Re: [Haskell-cafe] Re: what is inverse of mzero and return?
Ashley Yakeley wrote: In article [EMAIL PROTECTED], S. Alexander Jacobson [EMAIL PROTECTED] wrote: I assume there is a standard name for this class/function: instance Foo [] where foo [] = mzero foo (x:_) = return x instance Foo (Maybe x) where foo Nothing = mzero foo Just x = return x Surely they are incomplete monad definitions (has a return but no bind)... I don't believe so. I had to write my own classes to do this sort of thing. This is also a good opporunity to point out an ambiguity in the standard MonadPlus class. All instances satisfy these: mplus mzero a = a mplus a mzero = a But only some instances (such as []) satisfy this: (mplus a b) = c = mplus (a = c) (b = c) Other instances (IO, Maybe) satisfy this: mplus (return a) b = return a I think mplus should be separated into two functions. This code shows the difference a bit more clearly: do b - mplus (return True) (return False) if b then mzero else return () For the first kind this is the same as return (), for the second kind it's the same as mzero. But isnt the point of Monad plus, that to have a 'zero' implies failure (a normal monad cannot fail) - and failure implies choice (a `mplus` b) is a if a succeeds or b if a fails and b succeeds,or mzero if both fail. if you look at your first identity: mplus mzero a = a mplus a mzero = a This fits the above description, but I don't see how the following can be true: (mplus a b) = c = mplus (a = c) (b = c) The LHS says (if a fails run b) then run c. The RHS says if (a then c) fails run (b then c) Finally, mplus (return a) b = return a Is obvious because return a is not mzero, so it is true for all Monads that can fail. Or have I missed the point? Keean. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: what is inverse of mzero and return?
In article [EMAIL PROTECTED], Keean Schupke [EMAIL PROTECTED] wrote: This fits the above description, but I don't see how the following can be true: (mplus a b) = c = mplus (a = c) (b = c) Try it (and my test code) with [], which is an instance of MonadPlus. mplus is defined as (++) for []. -- Ashley Yakeley, Seattle WA ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Hugs for Zaurus
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Wow, hugs on my Z. Many thanks for that! - -- Rainer Volz Virtuelle Projekte -- www.vrtprj.de Virtual Projects -- www.vrtprj.com -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.0 (GNU/Linux) Comment: Processed by Mailcrypt 3.5.8 http://mailcrypt.sourceforge.net/ iD8DBQFB8mOgenjtlV2FwvQRAotuAJ0bY/RkqPiiW9N8pTz70J26yjXblACguSQ7 Kx5ExTM25TaGB1h7qi74JoM= =qsCT -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
Ashley Yakeley wrote: In article [EMAIL PROTECTED], Keean Schupke [EMAIL PROTECTED] wrote: This fits the above description, but I don't see how the following can be true: (mplus a b) = c = mplus (a = c) (b = c) Try it (and my test code) with [], which is an instance of MonadPlus. mplus is defined as (++) for []. but what if (a = c) causes c to fail, and (b = c) lets c succeed. In this case the LHS will fail, whereas the RHS will succeed I think? Keean. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
Am Samstag, 22. Januar 2005 21:20 schrieb Keean Schupke: Ashley Yakeley wrote: In article [EMAIL PROTECTED], Keean Schupke [EMAIL PROTECTED] wrote: This fits the above description, but I don't see how the following can be true: (mplus a b) = c = mplus (a = c) (b = c) Try it (and my test code) with [], which is an instance of MonadPlus. mplus is defined as (++) for []. but what if (a = c) causes c to fail, and (b = c) lets c succeed. In this case the LHS will fail, whereas the RHS will succeed I think? Keean. That's probably a misunderstanding due to the notation, in the [] monad, it's just concat (map c (a ++ b)) = concat (map c a) ++ concat (Map c b), which is easily seen to be true (if applying c to an element of a causes an error, neither side will go past that). Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hugs for Zaurus
Hi John, I'm curious if you know what the requirements (memory, model, etc.) for running Hugs on a Zaurus might be: I am not normally a PDA sort of person, but a quick search indicates that Zauruses (Zauri?) are available (used) for around US $30-50 (e.g., 2MB ZR 5800 through 64 MB SL-5500). I'm guessing the 2MB model wouldn't work, but even at the higher prices the temptation would be great. Anyway, thank you for your efforts, on behalf of current and future Zaurus users. -- Fritz PS: I realize that you might not know what the exact requirements are: I imagine you don't have a large range of test systems available :) . On Jan 21, 2005, at 12:15 PM, John Goerzen wrote: Hello, I have built a fixed Hugs for the Zaurus PDA running the OpenZaurus distribution. Download here: http://quux.org/devel/zaurus/hugs_hugs98-Nov2003-r1_arm.ipk Every module except Posix signals appears to be working. -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
