May 2009 - Haskell-Cafe

old Hugs libraries?
by Vasili I. Galchin 19 May '09

19 May '09

Hello, Do newSTArray, readSTArray, writeSTArray, etc. belong to an old deprecated Hugs library/module? If so, what is the Haskell 98 replacement? Thanks, Vasili

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RE: [Haskell-cafe] Request for feedback: HaskellDB + HList
by Brian Bloniarz 18 May '09

18 May '09

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showing a user defined type
by michael rice 18 May '09

18 May '09

I've been playing around with "The Little MLer" (pg. 95, 96) to try to improve my understanding of types. I can ask ML: - ints(0); val it = Link (1,fn) : chain - and Haskell: *Main> :t ints 0 ints 0 :: Chain *Main> ints 0 <interactive>:1:0: No instance for (Show Chain) arising from a use of `print' at <interactive>:1:0-5 Possible fix: add an instance declaration for (Show Chain) In a stmt of a 'do' expression: print it *Main> I think I need to write a show function for type Chain but not sure how to proceed. Michael =============== ;;ML datatype chain = Link of (int * (int -> chain)) fun ints(n) = Link(n + 1, ints) ;;Haskell data Chain = Link Int (Int -> Chain) ints :: Int -> Chain ints n = Link (n+1) ints

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showing a user defined type
by michael rice 18 May '09

18 May '09

I've been playing around with "The Little MLer" (pg. 95, 96) to try to improve my understanding of types. I can ask ML: - ints(0); val it = Link (1,fn) : chain - and Haskell: *Main> :t ints 0 ints 0 :: Chain *Main> ints 0 <interactive>:1:0: No instance for (Show Chain) arising from a use of `print' at <interactive>:1:0-5 Possible fix: add an instance declaration for (Show Chain) In a stmt of a 'do' expression: print it *Main> I think I need to write a show function for type Chain but not sure how to proceed. Michael =============== ;;ML datatype chain = Link of (int * (int -> chain)) fun ints(n) = Link(n + 1, ints) ;;Haskell data Chain = Link Int (Int -> Chain) ints :: Int -> Chain ints n = Link (n+1) ints

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Haskell in 3 Slides
by John Van Enk 18 May '09

18 May '09

Hi all, I'm giving a presentation to an IEEE group on Embedded DSL's and Haskell at the end of June. I need a 3 to 4 slide introduction to Haskell. What suggestions does the community have? Is such a short intro possible? It just needs to introduce the basics so I can show some code without alienating the audience. I'm hoping some one else has attempted this before, but if not, some boiler plate slides could be useful for every one! -- /jve

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ANN: Hieroglyph-2.21 and buster, buster-gtk, and buster-network-2.0
by Jeff Heard 18 May '09

18 May '09

Tons of relevant changes. Almost too many to count... Buster: - Fixed performance and bugs in buster, - split out buster, buster-network, and buster-gtk to make it easier to only build components of the system. - Added new functions in buster for selection and debugging. - Added behaviours in App.Behaviours.PrintEvents for printing only based on Group / Source / Name - Changed name to "ename" (for 'event name') - Added Association Lists to the buster EData type (EAssoc, EAssocL) - future now requires the bus to be passed as the first argument Hieroglyph - OpenGL-backended interactive buster is now functional and high performance if not complete. - Added "Dots" primitive for showing lots of points for things like scatterplots - Got rid of creating Attributes objects in favor of modifying common attributes with combinators; the combinators themselves have the same name as the old attributes, but they are mostly undocumented otherwise because I haven't had time yet. They are found in Graphics.Rendering.Hieroglyph.Visual - Got rid of greater/less detail combinator - Text now takes Text.PrettyPrint.Doc instead of String as the str, accepts Pango markup language, and uses full Layout functionality from Pango, including UTF-8 display. Added Pango markup combinators for Text.PrettyPrint.Docs, mark, bold, bigger, italic, strikethrough, subscript, superscript, smaller, monospace, underline, and span. See Pango documentation for their effect. - Points and Rects are now sensible instances of Num, Fractional, and Ord - Large scenes are now faster overall - Added object naming combinator to support object selection I will have more deails and examples on the blog soon... -- Jeff

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Re: [Haskell-cafe] tips on using monads
by Claus Reinke 18 May '09

18 May '09

>>> I've got one of those algorithms which "threatens to march off the right edge" (in the words of >>> Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. >>> So I presume I need StateT or MaybeT. However, I'm still (sdlowly) learning about monads from >>> first principles. I thought I might present my code and get some pointers... maybe someone could >>> actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in >>> action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything >>> will help. >> >> Perhaps this is useful: >> http://www.haskell.org/haskellwiki/Equational_reasoning_examples#Coding_sty… > I can't quite tell--is that example in the IO monad? Part of my difficulty is that I'm inside IO. > I know how to do this with Maybe, except that I have to combine Maybe and IO (use MaybeT?) It was in the GHC.Conc.STM monad, so yes, it used a MaybeT and Control.Monad.Trans.MonadTrans's lift (btw, the MonadTrans docs only point to [1], but [2] might also be of interest, if rather more compact/terse). Claus [1] http://web.cecs.pdx.edu/~mpj/pubs/springschool.html [2] http://web.cecs.pdx.edu/~mpj/pubs/modinterp.html

