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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: Re: Count how often a menu has been selected (Felipe Lessa) 2. Re: Sidebar to variables (Dean Herington) 3. Question on monads and laziness (Lakshmi Narasimhan Vaikuntam) 4. Re: uses of random number generators (Heinrich Apfelmus) 5. Getting the Takusen example code to compile - failed import problem (Daniel Everett) 6. Re: Question on monads and laziness (Dean Herington) 7. RE: Getting the Takusen example code to compile -failed import problem (Bayley, Alistair) ---------------------------------------------------------------------- Message: 1 Date: Sat, 25 Jul 2009 22:52:24 -0300 From: Felipe Lessa <felipe.le...@gmail.com> Subject: Re: [Haskell-beginners] Re: Count how often a menu has been selected To: beginners@haskell.org Message-ID: <20090726015224.ga9...@kira.casa> Content-Type: text/plain; charset=us-ascii On Sat, Jul 25, 2009 at 08:22:33PM +0200, Bernhard Lehnert wrote: > this works great except for the "print a" part which I replaced by: > > b <- readIORef a > print b Woops, sorry :). Tip: most haskellers would prefer either readIORef a >>= print or print =<< readIORef a even if you are already in a 'do' block. -- Felipe. ------------------------------ Message: 2 Date: Sat, 25 Jul 2009 21:53:29 -0400 From: Dean Herington <heringtonla...@mindspring.com> Subject: Re: [Haskell-beginners] Sidebar to variables To: Duke Normandin <dukeofp...@ml1.net>, Haskell Beginners <Beginners@haskell.org> Message-ID: <a06240800c691674b3...@[192.168.1.100]> Content-Type: text/plain; charset="us-ascii" ; format="flowed" At 7:42 AM -0600 7/25/09, Duke Normandin wrote: >My recent questions concerning "variables" spawned this observation: > >I take it that programming a solution using a functional language like >Haskell is really about "linking" a series of functions from which a > solution is derived. > >It seems to me that this approach would naturally encourage the use of >the "bottom-up" method of program development. I mean, breaking the >task down to the smallest possible segments, and then writing a function >for each segment. Refactoring the task until the smallest segments are >achieved. Very much like what I learned about programming in Forth (they >use the term "words" to mean "function") > >Am I on the right track here, in my view of the "Haskell approach" to >programming solutions? >-- >duke Yes, indeed. Haskell, like Forth but probably even more so, makes it easy and profitable to program small pieces that can be combined in ever-larger chunks. Dean ------------------------------ Message: 3 Date: Sun, 26 Jul 2009 09:23:33 +0530 From: Lakshmi Narasimhan Vaikuntam <lakshminaras2...@gmail.com> Subject: [Haskell-beginners] Question on monads and laziness To: Beginners@haskell.org Message-ID: <3cca9ddb0907252053u26db814et2eead20254a0e...@mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" Hello I am studying Real World Haskell chapter 9. Here is a snippet of code data Info = Info { infoPath :: FilePath , infoPerms :: Maybe Permissions , infoSize :: Maybe Integer , infoModTime :: Maybe ClockTime } deriving (Eq, Ord, Show) getInfo :: FilePath -> IO Info -- file: ch09/ControlledVisit.hs traverse order path = do names <- getUsefulContents path contents <- mapM getInfo (path : map (path </>) names) liftM concat $ forM (order contents) $ \info -> do if isDirectory info && infoPath info /= path then traverse order (infoPath info) else return [info] getUsefulContents :: FilePath -> IO [String] getUsefulContents path = do names <- getDirectoryContents path return (filter (`notElem` [".", ".."]) names) isDirectory :: Info -> Bool isDirectory = maybe False searchable . infoPerms When I read about IO in the previous chapter, I learnt that reading a file can be done lazily. Here my doubt is that whether the expression "order contents" would generate a list of directory contents (held in memory) for use in forM construct. Because in a subsequent para, I find this line. "If we are traversing a directory containing 100,000 files of which we care about three, we'll allocate a 100,000-element list before we have a chance to trim it down to the three we really want" Wouldn't laziness ensure that in the traverse function, when iterating over directory contents using the list generated by "order contents", it will generate just one element at a time and then free the memory for that entry immediately since we are not using the referencing list anymore in the rest of the function. Not sure whether I have understood the concept of laziness w.r.t monads correctly. Please clarify my doubts with regard to the code snippet. Thanks for your time. -- Regards Lakshmi Narasimhan T V -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.haskell.org/pipermail/beginners/attachments/20090725/d58439a0/attachment-0001.html ------------------------------ Message: 4 Date: Sun, 26 Jul 2009 10:08:54 +0200 From: Heinrich Apfelmus <apfel...@quantentunnel.de> Subject: [Haskell-beginners] Re: uses of random number generators To: beginners@haskell.org Message-ID: <h4h2tv$b4...@ger.gmane.org> Content-Type: text/plain; charset=ISO-8859-1 Michael P Mossey wrote: > I have an application for random numbers. So far, in my journey with > Haskell, I have learned basics and a few things about monads, and I was > hoping I could get some guidance how to employ random # gens. > > Can I get a few examples or pointers? I believe I will have to run this > in a State monad or the IO monad, will I not? Yes, the State StdGen monad for example. Note however that it's best to keep it abstract, i.e. to hide the fact that it happens to be a state monad. See also http://lukepalmer.wordpress.com/2009/01/17/use-monadrandom/ for making the case, http://en.wikibooks.org/wiki/Haskell/Understanding_monads for detailing the random number monad, and http://apfelmus.nfshost.com/random-permutations.html for an example use. Regards, apfelmus -- http://apfelmus.nfshost.com ------------------------------ Message: 5 Date: Fri, 24 Jul 2009 16:49:20 +0000 (GMT) From: Daniel Everett <djeinb...