- Haskell-Cafe - Haskell.org

Possible extension to Haskell overloading behavior
by Chris Smith 10 Jul '13

10 Jul '13

So I've been thinking about something, and I'm curious whether anyone (in particular, people involved with GHC) think this is a worthwhile idea. I'd like to implement an extension to GHC to offer a different behavior for literals with polymorphic types. The current behavior is something like: 1. Give the literal a polymorphic type, like (Integral a => a) 2. Type check the whole program, possibly giving the term a more constrained type. 3. If the type is still ambiguous, apply defaulting rules. I'd like to add the option to do this instead. 1. Take the polymorphic type, and immediately apply defaulting rules to get a monomorphic type. 2. Type check the program with the monomorphic type. Mostly, this would reduce the set of valid programs, since the type is chosen before considering whether it meets all the relevant constraints. So what's the purpose? To simplify type errors for programmers who don't understand type classes. What I have in mind is domain-specific dialects of Haskell that replace the Prelude and are aimed at less technical audiences - in my case, children around 10 to 13 years old; but I think the ideas apply elsewhere, too. Type classes are (debatably) the one feature of Haskell that tends to be tricky for non-technical audiences, and yet pops up in very simple programs (and more importantly, their error messages) even when the programmer wasn't aware of it's existence, because of its role in overloaded literals. In some cases, I think it's a good trade to remove overloaded literals, in exchange for simpler error messages. This leaves new programmers learning a very small, simple language, and not staring so much at cryptic error messages. At the same time, it's not really changing the language, except for the need to explicitly use type classes (via conversion functions like fromInteger) rather than get them thrown in implicitly. With GHC's extended defaulting rules that apply for OverloadedStrings, this could also be used to treat all string literals as Text, too, which might make some people happy, too. Of course, the disadvantage is that for numeric types, you would lose the convenience of overloaded operators, since this is only a sensible thing to do if you're replacing the Prelude with one that doesn't use type classes. But in at least my intended use, I prefer to have a single Number type anyway (and a single Text type that's not sometimes called [Char]). In the past, explaining these things has eaten up far too much time that I'd rather have spent on more general skills and creative activities.

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GHC bug? Let with guards loops
by Andreas Abel 10 Jul '13

10 Jul '13

Hi, is this a known bug or feature of GHC (7.4.1, 7.6.3)?: I got a looping behavior in one of my programs and could not explain why. When I rewrote an irrefutable let with guards to use a case instead, the loop disappeared. Cut-down: works = case Just 1 of { Just x | x > 0 -> x } loops = let Just x | x > 0 = Just 1 in x works returns 1, loops loops. If x is unused on the rhs, the non-termination disappears. works' = let Just x | x > 0 = Just 1 in 42 Is this intended by the Haskell semantics or is this a bug? I would have assumed that non-recursive let and single-branch case are interchangeable, but apparently, not... Cheers, Andreas -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel(a)ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/

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Understanding version differences
by Patrick Browne 10 Jul '13

10 Jul '13

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Boulder Haskell Programmers, Next Meeting July 17
by Peter Jones 10 Jul '13

10 Jul '13

The Boulder Haskell Programmer Group is getting together on July 17 at 6:30pm. All experience levels are welcome. Agenda: * 6:30--7:00: Social/beer time. * 7:00--7:30: What do we want from this group? * 7:30--8:30: Introduction to Haskell Types. * 8:30--9:00: Whatever! http://www.meetup.com/Boulder-Haskell-Programmers/events/125427022/ -- Peter Jones, Founder, Devalot.com Defending the honor of good code

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Open Source project suitable for 2-3 persons this fall?
by Anders Bech Mellson 10 Jul '13

10 Jul '13

Is there any project that needs working this fall which could be used as a university project? I am in the university (M.Sc. in software development), so I am mainly looking for project ideas (preferably concrete ones). We are 2-3 students that have ~10 hours pr week for 3 months to work on a project. Is there a listing somewhere with project ideas for contributing to the Haskell community? Thanks in advance, Anders

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ERP/CRM solution with Haskell ?
by Loïc Maury 09 Jul '13

09 Jul '13

Hello all, I don't know if it's the good mailing list to post, if not my apologize for that. I am actually looking for an ERP/CRM solution. Is there a solution (like dolibarr or openerp), but in Haskell to do that ? Thank you Loïc

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Harder Question - Chapter 4 - 4.12 - haskell the craft of functional programming - Second Edition
by Manoel Menezes 09 Jul '13

09 Jul '13

Hi everybody! I am trying to solve the question for a long time: [*4.12 Harder] Find out the maximum number of pieces we can get by making a given* *number of flat (that is planar) cuts through a solid block. It is not the same* *answer as we calculated for straight-line cuts of a flat piece of paper.* I find out that this function has the following results: f 0 = 1 f 1 = 2 f 2 = 4 f 3 = 8 That is, from 0 to 3, the flat cuts all the pieces in two other pieces, so the number of pieces is doubled. But, starting from f 4, the flat can not cuts all the pieces, in case of f 4, the flat can cut 6 out of the 8 pieces, resulting in 12 pieces plus 2 pieces 2 = 14 pieces. But I can not reach a general case. Can anybody help me to find out the solution? Thank you very much! Manoel Menezes. __________________________________ Manoel Messias da Silva Menezes Jr M.Sc.in Computer Science Federal University of Pernambuco System Analyst - Petrobras

