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You can reach the person managing the list at beginners-ow...@haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..." Today's Topics: 1. Re: small expression evaluator (Petr Novotnik) 2. Re: Recursion in monad (Adrian May) 3. Does haskell have to have values? (Mike Meyer) 4. Re: How to structure your Haskell installation? (frode k) 5. cabal dependencies question (Rob Nikander) 6. Re: cabal dependencies question (Daniel Fischer) 7. Re: Does haskell have to have values? (Patrick Lynch) ---------------------------------------------------------------------- Message: 1 Date: Wed, 23 Mar 2011 13:55:22 +0100 From: Petr Novotnik <pnovot...@googlemail.com> Subject: Re: [Haskell-beginners] small expression evaluator To: Henk-Jan van Tuyl <hjgt...@chello.nl> Cc: Haskell Beginners List <beginners@haskell.org> Message-ID: <4d89edba.2060...@googlemail.com> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Hello Henk-Jan, many thanks for your answer. Yes, I could do what you propose. But I still wonder if it is possible or not to the "lifting" as I mentioned earlier. Again thanks for answering, pete. On 03/22/2011 01:43 PM, Henk-Jan van Tuyl wrote: > On Tue, 22 Mar 2011 10:14:57 +0100, Henk-Jan van Tuyl > <hjgt...@chello.nl> wrote: > >> On Tue, 22 Mar 2011 08:56:45 +0100, Petr Novotnik >> <pnovot...@googlemail.com> wrote: >> >>> data Person = Person { >>> personName :: String >>> , personAge :: Int >>> } >>> deriving (Show) >>> >>> exampleExpr :: Bool >>> exampleExpr = (VConst 99) .==. (VFunc personAge) $ Person "pete" 99 >>> >>> >>> I was wondering, whether it'd be possible to enable defining >>> expression without the Value data constructors, i.e. >>> >>> >>> 99 .==. personAge $ Person "pete" 99 >> >> You can write: >> 99 == personAge (Person "pete" 99) >> > > Or you could write: > c .==. f = \x -> c == f x > > test = 99 .==. personAge $ Person "pete" 99 > > The .==. operator is not symmetrical in this case, of course > > Regards, > Henk-Jan van Tuyl > > ------------------------------ Message: 2 Date: Wed, 23 Mar 2011 23:46:31 +0800 From: Adrian May <adrian.alexander....@gmail.com> Subject: Re: [Haskell-beginners] Recursion in monad To: Chadda? Fouch? <chaddai.fou...@gmail.com> Cc: beginners@haskell.org Message-ID: <aanlktikyj_dinpqb7_mtubhdtsxm_jzlnqmoy0s2v...@mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hi Jedi (why not?) Thanks for the tips... > say i = putStrLn $ show i > > This already exist and is called "print" (though its type is more > general than your signature). > > So it is. > > walk i = randomRIO (0,1) >>= \r -> return (i+r*2-1) > > The >>= ... return is pretty ugly, it would rather be written as : > > > walk i = fmap (\r -> i + r * 2 - 1) $ randomRIO (0,1) > > Well, for the time being I like to see exactly what's happening with the monads. That's why I don't use do. There'll come a day when I already know and I'll bear it in mind for then. > > rep n i w s > > Passing say and walk as parameter seems a bit overkill, I seriously > doubt that you'll ever need this exact structure again, and even then > passing a single action should be enough : > > > rep n act i > > | n <= 0 = return () > > | otherwise = act i >>= \x -> rep (n-1) act x > > But what do I pass for act? walk.say? say>>walk? > rep may also be written with standard monad operations : > > rep n act i = foldM (\x _ -> act x) i $ replicate (n-1) () > > Cunning. But it seems a bit round-the-houses to me. I mean, there's no fundamental reason for that list of nothings; it's just plugging a hole in haskell. My counter looks naive but at least it's to the point. I guess it's a matter of taste. > Lastly it may be that the structure of the program itself, > particularly the use of randomRIO is suboptimal and a bit ugly, for a > throwaway program I would probably just use randomRs : > > > main = do > > g <- newStdGen > > mapM_ print . tail . scanl (\i r -> i+r*2-1) 50 . take 10 $ randomRs > (0,1) g > > Are there some extra dots in there? Actually, I can't let the number of random numbers control the number of iterations because my algorithm asks for random numbers when it feels like it. You can't predict how many unless you can predict the random numbers. My main loop is like this: step (w,i,o) = env o >>= \i' -> think w i' >>= \o' -> learn w i i' o o' >>= \w' -> return (w', i', o') I tried to tidy that up by factoring out that "remember the last iteration" business. (It's a Hebbian learning rule.) I figured I could use a comonad containing the current and previous values of i and o, a function that takes both and returns the new value, and let cobind shuffle them along. It didn't work though because I wasn't allowed to say: instance Comonad (a,a) where ... Apparently instance Comonad ((,) a) where ... is legal, but I really want to say that both elements of the tuple are the same, otherwise everything else barfs. Is there some workaround for that? The compiler seemed to imply that you can only ever have one parameter to a type in an instance declaration, but that would see rather limiting and arbitrary. What's the deal here? Adrian. