April 2009 - Haskell-Cafe

Optimizing unamb by determining the "state" of a thunk?
by Peter Verswyvelen 22 Apr '09

22 Apr '09

I was wandering if it would be possible to optimize unamb<http://hackage.haskell.org/packages/archive/unamb/0.1.9/doc/html/Data-Unamb…> by checking if a value is already evaluated to head normal form. So f `unamb` g would then be extremely fast if either f or g is already evaluated to head normal form. Maybe using some vacuum tricks? This function would need to be in IO since it is of course not referentially transparent. Although threads are lightweight in Haskell, forking/waiting/killing surely must have more overhead than just checking the thunk of an expression? Of course one could also make unamb a primitive :-)

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ANNOUNCE: dataenc 0.12.1.0
by Magnus Therning 22 Apr '09

22 Apr '09

I've just uploaded version 0.12.1.0 of dataenc with the following (visible) changes compared to the previous version (0.12): - implementation of a bunch of new encodings: - xxencode - hexadecimal - quoted-printable - python escaping - url encoding - squashing of a bug in the yEncoding implementation that only manifested on 32-bit systems - an attempt to conform to the guidelines for the Haskell Platform regarding versioning and dependencies This release brings the full list of encodings to - Base16 - Base32 - Base32Hex - Base64 - Base64Url - Base85 - Hexadecimal - Python string escaping - quoted-printable - URL encoding - uuencoding - xxencoding - yEncoding The hackage page[1] now reads: Versions 0.9, 0.10.1, 0.10.2, 0.11, 0.11.1, 0.12, 0.12.1.0 Dependencies array (>=0.2.0 && <0.3), base (>=4.0.0 && <4.1), containers (>=0.2.0 && <0.3) License BSD3 Copyright Magnus Therning, 2007-2009 Author Magnus Therning Maintainer magnus(a)therning.org Category Codec Home page http://www.haskell.org/haskellwiki/Library/Data_encoding Upload date Wed Apr 22 19:40:22 UTC 2009 Uploaded by MagnusTherning /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts

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

22 Apr '09

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/

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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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