@yahoo.co.uk> Subject: [Haskell-beginners] Getting the Takusen example code to compile - failed import problem To: beginners@haskell.org Message-ID: <182701.43131...@web23001.mail.ird.yahoo.com> Content-Type: text/plain; charset=utf-8 I have Takusen 0.8.5 installed and I'm trying to build the code found at http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html using ghc 6.10.3. This always fails with the error: Could not find module `Database.Oracle.Enumerator' The analogous errors are encountered if I substitute Oracle with Sqlite, Postgresql or ODBC. Any ideas how to resolve this? TIA, Daniel ------------------------------ Message: 6 Date: Mon, 27 Jul 2009 00:47:12 -0400 From: Dean Herington <heringtonla...@mindspring.com> Subject: Re: [Haskell-beginners] Question on monads and laziness To: Lakshmi Narasimhan Vaikuntam <lakshminaras2...@gmail.com>, Beginners@haskell.org Message-ID: <a06240800c6926cb99...@[192.168.1.100]> Content-Type: text/plain; charset="us-ascii" At 9:23 AM +0530 7/26/09, Lakshmi Narasimhan Vaikuntam wrote: >Hello >I am studying Real World Haskell chapter 9. Here is a snippet of code > >data Info = Info { > infoPath :: FilePath > , infoPerms :: Maybe Permissions > > , infoSize :: Maybe Integer > , infoModTime :: Maybe ClockTime > } deriving (Eq, Ord, Show) > >getInfo :: FilePath -> IO Info >-- file: ch09/ControlledVisit.hs > >traverse order path = do > names <- getUsefulContents path > contents <- mapM getInfo (path : map (path </>) names) > liftM concat $ forM (order contents) $ \info -> do > if isDirectory info && infoPath info /= path > > then traverse order (infoPath info) > else return [info] > >getUsefulContents :: FilePath -> IO [String] >getUsefulContents path = do > names <- getDirectoryContents path > return (filter (`notElem` [".", ".."]) names) > > >isDirectory :: Info -> Bool >isDirectory = maybe False searchable . infoPerms >When I read about IO in the previous chapter, I learnt that reading >a file can be done lazily. >Here my doubt is that whether the expression "order contents" would >generate a list of directory contents (held in memory) for use in >forM construct. Because in a subsequent para, I find this line. > >"If we are traversing a directory containing 100,000 files of which >we care about three, we'll allocate a 100,000-element list before we >have a chance to trim it down to the three we really want" > > >Wouldn't laziness ensure that in the traverse function, when >iterating over directory contents using the list generated by "order >contents", >it will generate just one element at a time and then free the memory >for that entry immediately since we are not using the referencing >list > >anymore in the rest of the function. > >Not sure whether I have understood the concept of laziness w.r.t >monads correctly. Please clarify my doubts with regard to the code >snippet. > >Thanks for your time. >-- >Regards >Lakshmi Narasimhan T V These issues are a bit subtle. Yes, a file can be read lazily. What this means is that there are I/O actions (readFile and getContents, for example) which produce lazy strings, for which the underlying read operations are done as needed as the successive characters of the strings are demanded. But this kind of "lazy I/O" is not at play here. I/O is a special monad. Its semantics are that I/O actions are performed in sequence, regardless of demands (or lack thereof) on the actions' results. (This is so that real-world side effects occur in determinstic order.) And the I/O actions themselves determine how much computation they do before offering their results. In the code you cited, `traverse` calls `getUsefulContents` to generate a list of names. `getUsefulContents` uses `getDirectoryContents`, which computes its result list completely before returning it. (The documentation doesn't say that explicitly. It is implied because it doesn't say that the result is returned lazily.) So both `names` and `contents` could be very large, as the authors note. Some describe these semantics by saying "the I/O monad is strict", but I don't think that's a precise statement, because strictness is a property of a function (and its arguments), not of a monad (at least as far as I understand). I hope this helps. Dean -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.haskell.org/pipermail/beginners/attachments/20090727/4162c9d2/attachment-0001.html ------------------------------ Message: 7 Date: Tue, 28 Jul 2009 09:21:05 +0100 From: "Bayley, Alistair" <alistair.bay...@invesco.com> Subject: RE: [Haskell-beginners] Getting the Takusen example code to compile -failed import problem To: "Daniel Everett" <djeinb...@yahoo.co.uk> Cc: beginners@haskell.org Message-ID: <125eacd0cae4d24abdb4d148c4593da911026...@gblonxmb02.corp.amvescap.net> Content-Type: text/plain; charset="us-ascii" > From: beginners-boun...@haskell.org > [mailto:beginners-boun...@haskell.org] On Behalf Of Daniel Everett > > I have Takusen 0.8.5 installed and I'm trying to build the > code found at > http://darcs.haskell.org/takusen/doc/html/Database-Enumerator. html using ghc 6.10.3. This always fails with the error: > > Could not find module `Database.Oracle.Enumerator' > > The analogous errors are encountered if I substitute Oracle > with Sqlite, Postgresql or ODBC. Any ideas how to resolve this? Hello Daniel, Could you please send me the output from "ghc-pkg describe Takusen" ? It might be that the various backend modules have not been installed. The output from the installation process might also be useful e.g. setup -v configure Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. 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