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Probabilistic functional programming with Baysig/BayesHive
by Tom Nielsen 09 Jul '13

09 Jul '13

Dear cafe, I would like to announce that the Baysig programming language and the BayesHive analytics environment (http://bayeshive.com) are now available for beta testers. Baysig is a new probabilistic, functional and typed programming language that attempts to realise the vision of "fully Bayesian computing". That is, in Baysig almost all the work in data processing consists of building a probabilistic model of the incoming data. Almost everything else -- optimal decisions, categorisation, measuring hidden parameters or states, forecasting, testing hypothesis -- becomes trivial. This paradigm can in principle be applied to a large number of domains, although for the moment we are focusing on models that are based on continuous parameters. It will therefore be of interest to users of statistics and dynamical systems models, including in finance, physics and life sciences. To analyse data in Baysig, you write a program in the random-number supply monad that generates simulated data. A special construct, "estimate", then applies Bayes' theorem to this program and returns the probability distribution of the model parameters given observed data. The "estimate" procedure is difficult to implement in Haskell or similar languages, which encouraged us to develop a new language. However, in many respects Baysig should feel like Haskell, and we hope that Baysig will encourage the Haskell community to experiment with statistical modelling. We have built a web-based environment to help users, including those with little-to-no programming experience, use Baysig, at bayeshive.com. This web application allows you to upload data from spreadsheets or timeseries, and to build statistical models with a point-and-click web interface which ends up generating Baysig code. Code and the results of running it are collected in shareable and editable literate programming documents. We would appreciate any feedback from the Haskell community before releasing our platform to unsuspecting statisticians and researchers. Almost everything is written in Haskell, including the Baysig compiler. The BayesHive website is written using Yesod, with which we are mostly happy. We also use the Stan package (mc-stan.org) and the web front-end is written using AngularJS. For the moment the Baysig language is only available through the BayesHive web interface, but that will change. If you want to run Baysig on your own computer, please send me an email at tomn(a)openbrain.org. Finally, both the BayesHive web application and the Baysig language implementation are still prototypes and very much work-in-progress. We promise to work hard at fixing the bugs you find! Links: BayesHive, including a few videos: http://bayeshive.com Baysig quick tour ("QuickBAYSIG"): http://bayeshive.com/helppage/Baysig%20quick%20tour:%20fundamentals More documentation: http://bayeshive.com/help Regards, Tom Nielsen OpenBrain Ltd.

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GHC trac
by Marios Titas 09 Jul '13

09 Jul '13

I just noticed that the GHC trac was moved to http://ghc.haskell.org/trac/ghc/ and I cannot login with my old credentials. Were the old accounts transferred to the new trac? Also, the registration page [1] does not work. [1] http://ghc.haskell.org/trac/ghc/register

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Parsec error message not making any sense
by Fredrik Karlsson 09 Jul '13

09 Jul '13

Dear list, I have a Parsec parser that fails and gives the following error message: *Main> parseFromFile textgridfile testFile Left "/Users/frkkan96/Documents/src/ume/umecore/testing/testdata/testdata.TextGrid" (line 35, column 5): unexpected "t" expecting "intervals [" Now, this is perfectly understandable, but line 35, col 5 in the file being parsed looks like the supplies image - there is no 't' there. Any ideas on what is going on? The parser I am using is: data VariableLine = VariableLine String String deriving Show data TierType = IntervalTier | PointTier deriving Show data Tier = Tier String deriving Show data LabelFile = LabelFile Double Double deriving Show data Label = Label String TierType Double Double String deriving Show haskelldef = makeTokenParser haskellDef textgridfile :: Parser (LabelFile, [[Label]]) textgridfile = do h <- header ll <- many1 tier return $ (h,ll) header :: Parser LabelFile header = do string headTS1 start <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) string "xmax = " end <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) string "tiers? <exists> \n" string "size = " integer haskelldef string "item []:" whiteSpace haskelldef return $ LabelFile start end tier :: Parser [Label] tier = do whiteSpace haskelldef string "item [" integer haskelldef string "]:" whiteSpace haskelldef try (string "class = \"IntervalTier\"") <|> string "class = \"TextTier\"" whiteSpace haskelldef string "name = " char '"' name <- many quotedChar char '"' <?> "quote at end of cell" whiteSpace haskelldef string "xmin = " try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) whiteSpace haskelldef string "xmax = " try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) string "intervals: size = " <|> string "points: size = " integer haskelldef whiteSpace haskelldef labelList <- many1 (interval name) return $ labelList interval :: String -> Parser Label interval tierName = do whiteSpace haskelldef string "intervals [" integer haskelldef string "]:" whiteSpace haskelldef string "xmin = " start <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) whiteSpace haskelldef string "xmax = " end <- try (float haskelldef) <|> (fmap fromInteger $ integer haskelldef ) whiteSpace haskelldef string "text = " char '"' text <- many quotedChar char '"' <?> "quote at end of cell" return $ Label tierName IntervalTier start end text which fails on the attached input file. I can't see how 't' is found?? What am I doing wrong? /Fredrik -- "Life is like a trumpet - if you don't put anything into it, you don't get anything out of it."

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