- Haskell-Cafe - Haskell.org

Problem with forall type in type declaration
by Magicloud Magiclouds 11 May '12

11 May '12

Hi, Assuming this: run :: Monad IO a -> IO a data Test = Test { f } Here I'd like to set f to run, like "Test run". Then what is the type of f? The confusing (me) part is that, the argument pass to f is not fixed on return type, like "f1 :: Monad IO ()", "f2 :: Monad IO Int". So "data Test a = Test { f :: Monad IO a -> IO a} does not work. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

4 6

Oxford Haskell Users Group
by Nicolas Wu 10 May '12

10 May '12

Dear all, I'd like to announce that the Oxford Haskell User's Group will be meeting up next week at 7pm on Tuesday 15th May. As is our custom, we'll be catching up over a few drinks in a local pub, this time, it's at the Three Goats Heads, St Michael's Street. Discussion is informal, with the occasional presentation about some interesting aspect of Haskell. All are welcome! If you'd like to be kept up to date about future meetings, which are roughly once a week during term time, then do join our mailing list, which can be found under OxHUG at google groups [1]. All the best, Nick [1]: http://groups.google.com/group/oxhug

1 0

ANNOUNCE: haskell-src-exts 1.13.3
by Niklas Broberg 10 May '12

10 May '12

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.13.3! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts This release attempts to fix a bunch of long-standing (and a few new) bugs, while still avoiding a new major version bump. Changelog: 1.13.2 --> 1.13.3 =============== * Fundep premises are now allowed to be empty. * Fix the bug where the lexer would crash on a LINE pragma that did not include a line number. * Fix the bug where the lexer would require the # of a MagicHash-style type constructor to be succeeded by at least one character in the file. * Fix really long-standing bug where the parser would crash with an ugly "Internal error" error message if encountering an extra }. * Report errors at the right place for function arity mismatches. Earlier they were reported at end of file, now they are reported where the function is declared. * Lexer now properly fails on line-breaks in string literals. * Lexer now handles character escapes up to 0x10FFFF (unicode). Cheers, /Niklas

1 0

Gentle reminder: Summer school on Applied Functional Programming at Utrecht University; deadline for registration May 20
by S D Swierstra 10 May '12

10 May '12

Again we will teach an "Applied Functional Programming Summer in Haskell" school this year at Utrecht University. In the previous two occasions students were all very happy with the school and we plan to repeat this success this year. The intended audience are prospective master students who have been in contact with Functional Programming, e.g. by taking a general course on programming languages, and want to learn more about Haskell and its typical programming patterns. In the previous two years we have taught an introductory part (advanced bachelor level), an advanced part (beginning master level) and a shared part for both groups. Topics covered are, besides some examples of domain specific languages, also monads, monad transformers, arrows, parser combinators and self-analysing programs, underlying principles, type inferencing, etc. Half of the course time is spent on a larger programming exercise; you can also come with a problem of your own if you want, and get help from the Utrecht University Software Technology group in finding the proper Haskell idioms, tools and libraries, for solving it. Important links: -- our own page where we supply information based on questions asked http://www.cs.uu.nl/wiki/bin/view/USCS2012/WebHome -- the poster you can print and hang somewhere (why not your office door): http://www.cs.uu.nl/wiki/pub/USCS2012/WebHome/USCSpos12.pdf -- the official summerschool site where you can register: http://www.utrechtsummerschool.nl/index.php?type=courses&code=H9 Furthermore we ask for your cooperation to bring this announcement under the attention of potential participants. Best, Doaitse Swierstra PS: apologies if you get this mail more than once _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Template Haskell: Generate annotated function of a typeclass
by Ismael Figueroa Palet 10 May '12

10 May '12

Hi all, I think this is the right place for the following questions and I thank beforehand for your answers :-) I'm experimenting with typeclasses and TH, and I want to define a 'macro' that works more or less like this: Given the name of a typeclass and a function, return the expressions corresponding to the type-annotated instances, for instance $(foo Show show) should translate to: ["(show :: Int -> String)", "(show :: Bool -> String)", ....] for all instances currently in scope. I'm currently playing with the isInstance function (I'm running GHC 7.4.1) and can get a list of instances, and check if a given type is part of a typeclass or not. But I don't know how to create the expression corresponding to "instantiated function", as above. Thanks! -- Ismael

2 3

anyone else driven mad by trying to setup a gmp free version of haskell platform?
by Anatoly Yakovenko 10 May '12