Daniel Fischer wrote: That's probably a misunderstanding due to the notation, in the [] monad, it's just concat (map c (a ++ b)) = concat (map c a) ++ concat (Map c b), which is easily seen to be true (if applying c to an element of a causes an error, neither side will go past that). Daniel So do we consider [] to be fail?, Monad.hs defines: instance MonadPlus [] where mzero = [] mplus = (++) What would happen if this was the definition? instance MonadPlus [] where mzero = [] mplus a b | a == [] = b | otherwise = a Keean. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
But only some instances (such as []) satisfy this: (mplus a b) = c = mplus (a = c) (b = c) Other instances (IO, Maybe) satisfy this: mplus (return a) b = return a I think mplus should be separated into two functions. How would we implement the first kind in the Maybe instance of MonadPlus? J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: what is inverse of mzero and return?
snip Only the monoid Maybe a is not very nice (nor is the monoid IO a),since the second argument of the composition is in general ignored. snip So I think, rather than separating mplus, one should think about whether it is sensible to make Maybe and IO instances of MonadPlus in the first place. I don't know nearly enough of the innards of Haskell to form a valuable opinion of that, but perhaps somebody could enlighten me? My humble opinion follows. It's still a Monoid, being boring should be no reason not to include it [*]. By taking advantage of typeclasses, we can easily alternate between more elaborate approaches and dull ones (if we have the dull ones available too). One common example is using MonadPlus for some backtracking algorithm, then instantiatiating it to Maybe or List instance depending on wether you just want one solution or all of them. [*] For instance, I've missed Maybe being an instance of MonadError. Greetings, J.A. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hugs for Zaurus
On Sat, Jan 22, 2005 at 01:52:58PM -0800, Fritz Ruehr wrote: I'm curious if you know what the requirements (memory, model, etc.) for running Hugs on a Zaurus might be: I am not normally a PDA sort of person, but a quick search indicates that Zauruses (Zauri?) are available (used) for around US $30-50 (e.g., 2MB ZR 5800 through 64 MB SL-5500). I'm guessing the 2MB model wouldn't work, but even at the higher prices the temptation would be great. I can give you some idea, at least. When started from scratch, hugs consumes about 4MB RAM on my system. for those of you not familiar with the Zaurus, it is a PDA from Sharp that runs a version of the Linux kernel and the Qt-derived Qtopia environment from the factory. There arealso numerous projects that have developed alternative Zaurus ROMs, such as OpenZaurus. In addition to the Zaurus models you mentioned, there is also the 640x480 cxx0 series (c760, c860, c700, c750/ etc). These models were only sold in Japan, but are widely available in the rest of the world through ebay and sites like dynamism.com and conics.net.. I have a c760 and it has 128MB RAM IIRC. Most Zaurii can take SD and CF cards. Debian has been ported to the Zaurus also, and can run from a memory card. nhc98 works in such a setting. Of course, hugs already works that way. My effort simply makes it work outside a debian chroot. My understanding is that ghc is currently broken on arm, but if/when that is fixed, you could actually run ghc itself on the zaurus in Debian. Thouhg it sometimes uses almost 1GB of RAM -- that won't do :-) The OpenEmbedded folks have a cross-compilation system built up, and packages of about 1200 well-known unix programs that build in that environment . This is how OpenZaurus itself gets bilt.A They do have a hugs package, but it was broken due to some incompatibilities with cross-compiling. Hope this helps. Feel free to ask if you have any other questions. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] File path programme
You might be interested in the new FilePath module that's in the works. There's been a lot of work to make these functions portable. http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/libraries/base/System/FilePath.hs peace, isaac ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
