July 2010 - Haskell-Cafe

[romildo@malaquias.DHCP-GERAL: Re: [Haskell-cafe] Tiger compiler in Haskell: annotating abstract syntax tree]
by José Romildo Malaquias 19 Jul '10

19 Jul '10

Forgot the attachment. Romildo

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possible bug in default module lookup scheme / or invalid haskell?
by Carter Schonwald 19 Jul '10

19 Jul '10

Hello All, I'm not sure if this either a bug in how ghc does path/module lookup or it simply is invalid haskell: consider modules A, A.B and A.B.C where A imports A.B, and A.B imports A.B.C with the following file system layout A.hs A/B.hs A/B/C.hs minimal file examples: module A where import A.B testA = "will it really really work? ------------ module A.B where import A.B.C testB = "will it work ----------------- module A.B.C where testC = "will this work?" ---------- if i run ghci A.hs everything's fine but if in directory B i rune ghci B.hs, i get A/B.hs:2:8: Could not find module `A.B.C': Use -v to see a list of the files searched for. ----------- it seems to me that if the default search path for ghc(i) includes the current directory (which according to docs it does), this shouldn't be happening. (or is there some why this is good Behavior?) thanks! -Carter

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qtHaskell
by Ali Razavi 19 Jul '10

19 Jul '10

Greetings, I have only used the wxHaskell library before, but I am looking into trying one of these more 'advanced' frameworks. To serve my proclivity for QT, I would like to know how its Haskell binding, qtHaskell, compares to that of Gtk. Regards, Ali Razavi

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MonadLib usage
by Emil Melnikov 19 Jul '10

19 Jul '10

Recently I discovered for myself "monadLib" as an alternative for mtl-like packages. It seems that "monadLib" has better interface, but I'm quite confused with it's type classes: they have the monad type variable before the effect type variable (e.g. "ReaderM m i" instead of "ReaderM i m"). AFAIU, this ordering makes impossible to use these type classes in conjunction with GHC's cunning newtype deriving extension. Of course one can easily derive all instances using functionality provided by "MonadLib.Derive" module, but this is a boilerplate. Is there any way to avoid this boilerplate? -- Emil.

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[Newbie] What to improve in my code
by Frank1981 19 Jul '10

19 Jul '10

First of all: I'm not sure if this question is allowed here. If not, I apologize I'm trying to solve the following problem: For each word in a text find the number of occurences for each unique word in the text. i've come up with the following steps to solve this: * remove all punctuation except for whitespace and make the text lowercase * find all unique words in the text * for each unique word, count the number of occurences. This has resulted in the following code: removePunctuation :: [Char] -> [Char] removePunctuation str = filter (\c -> elem c (['a'..'z'] ++ ['A'..'Z'] ++ ['\t', ' ', '\n'])) str process :: [Char] -> [String] process str = words (map toLower (removePunctuation str)) unique :: (Eq a) => [a] -> [a] unique [] = [] unique (x:xs) = [x] ++ unique (filter (\s -> x /= s) xs) occurenceCount :: (Eq a) => a -> [a] -> Int occurenceCount _ [] = 0 occurenceCount x (y:ys) | x == y = 1 + occurenceCount x ys | otherwise = occurenceCount x ys occurenceCount' :: [String] -> [String] -> [(String, Int)] occurenceCount' [] _ = [("", 0)] occurenceCount' (u:us) xs = [(u, occurenceCount u xs)] ++ occurenceCount' us xs Please remember i've only been playing with Haskell for three afternoons now and i'm happy that the above code is working correctly. However i've got three questions: 1) occurenceCount' [] _ = [("", 0)] is plain ugly and also adds a useless tuple to the end result. Is there a better way to solve this? 2) I'm forcing elements into a singleton list on two occasions, both in my unique function and in my occurenceCount' function. Once again this seems ugly and I'm wondering if there is a better solution. 3) The whole process as i'm doing it now feels pretty imperatively (been working for years as a Java / PHP programmer). I've got this feeling that the occurenceCount' function could be implemented using a mapping function. What ways are there to make this more "functional"? -- View this message in context: http://old.nabble.com/-Newbie--What-to-improve-in-my-code-tp29156025p291560… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Running tests in Cabal
by Magnus Therning 19 Jul '10

19 Jul '10

In my Cabal file I have defined a flag that controls whether tests are built or not. Now I'd like to hook it up a bit more so that './Setup.hs test' actually runs the tests. I haven't found a way to access that configuration flag in my hook though. Is it not available in some way? Currently I'm settling for calling 'withExeLBI' and triggering the running based on the exe name. That feels somewhat circuitous though. Is there some better approach? Cheers, M

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Hierarchical modules issue
by Jonathan Geddes 19 Jul '10

