- Haskell-Cafe - Haskell.org

astronomy projects in haskell
by Michael Litchard 22 Apr '09

22 Apr '09

I remember reading some website, that dons (probably) posted once. I'd like to find them again for a report I'm doing. So, if you know of any astronomy websites that talk about projects using haskell, please let me know. thanks Michael Litchard

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haxr build failure from cabal
by Jeff Heard 22 Apr '09

22 Apr '09

haxr will no longer compile from cabal because of the dependency marked HTTP > 1.0. The current version of the library requires HTTP < 4000.0.0 as it stands. Can it be updated real quick in hackage? -- Jeff

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Using Data.Complex
by michael rice 22 Apr '09

22 Apr '09

Just exploring. How to load? Michael [michael@localhost ~]$ ghci Data.Complex GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <no location info>: module `Data.Complex' is a package module Failed, modules loaded: none. Prelude>

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Re: [Haskell-cafe] Getting the x out
by michael rice 22 Apr '09

22 Apr '09

Hi Neil, I just tried out Hoogle for the first time and it works great. It seems there are a lot of resources available for Haskell; it just takes time to find out about them and how they work. Thanks. Michael --- On Wed, 4/22/09, Neil Mitchell <ndmitchell(a)gmail.com> wrote: From: Neil Mitchell <ndmitchell(a)gmail.com> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: "Tony Morris" <tonymorris(a)gmail.com>, haskell-cafe(a)haskell.org Date: Wednesday, April 22, 2009, 1:26 AM Hi It's not too hard. You wanted a function that converted Maybe a -> a, you just Hoogle for it: http://haskell.org/hoogle/?hoogle=Maybe+a+->+a Thanks Neil On Wed, Apr 22, 2009 at 2:07 AM, michael rice <nowgate(a)yahoo.com> wrote: > Got it! I figured there must be some way to unpack it. > > My goodness, there are so many functions I'm not even aware of. Has anyone > ever counted them all? > > Thanks. > > Michael > > --- On Tue, 4/21/09, Tony Morris <tonymorris(a)gmail.com> wrote: > > From: Tony Morris <tonymorris(a)gmail.com> > Subject: Re: [Haskell-cafe] Getting the x out > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Tuesday, April 21, 2009, 8:54 PM > > You mean, the x out of *Maybe* x even. In the very literal sense, the > assumption that there is an x in "Maybe x" is false -- there may not be > one since it is maybe, but not necessarily, x. IT's a bit like the use > of null that you might have seen in other languages where you might have > a value or you might have null. What you can do however, is say "give me > the x if there is one, otherwise, use this value". > > This is the fromMaybe function. > > Prelude Data.Maybe> let safeDivision x y = if y == 0 then Nothing else > Just (x/y) > Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 5) > 5.0 > Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 0) > 45.0 > > > > michael rice wrote: >> How do I get the x out of Just x? >> >> Michael >> >> ============= >> >> safeDivision :: Float -> Float -> Maybe Float >> safeDivision x y = if y == 0 then Nothing else Just (x/y) >> >> *Main Data.List> safeDivision 10 5 >> Just 2.0 >> *Main Data.List> 3 + (safeDivision 10 5) >> >> <interactive>:1:0: >> No instance for (Num (Maybe Float)) >> arising from a use of `+' at <interactive>:1:0-22 >> Possible fix: add an instance declaration for (Num (Maybe Float)) >> In the expression: 3 + (safeDivision 10 5) >> In the definition of `it': it = 3 + (safeDivision 10 5) >> *Main Data.List> >> >> >> ------------------------------------------------------------------------ >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > -- > Tony Morris > http://tmorris.net/ > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] Getting the x out
by michael rice 22 Apr '09

22 Apr '09

It makes sense, once you understand the terminology. Thanks. Michael --- On Wed, 4/22/09, Cristiano Paris <frodo(a)theshire.org> wrote: From: Cristiano Paris <frodo(a)theshire.org> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, April 22, 2009, 7:55 AM On Wed, Apr 22, 2009 at 2:49 AM, michael rice <nowgate(a)yahoo.com> wrote: > How do I get the x out of Just x? Hi Michael, in your code you're using Maybe to inform the caller of safeDivision about an exceptional situation. This way, you made a full coverage of all the input cases and nothing is left out, i.e. you created a total function (which is GOOD). If you introduced the Nothing case, you just don't want to ignore it. Also, the type system is forcing you to take the Nothing case into account so you can handle it properly. Hence, you might try something like the "maybe" function, which accounts for the Nothing case. If you'd use fromJust, the Nothing case would remain uncovered, leading you to an unhandled exception, which conflicts with your safeDivision definition. Cristiano