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tips on using monads
by Michael P Mossey 18 May '09

18 May '09

I've got one of those algorithms which "threatens to march off the right edge" (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (slowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help. Here's the program: {- This is a program which starts with a document containing "notes" about software requirements (in a particular format) and puts them into a database. Notes include details such as the "source" of the requirement (who gave it), the topic(s) to which it pertains, the date, etc. I have written a parser to take a text document typed up by me during a meeting and parse it into a NoteRecord structure. Here is the structure: -} data NoteRecord = NoteRecord { recordSource :: String, -- Name of person who gave req. recordDate :: [Int], -- Date in [<year>,<month>,<date>] recordSourceType :: String, -- "meeting", "phone", "email", etc. recordBugNum :: Maybe Int, -- Bugzilla # (if relevant) recordTopics :: [String], -- list of official topics pertaining recordText :: String } -- the text of the note itself deriving (Show) {- One other wrinkle. The source (person name) and topic must be one of a set of pre-determined strings. A person has an official full name which is stored in the database. Topics also have official descriptive strings. If I wasn't clever, then the note, as I type it up, must have the exact name and topic. But I hate trying to remember things like that. So I have implemented a "fuzzy string match" system so that I can type part of someone's name (or even misspell it) or part of a topic string, and the system will find the best match to an official string. In pseudocode, the function to insert a note in the database must do this: This function starts with a NoteRecord. - If text already exists in the database, give an error and skip to end. - Fuzzy-match strings to topics and source. - If no potential match can be found to some of topics or source, give error and skip to end. - Ask user to confirm if the matched topics and source look okay. - if user says no, skip to end. - Actually insert the record. -} insertNote :: NoteRecord -> Connection -> IO () insertNote nr conn = do -- Check if it exists in the database already. status <- checkPreExistingText nr conn if status then putStrLn "Skipping... text exists already." else do -- Find best fit for all topics and source. -- See type signatures below. bestFitTopics <- fitTopics nr conn bestFitSource <- fitSource nr conn case any isNothing bestFitTopics of True -> putStrLn "Error... some topic couldn't be matched." False -> case bestFitSource of Nothing -> putStrLn "Error.. source couldn't be matched." _ -> do b <- isUserOkay nr bestFitTopics bestFitSource if b then do -- Create a new NoteRecord with matched -- and validated topics/source. nrValidated = nr { recordTopics = bestFitTopics , recordSource = bestFitSource } insertRow nrValidated conn else putStrLn "Abort due to user request." checkPreExistingText :: NoteRecord -> Connection -> Bool -- There are multiple topics in the NoteRecord. For each one, -- find the best fuzzy match, or indicate if there is no plausible -- match at all. fitTopics :: NoteRecord -> Connection -> [Maybe String] -- There is one source. Try to find fuzzy match. fitSource :: NoteRecord -> Connection -> Maybe String -- Present user with all fuzzy matches and get a yes/no response if it's -- okay to proceed. isUserOkay :: NoteRecord -> [Maybe String] -> Maybe String -> Bool -- Do actual insert into database. insertRow :: NoteRecord -> Connection -> IO ()

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Trying to Express Constraints using a data structure
by Günther Schmidt 18 May '09

18 May '09

Hi all, I'm trying to express a constraint using a data structure. Let's say I'd want to express a "mapping" of a to b, c to b, d to b and e to f. A mapping can also be from a to a, b to b and so on. The constraint is that one cannot map a to b if b was already "mapped" to let's say c. I'd like to express this constraint in a data structure, but haven't figured out how, yet. Any suggestions? Günther

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Removing mtl from the Haskell Platform
by roconnor＠theorem.ca 18 May '09

18 May '09

I wanted to pass this idea around the cafe to get some thoughts before submitting a trac on this topic. I'd like to see the mtl removed from the Haskell Platform. The mtl was a tremendous step forward when it was developed. However, we have learned a few things about monad transformers since the development of the mtl, and it is time that we moved forward. There are at least 3 significant problem with the mtl. 1) `pass' should not be a member functions of the MonadWriter class. It is my understanding that there is no `MonadWriter w m => MonadWriter w (ContT s m)' instance because the `pass' function cannot be implemented. I'm also highly suspicious of some other methods too (I'm looking at you `local'). 2) The `StateT s (Cont r a)' instance of callCC is wrong. The paper on modular monad transformers <http://www.cs.nott.ac.uk/~mjj/pubs/mmt/mmt.pdf> describes why this is wrong. 3) I am told by many people that the order of the state and value pair in `State' is backwards. Actually, I'm not entirely sure what the issue is here, but I trust the people who say this. I think that use of the mtl should be deprecated so that we move on to improved monad transformer libraries. Having the mtl in the Haskell Platform does the opposite by further entrenching its use, possibly to the point where we may not be able to get rid of it for years. If I had to recommend a replace library, I would pick monadLib. However, there are other libraries, such as the mmtl and transformers and it's related packages that I haven't looked at, and may also make fine replacements for the mtl. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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