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20110323/d7c9ce65/attachment-0001.htm> ------------------------------ Message: 3 Date: Wed, 23 Mar 2011 16:17:57 -0400 From: Mike Meyer <m...@mired.org> Subject: [Haskell-beginners] Does haskell have to have values? To: Haskell Beginners List <beginners@haskell.org> Message-ID: <20110323161757.447f2...@bhuda.mired.org> Content-Type: text/plain; charset=US-ASCII I'm working my way through Real World haskell, and so far have found the experience quite pleasant (though the harder exercises seem to require things net yet covered). Among the comments in the IO chapter was a discussion of whether or not some monad was or was not a function, which got me thinking. Values in haskell aren't evaluated until they're needed. They're implemented as thunks, meaning they're roughly zeroadic functions that will return the value when called. The syntax of the language seems to make treating values as zeroadic functions that return the value in question a reasonable interpretation as a degenerate case: (+) accepts two arguments and returns their sum. (+ 5) accepts one argument and returns that plus 5. (3 + 5) accepts zero arguments and returns 8. or (more pedantically): (+) accepts one argument and returns a function that accepts one argument and returns a zeroadic function that returns the value of the sum of the two arguments. (+ 5) accepts one argument and returns a zeroadic function that ... (3 + 5) a zeroadic function that returns 8 So the question is - is there any advantage or harm in this way of looking at values? Thanks, <mike -- Mike Meyer <m...@mired.org> http://www.mired.org/consulting.html Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org ------------------------------ Message: 4 Date: Wed, 23 Mar 2011 21:56:06 +0100 From: frode k <mailingl...@klevstul.com> Subject: Re: [Haskell-beginners] How to structure your Haskell installation? To: beginners <beginners@haskell.org> Message-ID: <AANLkTimjJXMkz-9puGZrUq6C=D=8ebtf0a7dvbhye...@mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" Thanks for your feedback, it helped me setting up both GHC 7.0.2 and 6.12.3 in parallel, and Cabal-install with the edited configuration file made it all much easier. I only needed GHC 6.12.3 for building 7.0.2, hence I did not bother installing / building Cabal-install for 6.12.3. I did document the entire process here: http://klevstul.posterous.com/haskell-ghc-702-on-centos-55 [k] -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://www.haskell.org/pipermail/beginners/attachments/20110323/466c955e/attachment-0001.htm> ------------------------------ Message: 5 Date: Wed, 23 Mar 2011 17:08:25 -0400 From: Rob Nikander <rob.nikan...@gmail.com> Subject: [Haskell-beginners] cabal dependencies question To: beginners <beginners@haskell.org> Message-ID: <aanlktikplnq+zeme2f0vejyou8az7+82jbxua2snb...@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Hi, I was unable to normally install the "mime" package -- I got an error that it needed "base <= 4". I have 4.3.1.0. So I ran `cabal unpack mime`, edited mime.cabal to say "base <= 4.3.1.0" and from that file's directory ran `cabal install'. It appeared to install it okay. But when I try to install another package that needs mime, I get the same kind of error: > cabal install http-server Resolving dependencies... cabal: cannot configure mime-0.3.2. It requires base >=3 && <=4 For the dependency on base >=3 && <=4 there are these packages: base-3.0.3.1 and base-3.0.3.2. However none of them are available. base-3.0.3.1 was excluded because base-4.3.1.0 was selected instead I'm wondering why it thinks the mime package wants base <= 4 when I just installed with a .cabal file that says base <= 4.3.1.0. ?? thanks, Rob ------------------------------ Message: 6 Date: Wed, 23 Mar 2011 22:31:31 +0100 From: Daniel Fischer <daniel.is.fisc...@googlemail.com> Subject: Re: [Haskell-beginners] cabal dependencies question To: beginners@haskell.org Message-ID: <201103232231.32164.daniel.is.fisc...