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Getting the x out
by michael rice 22 Apr '09

22 Apr '09

How do I get the x out of Just x? Michael ============= safeDivision :: Float -> Float -> Maybe Float safeDivision x y = if y == 0 then Nothing else Just (x/y) *Main Data.List> safeDivision 10 5 Just 2.0 *Main Data.List> 3 + (safeDivision 10 5) <interactive>:1:0: No instance for (Num (Maybe Float)) arising from a use of `+' at <interactive>:1:0-22 Possible fix: add an instance declaration for (Num (Maybe Float)) In the expression: 3 + (safeDivision 10 5) In the definition of `it': it = 3 + (safeDivision 10 5) *Main Data.List>

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Issues with IO and FFIs
by Jon Harrop 22 Apr '09

22 Apr '09

Does anyone have any comments on the following criticism of some difficulties with FFI, including IO, in Haskell: http://groups.google.com/group/comp.lang.functional/msg/6d650c086b2c8a49?hl… In particular, is it not always possible to write IO libraries safely in Haskell? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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ANN: list-tries-0.0 - first release
by Matti Niemenmaa 22 Apr '09

22 Apr '09

Announcing the first public release of list-tries! Version 0.0, woo! list-tries is a library providing implementations of finite sets and maps for list keys using tries, both simple and of the Patricia kind. The data types are parametrized over the map type they use internally to store the child nodes: this allows extending them to support different kinds of key types or increasing efficiency. To get at it, head over either to Hackage or my homepage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/list-tries http://iki.fi/matti.niemenmaa/list-tries/ It's Hugs-compliant to boot! I haven't tested UHC, but I suspect it'd work as well. I've tested compilation with both GHC 6.10 releases; 6.8 should also be fine. Being at version 0.0, I'm happy to get feedback, although I'm fairly busy with school for the foreseeable future so I can't promise I'll act on it very quickly. In particular, the API might be a bit unobvious due to the map type parametrization (and a bit cluttered due to the strict versions of just about every operation), but I hope it's not much trouble. I've actually had this release ready for a while now. I originally planned to coincide its release with that of GHC 6.10.2 since it needs a new version of 'containers' to build (see "Warning", below), but of course 'containers' wasn't updated since the changes that list-tries requires are API changes. I figured I might as well wait until 6.12.1, but with some recent buzz regarding tries I decided to just shove it out now, even as a crippled version due to the lack of a new enough 'containers'. Warning ======= In order to run properly, list-tries needs a version of 'containers' that hasn't yet been released. I incorporated a little hack which makes it compile even with 0.2, but some calls will fail by calling 'error': 30 of my 1014 test cases do so. If you have the latest Darcs version of 'containers' (which you probably do if you're using the GHC HEAD, for instance), make sure it's registered as a 0.3 version and then ask Cabal to build list-tries with the '-fcontainers03' flag. That way you'll get a fully-featured version of list-tries! Unfinished ========== I had hoped to get some benchmarks out before I released list-tries, to see if there's still much work to be done with respect to performance (and if there's any point in releasing it), but I didn't find the time. Data.ListTrie.Base.Map is largely undocumented, but it's not meant to be used much anyway so that shouldn't matter a great deal.

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Books recommendation for LINQ?
by Günther Schmidt 22 Apr '09

22 Apr '09

Hi, I'll be trying Sigbjorns haskell-dotnet package together with LINQ. What's a good book for LINQ? Günther

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ANN: HTTP-4000.0.6
by Sigbjorn Finne 22 Apr '09

22 Apr '09

Hi, a refresh release of the HTTP package has been uploaded to Hackage, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP Robust handling of ill-formed cookies + squash a bug in normalization of certain proxy-bound requests would be the (minor) headlines of this release. enjoy --sigbjorn

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