January 2009 - Haskell-Cafe

The problem with Monads...
by Rafael Gustavo da Cunha Pereira Pinto 14 Jan '09

14 Jan '09

Last night I was thinking on what makes monads so hard to take, and came to a conclusion: the lack of a guided tour on the implemented monads. Let's take the Writer monad documentation: all it says is: Inspired by the paper "Functional Programming with Overloading and Higher-Order Polymorphism", Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995. SO WHAT? The best approach is the Part II of the "All About Monads" tutorial. There you have the almost ideal approach, except that the examples are just thrown there, with no step-by-step explanation. Of course one could copy, paste and run it, but this gives pretty much a "is it right?' feeling. Questions like "if a Reader is an application, why don't use a regular function instead?" or "what bind means for a State monad?". I will try to work on a "Part II" extended version on my vacations... maybe a WikiMonad... or MonadPedia... :-) After all, it is my duty as a haskell noob to write another monad tutorial! :D Cheers! -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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endian-ness ....
by Galchin, Vasili 14 Jan '09

14 Jan '09

Hello, I am used to network-neutral endianness (TCP/IP) As far as "persistent" store, endian neutralness relies on a convention in a particular marshalling/serializing situation??? Sorry ... probably dumb question .. and I think I know the answer ... but "what the hey". I like to get a consensus answer. Regards, Vasili

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posting newspaper article?
by Galchin, Vasili 14 Jan '09

14 Jan '09

Hello, I don't want to risk the ire of the Haskell Cafe community. Is there a moderator? I want to post an editorial by the Big Blue CEO which seems to me to be very interesting vis-a-vis the FPL community and Haskell in general. I have an online subscription and can post to stimulate discussion. ???? Vasili

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ANN: json-0.4.1
by Sigbjorn Finne 13 Jan '09

13 Jan '09

Hi, a new release of the 'json' package is now available via hackage, version 0.4.1 http://hackage.haskell.org/cgi-bin/hackage-scripts/package/json [no claims that it represents rocket science, but a number of downstream codebases depend on this package for their operation, hence the announcement here.] New in this release is a generic JSON encoder contributed by Lennart Augustsson (Text.JSON.Generic ; thanks Lennart!) along with a number of other, smaller changes (including changes to the default encoding for constructors and prelude types - no longer down-cased; see release notes for more.) Enjoy --sigbjorn

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Real World Haskell: confusion
by Peter Verswyvelen 13 Jan '09

13 Jan '09

On page 102: "partial function application is named currying" I thought "currying" or "to curry" means converting f :: (a,b) ->c into g :: a -> b -> c by applying "curry" (mmm, are Asian people good at Haskell? :-) g = curry f

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Adding Authentication and Authorization to a service implemented in Haskell
by Daryoush Mehrtash 13 Jan '09

13 Jan '09

I am trying to figure out a clean way to add authentication and authorization in a webservice. The services of the web services are implemented as Haskell functions. The request to the service contains the user authentication information. I want to authenticate the user by verifying his authentication informaton, assign him roles, and then based on the roles decide on the functions and data that he can have access too. Is there a clean way to do this in Haskell? Thanks Daryoush

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Request for help testing HDBC-postgresql
by John Goerzen 13 Jan '09

13 Jan '09

Hi folks, I've pushed to the Git repo a bunch of new code for HDBC-postgresql. Specifically, it: 1) Removes autoconf in favor of Duncan's Setup.lhs that should work on all combinations of GHC 6.8, GHC 6.10, POSIX, and Windows 2) Adds support for UTF-8 encoding of strings 3) Adds support for translation of dates/times into a PostgreSQL format I would appreciate people testing it on GHC 6.8, 6.10, and Windows before I upload it to hackage. Please reply with thumbs up/thumbs down. Get it with: git clone git://git.complete.org/hdbc-postgresql Thanks! - John

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ByteString intercalate semantics??
by Galchin, Vasili 13 Jan '09

13 Jan '09

Hello, From Hoogle (my friend) .... *intercalate* :: ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>-> [ ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>] -> ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…> Source<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/src…> *O(n)* The intercalate<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>function takes a ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>and a list of ByteString<http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Dat…>s and concatenates the list after interspersing the first argument between each element of the list. So intercalate doesn't do a simple concatenation? Vasili

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unfoldr [ANN: HLint 1.2]
by Andrew Coppin 13 Jan '09

13 Jan '09

Neil Mitchell wrote: > Hi Andrew, > > >>> HLint will automatically detect if you should have used a map, a foldr >>> or a foldl and suggest how to change your code. In the GHC, darcs and >>> Hoogle code bases there are no obvious map-like functions, which is a >>> good sign :-) >>> >>> >> ...What an intriguing idea. Clearly I'm going to have to play with this >> sometime soon. >> >> Does it suggest unfoldr too? >> > > No, but it could do in the future - I never use unfold's in my code, > so am not really familiar with them. But I will look into them, do you > have any examples where it should suggest unfoldr so I can play with > them? > Off the top of my head, try this: convert b 0 = [] convert b n = n `mod` b : convert b (n `div` b) (Takes a number and yields the radix-B representation of it. Backwards.) convert b = unfoldr (\n -> if n > 0 then Just (n `mod` b, n `div` b) else Nothing) It's also useful for converting a data structure into a list: heap_to_list = unfoldr (\h -> if heap_empty h then Nothing else Just (heap_top h, heap_delete_top h)) Those are the major uses I'm aware of. I'm sure somebody else will think of others. (IIRC, unstream is implemented as an unfold...?)

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Re: [Haskell] ANN: ghci-haskeline 0.1
by Andrew Hunter 13 Jan '09

13 Jan '09

On Mon, Jan 12, 2009 at 12:57:57PM -0800, Judah Jacobson wrote: > I'm pleased to announce the first release of ghci-haskeline. This > package uses the GHC API to reimplement ghci with the Haskeline > library as a backend. Haskeline is a library for line input in > command-line programs, similar to readline or editline, which is > written in Haskell and thus (hopefully) more easily integrated into > other Haskell programs. > Perhaps this has already been discussed at length, in which case I apologize but, well, why provide line input editing at all? A number of languages/programs (off the top of my head: sml, most Schemes) don't; the standard method to get line editing is rlwrap. And this works (in my limited experience) quite well. The disadvantage as I see it of using editline or Haskeline or whatever is that it's going to be sutbly different than other methods; presumably, people won't like the changes in behavior. It seems to me that from a UNIX-y separation of concern view, the right thing to do (as many languages have chosen) is to /not/ provide line editing, and just let the user do that with any number of convenient tools that focus on getting/that/ right (like rlwrap.) Is there a reason we've not taken that approach? Thanks, AHH

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