@googlemail.com> Content-Type: Text/Plain; charset="iso-8859-1" On Wednesday 23 March 2011 22:08:25, Rob Nikander wrote: > Hi, > > I was unable to normally install the "mime" package -- I got an error > that it needed "base <= 4". I have 4.3.1.0. So I ran `cabal unpack > mime`, edited mime.cabal to say "base <= 4.3.1.0" and from that file's > directory ran `cabal install'. It appeared to install it okay. But > when I try to install another package that needs mime, I get the same > > kind of error: > > cabal install http-server > > Resolving dependencies... > cabal: cannot configure mime-0.3.2. It requires base >=3 && <=4 > For the dependency on base >=3 && <=4 there are these packages: > base-3.0.3.1 and base-3.0.3.2. However none of them are available. > base-3.0.3.1 was excluded because base-4.3.1.0 was selected instead > > I'm wondering why it thinks the mime package wants base <= 4 when I > just installed with a .cabal file that says base <= 4.3.1.0. Because it tries to make a consistent install plan for the packages incuding mime-0.3.2. For that it looks at the dependencies listed in the package-index (because it knows where that is). That says mime-0.3.2 needs base <= 4, so for a consistent install it would have to reinstall mime-0.3.2, since a missing dependency indicates a broken package. The solution (apart from prodding the maintainer to release a new version compatible with base-4.3) is to bump the version of mime in your local install, so $ cd mime -- edit mime.cabal to say the version is 0.3.2.1, save $ cabal install Now you have mime-0.3.2.1 installed and the package-index doesn't know about that package, so cabal assumes that one is okay for further installs. It won't work if the packages specify a dependency mime <= 0.3.2, in which case you'd have to edit their .cabal files too (and don't forget to do a minor-minor version bump for those). Remember, everyone: if you edit a .cabal file to repair the build-depends, bump the version or cabal-install will think it's broken. > > ?? > > thanks, > Rob HTH, Daniel ------------------------------ Message: 7 Date: Wed, 23 Mar 2011 17:34:54 -0400 From: "Patrick Lynch" <kmandpjly...@verizon.net> Subject: Re: [Haskell-beginners] Does haskell have to have values? To: "Mike Meyer" <m...@mired.org>, "Haskell Beginners List" <beginners@haskell.org> Message-ID: <5D1F4F6A08E441D8A755D579557760C9@UserPC> Content-Type: text/plain; format=flowed; charset=iso-8859-1; reply-type=original Hi, I'm going thru RWH [Real World Haskell] too... I'm finding it to be an excellent way to learn Haskell. I prefer it to the Computer Scientist geared books that I'm also reading... I also found "Learn You a Haskell for Great Good!" to be very worthwhile - I've ordered a copy of it and it should be availble at the end of April, see link: http://book.realworldhaskell.org/read/code-case-study-parsing-a-binary-data-format.html I found a link for RWH too - it is: http://fldit-www.cs.uni-dortmund.de/~peter/RealWorldHaskell.pdf BTW: I'm also an independent software consultant -- been doing it for 30+ years... As for your question, I'm not, as yet, prepared to answer it...but it seems to me that 'Sections' and 'Currying' are very important in the study of Haskell...but I'm still in 'newbie' status... I tried to install TK on my Windows Vista PC but was unable to do so...I will try it again, using RWH - if you're heading there too, please let me know - perhaps we can work this together... I also took a look at Category Theory books but so far I'm stumped with it - it's starting to make a bit of sense but I don't know if I'll be able to link it to Haskell... Good luck to both of us... Pat ----- Original Message ----- From: "Mike Meyer" <m...@mired.org> To: "Haskell Beginners List" <beginners@haskell.org> Sent: Wednesday, March 23, 2011 4:17 PM Subject: [Haskell-beginners] Does haskell have to have values? > I'm working my way through Real World haskell, and so far have found > the experience quite pleasant (though the harder exercises seem to > require things net yet covered). > > Among the comments in the IO chapter was a discussion of whether or > not some monad was or was not a function, which got me thinking. > > Values in haskell aren't evaluated until they're needed. They're > implemented as thunks, meaning they're roughly zeroadic functions that > will return the value when called. > > The syntax of the language seems to make treating values as zeroadic > functions that return the value in question a reasonable > interpretation as a degenerate case: > > (+) accepts two arguments and returns their sum. > (+ 5) accepts one argument and returns that plus 5. > (3 + 5) accepts zero arguments and returns 8. > > or (more pedantically): > > (+) accepts one argument and returns a function that accepts one > argument and returns a zeroadic function that returns the > value of the sum of the two arguments. > (+ 5) accepts one argument and returns a zeroadic function that ... > (3 + 5) a zeroadic function that returns 8 > > So the question is - is there any advantage or harm in this way of > looking at values? > > Thanks, > <mike > -- > Mike Meyer <m...@mired.org> http://www.mired.org/consulting.html > Independent Software developer/SCM consultant, email for more information. > > O< ascii ribbon campaign - stop html mail - www.asciiribbon.org > > _______________________________________________ > Beginners mailing list > Beginners@haskell.org > http://www.haskell.org/mailman/listinfo/beginners > ------------------------------ _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners End of Beginners Digest, Vol 33, Issue 32 *****************************************