10 May '12

i would really like to be able to ship haskell based linux binaries, but the gmp dependency makes it virtually impossible. so what version of host os, host ghc, and haskell-platform sources are known to build a working compiler? any reason why the dependency on gmp is static? are the interfaces between versions actually different? or can we build a version fo the compiler that loads the library via dlopen on demand? Thanks, Anatoly

3 2

Re: [Haskell-cafe] [Q] multiparam class undecidable types
by oleg＠okmij.org 10 May '12

10 May '12

> | instance (Table a c, Show c) => Show a where > I would have thought that there is on overlap: the instance in my code > above defines how to show a table if the cell is showable; No, the instance defines how to show values of any type; that type must be an instance of Table. There is no `if' here: instances are selected regardless of the context such as (Table a c, Show c) above. The constraints in the context apply after the selection, not during. Please see ``Choosing a type-class instance based on the context'' http://okmij.org/ftp/Haskell/TypeClass.html#class-based-overloading for the explanation and the solution to essentially the same problem. There are other problems with the instance: | instance (Table a c, Show c) => Show a where For example, there is no information for the type checker to determine the type c. Presumably there could be instances Table [[Int]] Int Table [[Int]] Bool So, when the a in (Table a c) is instantiated to [[Int]] there could be two possibilities for c: Int and Bool. You can argue that the type of the table uniquely determines the type of its cells. You should tell the type checker of that, using functional dependencies or associated types: class Table table where type Cell table :: * toLists :: table -> [[Cell table]]

2 2

Re: [Haskell-cafe] Datapacker?
by sharon kimble 09 May '12

09 May '12

On Thursday 10 May 2012 04:09:33 sharon kimble wrote: > On Thursday 10 May 2012 04:01:56 you wrote: > > What is datapacker? Can you summarize the sort of problem you are having? > > https://github.com/jgoerzen/datapacker/wiki ............ datapacker is a > tool to group files by size. It is perhaps most often used to fit a set of > files onto the minimum number of CDs or DVDs. > > I just want to check the syntax of the command before i commit it to stone > for backing up my mp3s? > > Thanks > Sharon. > > > On May 9, 2012 9:34 PM, "sharon kimble" <boudiccas(a)talktalk.net> wrote: > > > Is this the right place to ask questions about datapacker please? If > > > not, can > > > anyone tell me where is please? > > > > > > Thanks > > > Sharon. > > > -- -- A taste of linux = http://www.sharons.org.uk/taste/index.html efever = http://www.efever.blogspot.com/ efever = http://sharon04.livejournal.com/ Debian 6,0.4, Gnome 1:2.30+7, LibreOffice 3.4.6 Registered Linux user 334501

1 0

Re: [Haskell-cafe] Datapacker?
by Antoine Latter 09 May '12

09 May '12

On Wed, May 9, 2012 at 10:09 PM, sharon kimble <boudiccas(a)talktalk.net> wrote: > On Thursday 10 May 2012 04:01:56 you wrote: >> What is datapacker? Can you summarize the sort of problem you are having? > > https://github.com/jgoerzen/datapacker/wiki ............ datapacker is a tool > to group files by size. It is perhaps most often used to fit a set of files onto > the minimum number of CDs or DVDs. > > I just want to check the syntax of the command before i commit it to stone for > backing up my mp3s? > You should probably ask the author - that link is a part of the author's personal github page. Here is some contact information: http://www.complete.org/JohnGoerzen Antoine > Thanks > Sharon. >> >> On May 9, 2012 9:34 PM, "sharon kimble" <boudiccas(a)talktalk.net> wrote: >> > Is this the right place to ask questions about datapacker please? If not, >> > can >> > anyone tell me where is please? >> > >> > Thanks >> > Sharon. >> > -- > > -- > A taste of linux = http://www.sharons.org.uk/taste/index.html > efever = http://www.efever.blogspot.com/ > efever = http://sharon04.livejournal.com/ > Debian 6,0.4, Gnome 1:2.30+7, LibreOffice 3.4.6 > Registered Linux user 334501

1 0

Datapacker?
by sharon kimble 09 May '12

09 May '12

Is this the right place to ask questions about datapacker please? If not, can anyone tell me where is please? Thanks Sharon. -- A taste of linux = http://www.sharons.org.uk/taste/index.html efever = http://www.efever.blogspot.com/ efever = http://sharon04.livejournal.com/ Debian 6,0.4, Gnome 1:2.30+7, LibreOffice 3.4.6 Registered Linux user 334501

2 1