March 2014 - Haskell-Cafe

Regular Expression Simplification
by Berg, Matias Juho 24 Mar '14

24 Mar '14

?Hi all, I am a final year undergraduate student at a university and I am doing my final honours project on natural language generation from regular expressions. For this to work efficiently I need to simplify the regular expressions before I translate them. It seems that there is some previous work done on this in Haskell but I have only been able to find this code (http://hackage.haskell.org/package/HaLeX-1.1/docs/src/Language-HaLex-RegExp…) which does some elementary simplification. Does anyone have any suggestions on where to look for more examples so I can see what kinds of attempts people have used to try and solve this problem? Also if someone has worked on this kind of problem was Kleene algebra a good starting point? Best regards, Matias

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[ANN] rainbox - print colorful text boxes - good for tabular data
by Omari Norman 23 Mar '14

23 Mar '14

Rainbox helps you create (possibly) colorful text boxes for printing to a UNIX terminal. It uses the rainbow [1] library to permit use of 8 or 256 colors. Rainbox is great for formatting tabular data. One module, Rainbox, permits you to make simple grids--like spreadsheets that do not allow you to split or merge cells. Another module, Rainbox.Box, lets you create boxes of arbitrary complexity. Rainbox is similar to the boxes library [2] but unlike Boxes it's easy to use colors with Rainbox. The Haddocks for both modules will get you started; there is also a tutorial for the simple grid model in the Rainbox.Tutorial module. That file is written in literate Haskell, which HsColour does not fare so well with, so it makes more sense in your text editor or on Github: https://github.com/massysett/rainbox/blob/master/lib/Rainbox/Tutorial.lhs Rainbox is here: http://hackage.haskell.org/package/rainbox --Omari [1] http://hackage.haskell.org/package/rainbow [2] http://hackage.haskell.org/package/boxes

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[ANN] persistent-odbc
by grant weyburne 23 Mar '14

23 Mar '14

Hi, This package contains Persistent-compatible database backends using ODBC. It currently supports the following databases: MSSQL, MySql, Oracle, Sqlite, DB2 and Postgres. https://github.com/gbwey/persistent-odbc http://hackage.haskell.org/package/persistent-odbc-0.1.2.0/candidate Let me know if you have any problems or suggestions to improve it. Thanks, Grant

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No instance for (Eq v) trouble
by martin 22 Mar '14

22 Mar '14

Hello all, how can I make a type an instance of the Show class, if the functions used to implement show require e.g. Eq ? -------------------------------------------- I have a class: class Behavior b where at :: b a -> Day -> a and an instance data Change v = Change v [(Day,v)] deriving(Show) instance Behavior Change where ... and another instance (a Behavior of Behavior) data WeekdayPattern v = WeekdayPattern (Change v) instance Behavior WeekdayPattern where at (WeekdayPattern change) day = at change (day `mod` 7) And I wanted to write my own show function for WeekdayPattern. I started off like this: instance Show (WeekdayPattern v) where show wdp = show $ groupBy sameValue expanded where expanded = zip (map (at wdp) [0..6]) [0..6] sameValue x y = (fst x) == (fst y) ------------------------------------------ But I get: No instance for (Eq v) arising from a use of `sameValue' I believe the compiler wants to tell me, that it has no reason to assume that the values (the "v"s) can be compared for equality. Indeed the class itself and its "at" function do not require this (but it wouldn't hurt to add a constraint) I tried inserting an (Eq v) => in various places of the class or data declaration, but the compiler either didn't like that or it did not solve the problem. There is no problem just writing a show-like method as in xshow :: (Eq v, Show v) => (WeekdayPattern v) -> String but I cannot make this the "show" function of the Show class.

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Listing native package requirements based on cabal information
by David Thomas 22 Mar '14

22 Mar '14

Is there a way to extract this? I'm looking to make it easier for newcomers to my project to get things building across different linux distros.

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Re: [Haskell-cafe] Regular Expression Simplification
by Doug McIlroy 21 Mar '14

21 Mar '14

Apropos of regular expression simplification, > My first thought was: Maybe it would be possible to convert the regular expression to something like an NFA, > I'm slightly worried that the overhead of converting to an NFA is far too great for complicated regular expressions "Far too great" is only quadratic in the number of nonterminal symbols (with repetitions counted) in the regular expression. And the result is a particularly nice NFA, with one state for each of those symbols, and no epsilon moves. So I think there's merit in the proposal. See my Functional Pearl, JFP 14 (2004) 503-518, http://www.cs.dartmouth.edu/~doug/pubs.html Doug McIlroy ok(a)cs.otago.ac.nz

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SQL-like DSL for haskell List of Records
by martin 21 Mar '14

21 Mar '14

Hello all, I am currently prototyping some DB-related ideas in haskell. To make the next step I would have to write some joins. I am happy to work with just Lists of Records. Is there any DSL which relieves me from writing nested loops and all that caboodle? Doesn't have to provide SQL syntax either, just some expressiveness which comes close to basic SQL. Data volume will be miniscule.

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Re: [Haskell-cafe] Regular Expression Simplification
by Doug McIlroy 21 Mar '14

21 Mar '14

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How to obtain an expression type in a GHC compiler plugin?
by J. Stutterheim 21 Mar '14

21 Mar '14

Hi all, I'm trying to familiarise myself with writing GHC compiler plugins, but the GHC API is rather overwhelming, hence my question here. During compilation, I'm trying to determine whether a top-level function in my program has a certain type. Initially, if this is the case, I just want to print the function name. For me to do this check, I have to obtain the type for each top-level function. How can I achieve this? I already have a plugin with which I can print the name for a Bind, so I'm mainly interested in knowing how to obtain its type. Cheers, Jurriën

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PhD position at IT University of Copenhagen
by Iago Abal 21 Mar '14

21 Mar '14

The VARIETE project seeks an excellent PhD student to work on analysis methods for code and models found in highly configurable software systems (software product lines), especially in projects that use simple code generators and domain specific models. The objectives are to work on extensions of model checking and static analysis techniques for the verification of software systems implemented using model transformations. The ideal candidate has a solid background in semantics of programming languages and in algorithmic verification techniques (model checking, type checking, static analysis, satisfiability solving), combined with an appreciation for problem solving stemming from practice of software development. The project develops theories as well as tools. Functional programming skills are an advantage (but not essential). VARIETE is a highly prestigious research project awarded by the Danish Independent Research Council, within the Sapere Aude program. Project website: https://variete.wikit.itu.dk/ Positions are based in Copenhagen Denmark within the modern and lively research environment of IT University of Copenhagen (http://www.itu.dk <http://www.itu.dk/>). The position comes with entry level public servant salary (ca. 2200 EUR per month after taxes are deducted), social benefits (like paid vacation time, health care and pension saving scheme) along with a travel budget and all other support funds required for executing the research project. Prospective starting date is in fall 2014. Duration of the scholarship is 3 or 4 years. Personal contact via email prior to applying is strongly encouraged. Contact: Associate Professor Andrzej Wąsowski (wasowski(a)itu.dk) Postdoc Aleksandar Dimovski (adim(a)itu.dk) Research Group: Process and System Models < https://pure.itu.dk/portal/en/organisations/process-and-system-models%28780… >; Software and Systems Section <http://itu.dk/en/Om-IT-Universitetet/Organisation_tal_og_fakta> Deadline for application is April 23 at 23:59 CET. Please see the conditions of the call at https://delta.hr-manager.net/ApplicationInit.aspx?cid=119&ProjectId=168671&… Iago Abal Rivas

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