small unit library to excercise my barely existing Haskell skills. However I can't figure out how to make Haskell accept any type of a specific class. This is what I've got so far: <code> class Unit u where shortName :: u -> String data Meter = Meter instance Unit Meter where shortName u = "m" data UnitValue = UnitValue {uUnit :: Unit u => u, uValue :: Num n => n} </code> Meter is supposed to be a type in the Unit class. UnitValue is supposed to represent unit values regardless of the Unit type. Meter appears to be a subclass of Unit, so far so good: *Units> shortName Meter "m" But I can't get UnitValue to accept Meter as a Unit: *Units> UnitValue Meter 10 <interactive>:1:10: Couldn't match the rigid variable `u' against `Meter' `u' is bound by the polymorphic type `forall u. (Unit u) => u' at <interactive>:1:0-17 Expected type: u Inferred type: Meter In the first argument of `UnitValue', namely `Meter' In the definition of `it': it = UnitValue Meter 10 I strongly suspect that I've got the declaration of UnitValue wrong but I've spent two hours searching the web and trying to get it to work without any success.

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Notes and questions from a first-time Cabal user.
by Brian Smith 14 Jun '05

14 Jun '05

I succeeded in getting my project to build under Cabal. As I converted my makefile to Cabal, I took some notes about things that I found unintuitive, so that the documentation can be improved for future new users. There are some questions mixed in as well. I am using GHC, Cabal, Happy, Alex, and Haddock, all from the CVS head, all built with GHC 6.2.2, all executing under MSYS 1.0's bash 2.04.0(1) on Windows XP Pro SP2. * If I use "Executable: check-ocl2," then under Windows the executable's name really becomes "check-ocl2.exe." This is desireable behavior. But, this causes "Setup.hs copy" and "Setup.hs install" to fail because it tries to copy a file named "check-ocl2" (without the ".exe"), which obviously doesn't exist. How can I get Cabal to generate "check-ocl2.exe" on Windows and "check-ocl2" on all other platforms, and have the "copy" and "install" commands work, without using autoconf? * What is the easiest way to create a "Setup.hs all" command, analogous to "make all" that simply executes "configure." "build," "haddock," and "test" in sequence, stopping on the first failed step? * The tutorial documentation should explain how to use tools like alex and happy. As far as I could tell, the way to do this is to add "Other-Modules: Module.Name.Of.Parser.y Module.Name.Of.Lexer.x" to each executable/library stanza. But, I am not sure why this is necessary, as it seems like it should be able to find them automatically (i.e. without having to list them in the Other-Modules: field) * Is there some way to tell Cabal to generate all intermediate files into a directory that is different than the source directory? I usually keep all my sources in a "src" directory, and then generate all .hi, .o, and .hs files from Happy and Alex into a "build" directory alongside it. This way, my "src" directory only contains stuff that belongs in my version control repository, and "make clean" was simply "rm -Rf build." * If I run "runghc ./Setup.hs" I get this message: No command given (try --help). This is confusing because it isn't clear that it is a cabal error message and not a runghc error message. That is, it isn't clear whether to run "runghc --help" or "runghc Setup.hs --help," and both commands are valid. The message should be improved, or "--help" should be the default command. * Chapter 2 needs to be updated to replace "Hs-Source-Dir" with "Hs-Source-Dirs," since Hs-Source-Dir is deprecated. * It is unintuitive that the examples in chapter 2 use mixed case (e.g. "Hs-Source-Dir") while the program issues error messages using lowercase only ("hs-source-dir"). * Chapter 2 of the documentation states that identifiers can contain underscores. However, Cabal gives "*** Exception: Line 1: Parse of field 'name' failed" if I use "Name: ocl2_haskell" whereas "Name: ocl2haskell" works fine. * Chapter 2 of the documentation states that identifiers must start with a letter. However, "Name: 2" is parsed without complaint. * Chapter 2 of the documentation states that idenfiers must start with a letter. However, "letter" is not defined. Presumably the intention is to restrict identifiers to English letters only? * I originally put my "Build-depends: base" in an "Executable:" stanza. This seems reasonable because presumably different executables (and libraries) could have different dependencies, and it is advantageous to keep track of dependencies on a fine-grained basis. Yet, Cabal requires the Build-depends to be in the main stanza. Why is this? Couldn't Cabal combine all the Build-Depends it finds into the one that it puts into the final package specification? For example, my unit tests require HUnit, but nothing else does. Should I put my unit tests into a seperate package? * The error message for putting "Build-depends: base" in an "Executable: " stanza is: "*** Exception: Line 10: Unknown field 'build-depends' but I think it would be better to say *** Exception: Line 10: field 'build-depends' does not apply to executables or even better: *** Exception: Line 10: field 'build-depends' applies only to the main stanza (and similarly for other fields, e.g. "field 'main-is" applies only to executables") * Chapter 2 of the documentation states that I should be able to use syntax like "foo > 1.2" in Build-depends, but I could not get "HUnit > 1.1" or "HUnit >= 1.1" to work; they fail with parser errors. Regards, Brian

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RE: [Haskell-cafe] [Newbie] Quest for inheritance
by Ralf Lammel 14 Jun '05

14 Jun '05

> On 6/6/05, Gracjan Polak <gracjan(a)acchsh.com> wrote: > > If you stick to single inheritance there is other way to simulate OO in > > Haskell. Look for "phantom types". Whole wxHaskell (for example) is > > based on this concept. > > I heard about them indeed but barely found clear explanations of it. > Any useful pointer you're aware of maybe ? > > Cédric Hi Cedric, See Section 7.3 of the latest revision of "Haskell's overlooked object system". There are pointers and explanations. This discussion also clarifies that the phantom approach is tailored for foreign library and component import (but it can be generalized and has been indeed, by Burton in a paper from 1990). Regards, Ralf

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foldl and space problems
by Gracjan Polak 14 Jun '05

14 Jun '05

Hello, My space problems continued... I have foldl that produces list, some combining function and quite large source list: let xyz = foldl f state myBigList This setting should lazyli consume myBigList when next elements of xyz are demanded. Except that it seems that myBigList is held by state to the end of computation :( Question: is there any way to see what is holding my source list? I did try to guess, but without results as of now:( How do I debug and/or reason about such situation? -- Gracjan

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Generalized version of `words'?
by Dimitry Golubovsky 13 Jun '05

13 Jun '05

Does there exist a generalized version of the `words' function i. e. one that breaks an arbitrary list into parts by an arbitrary predicate? splitAt is not what I need. I had to write my own: -- A version of words, but works with any lists on any predicate. parts pred s = case dropWhile pred s of [] -> [] s' -> w : parts pred s'' where (w, s'') = break pred s' (just by parameterizing `words' found in Data.List with a predicate passed as a parameter). In case such a function already exists, what is its name? In the opposite case, can such a function be added to the standard library? (or why didn't it exist?) Dimitry Golubovsky Middletown, CT

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