19 Jul '10

When I'm developing Haskell code I like to frequently compile the file I'm currently working on so that the compiler will tell me if and where I've made mistakes. In my text editor I have a key mapped to do just that. I have another key mapped to fire up ghci with the current file so I can play around with the functions I've defined. I find this workflow to be very productive. I recently reorganized a project to take advantage of hierarchical modules and it broke my workflow. Now when I'm working in a subdirectory, say Foo where the modules are named Foo.Bar, Foo.Baz, Foo.etc., and I try to compile Bar.hs which imports Foo.Baz, the compiler looks for the file Foo/Baz.hs. The problem is that I'm already in the directory Foo and so the compiler fails to find the file for the module Foo.Baz. If I compile the whole project from the root project directory where the main file is, the compiler finds every module just fine. It seams very strange to me that when the compiler is compiling a module named Foo.Bar and there is an import for Foo.Baz, the compiler doesn't look in the directory where Foo.Bar is located (the current dir)! My temporary soluation has been to create symlinks to help the compiler find what it's looking for. For example in the Foo directory I have a symlink to that same Foo directory so that when compiling Bar.hs, the compiler finds Foo/Baz.hs when it looks for it. However, this seems like a dirty hack and it's not portable besides. Is this a known issue? Is there a better solution than mine? Is there something I'm missing? --Jargv

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Refactoring type-class madness
by Andrew Webb 18 Jul '10

18 Jul '10

Hi there haskellers, I have thoroughly confused myself with type-classes in a haskell system I am writing, and I was wondering if anyone had some useful suggestions to get me out of my mess. I apologise if this is all long and rambling, but that maybe why I can't solve it... The system itself performs analyses on data from human eye-tracking experiments, where the tracking data is simplified from raw positional data into "events" which are either fixations or saccades. This is easy to do: data Event = Fixation {...} | Saccade {...} Because, at the basic level all of the experiments share this type of data, it seems that I should be able to write analysis functions that work for any experiment. However, the experiments differ in the stimuli used, and associated with each stimulus set is a set of "milestones" that give times at which important things happen in the stimuli, and "regions of interest" that give areas of the visual scene that are considered important. For a single experiment I would have: data Experiment = Exp [Trial] data Trial = Trial [Event] (Map MileStone Time) data MileStone = M1 | M2 | ... and the analysis functions can take an experiment, use the events and the milestones, and return whatever. The problem arises when I want to represent a second experiment. I assumed the way to solve this was with a type class: class ExperimentClass a data ExperimentOne = Exp [Trial] instance ExperimentClass ExperimentOne and have the analysis functions take (ExperimentClass a) => a -> ... This, however, necessitates a change to Trial as well, because the MileStones for each experiment are different: class MileStoneClass a data (MileStoneClass a) => Trial a = Trial [Event] (Map a Time) which means that: data (MileStoneClass a) => ExperimentOne a = Exp [Trial a] and maybe even: class (MileStoneClass b) => ExperimentClass a b instance ExperimentClass ExperimentOne ExOneMileStones which all seems fine. But then I need to add regions of interest, and a corresponding type parameter, and a couple of more things with type parameters, and I get something like: class (MileStoneClass b, RegionClass c, ... d, ... e) => ExperimentClass a b c d e which is a lot of type parameters that are all dependent on a, since the experiment defines the milestones etc. I can use fundeps to enforce that in the type system, but it is still quite messy. So, I was wondering whether there was something wrong with my basic model which leads to this ugly type class, or whether this is the proper way forward. Either is fine, really, it would just be nice to know for certain. Thanks in advance, Andrew

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Docs on the current and future constraint solver?
by Corey O'Connor 18 Jul '10

18 Jul '10

I believe I have run headlong into issue #3064 in ghc (http://hackage.haskell.org/trac/ghc/ticket/3064) All I think I know is this: * this is a performance issue with the system used to solve type constraints. * the solver is undergoing an overhaul to resolve performance issues in addition to other issues. * An efficient constraint solver is difficult. NP-Complete in the general case? Beyond that I'm at a loss. What can I read to understand the constraint satisfaction problem as it stands in GHC? Is there a paper on the implementation of the current solver? Anything on how the new solver will differ from the current? I think I located one page on the new solver: http://hackage.haskell.org/trac/ghc/wiki/TypeFunctionsSolving Cheers, Corey O'Connor

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Fwd: [Haskell-cafe] Bytestrings and [Char]
by Alberto G. Corona 18 Jul '10

18 Jul '10

The question can be generalized via type classes: Is there any common set of primitives encapsulated into a single type class that has instances for Strings (Data.List) ByteStrings, Data.Text, Lazy bytestrings, Arrays, vectors and wathever container that can store an boxed, unboxed, packed unpacked sequence of wathever including chars? All of them have folds, heads, tails and a lot of common functions with the same name, but since there is not a single type class, the library programmer can not abstract his code when it is possible, so the library user can chose the particular instance for his particular problem. 2010/3/23 Johann Höchtl <johann.hoechtl(a)gmail.com> On 22.03.2010 14:15, Ivan Miljenovic wrote: > >> On 23 March 2010 00:10, Johan Tibell<johan.tibell(a)gmail.com> wrote: >> >> >>> A sequence of bytes is not the same thing as a sequence of Unicode >>> code points. If you want to replace String by something more efficient >>> have a look at Data.Text. >>> >>> >> Though Data.Text still has the disadvantage of not being as nice to >> deal with as String, since you can't pattern match on it, etc. >> >> Whilst it may degrade performance, treating String as a list of >> characters rather than an array provides you with greater flexibility >> of how to deal with it. >> >> >> > But doesn't this basically mean sacrifice performance / applicability for > algorithmic beauty? Sure, pattern match comes in handy, but in the case of > strings at a high price > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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