Re: [Haskell-cafe] MySQL and HDBC?
2009/1/23 Galchin, Vasili vigalc...@gmail.com Hello, Real World Haskell seems to say that the abstraction layer HDBC doesn't support MySQL. If so, in what sense doesn't HDBC support MySQL?? It doesn't have a MySQL backend. However, it does have an ODBC backend which should work fine with MySQL. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Does readFile /proc/mounts hang for you?
On 2009 Jan 21, at 21:13, Thomas DuBuisson wrote:
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x77470b70) = -1 ENOTTY
(Inappropriate ioctl for device)
This is just the test for buffering: terminal-like devices (TCGETS)
are line buffered, others are block buffered.
select(4, [3], [], NULL, {0, 0})= 0 (Timeout)
select(4, [3], [], NULL, {134, 217727} unfinished ...
I'm too rusty on any sort of low level work to draw meaning out of
this without work, but...
See that failed ioctl that seems to be completely ignored, right
before hanging in our 'select' call?
And see the select call that says 'wait 134 seconds'? If you wait it
out you get:
The ioctl() is a red herring, as stated above. I think the real
problem is that files in /proc are all simulated by the kernel, and
the simulation probably doesn't work very well if they're opened non-
blocking and select()ed on.
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH
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Re: [Haskell-cafe] Employment
On Fri, 2009-01-23 at 16:19 +1100, Toby Hutton wrote: On Fri, Jan 23, 2009 at 11:56 AM, Jonathan Cast jonathancc...@fastmail.fm wrote: Not really. My company *advertises* for Haskell developers, and then when they come in to interview, informs them that the code base is actually written in Perl. Works, too --- we have several Haskellers working here. If all you care about is the quality of the developers, and not their productivity once you've got them, you don't actually need to let them use Haskell after the interview is over... I saw this trick recently for a job advertised locally to me, in Melbourne. I was initially pretty excited that someone in this city was actually advertising for Haskell programmers, until I realised they needed to be good at Javascript and Perl so they could work on their web apps. Argh, I didn't bother applying. I think I saw the same advertisement; unfortunately it was on a joint Australasian website so I was in the wrong country anyway :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why monoids will abide...
On 2009 Jan 22, at 10:09, Andrew Wagner wrote: See, that's the kind of name we need! StructureWithAssociativeOperationAndIdentity -- make both the mathematicians AND the non-mathematicians mad! SimpleArithmetic (you have numbers and a single arithmetic operation on them). You can play similar games with the mathematical concepts of groups and rings. (But you get into trouble with magmas and semigroups.) In any case, my response to bikeshedding these days is to present a fait accompli so people can just get stuff done instead of waiting for many-legs-and-no-brain (otherwise known as a committee) to do something. The math terms have at least the advantage of already being well defined. Yes, this means you get to learn some abstract math --- but then, you're going to be faced with that the first time you encounter (or need!) type-level Peano numbers anyway. Or fix/mfix (least defined fixed point). -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Liskell installation configuration problems in Cygwin on Windows XP Professional, Service Pack 2
Yesterday, I downloaded Darcs 2.2 and Liskell for Windows XP Professional, Service Pack 2, but the following error message appeared when I ran ./Setup.lhs configure in Cygwin: Configuring liskell-0.1... Setup.lhs: At least the following dependencies are missing: ghc-paths -any Does anybody know how this issue should be resolved? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 Furuike ya, kawazu tobikomu mizu no oto. -- Matsuo Basho^ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Liskell installation configuration problems in Cygwin on Windows XP Professional, Service Pack 2
On Fri, 23 Jan 2009 18:10:16 +0900, Benjamin L.Russell dekudekup...@yahoo.com wrote: Yesterday, I downloaded Darcs 2.2 and Liskell for Windows XP Professional, Service Pack 2, but the following error message appeared when I ran ./Setup.lhs configure in Cygwin: Configuring liskell-0.1... Setup.lhs: At least the following dependencies are missing: ghc-paths -any Does anybody know how this issue should be resolved? -- Benjamin L. Russell I forgot to mention that I downloaded Liskell by following the instructions on CFruhwirth's Weblog: Liskell standalone (see http://blog.clemens.endorphin.org/2009/01/liskell-standalone.html). I also posted this question yesterday in a comment there, but nobody has responded yet. Incidentally, the Liskell home page at http://liskell.org/ seems to be down (instead of the original site, an advertisement page, with the error message Sorry, access forbidden, error 403, appears at the bottom of the displayed site). Does anybody know what's up (or down?) with Liskell? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 Furuike ya, kawazu tobikomu mizu no oto. -- Matsuo Basho^ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MySQL and HDBC?
On Fri, 2009-01-23 at 08:06 +, Sebastian Sylvan wrote: 2009/1/23 Galchin, Vasili vigalc...@gmail.com Hello, Real World Haskell seems to say that the abstraction layer HDBC doesn't support MySQL. If so, in what sense doesn't HDBC support MySQL?? It doesn't have a MySQL backend. However, it does have an ODBC backend which should work fine with MySQL. This was uploaded to hackage yesterday: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql-0.1 You might like to test it and give feedback to the author. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Liskell installation configuration problems in Cygwin on Windows XP Professional, Service Pack 2
Hi Benjamin, Try: cabal install ghc-paths If you want to install packages manually you can also get it from http://hackage.haskell.org Thanks Neil On Fri, Jan 23, 2009 at 9:16 AM, Benjamin L. Russell dekudekup...@yahoo.com wrote: On Fri, 23 Jan 2009 18:10:16 +0900, Benjamin L.Russell dekudekup...@yahoo.com wrote: Yesterday, I downloaded Darcs 2.2 and Liskell for Windows XP Professional, Service Pack 2, but the following error message appeared when I ran ./Setup.lhs configure in Cygwin: Configuring liskell-0.1... Setup.lhs: At least the following dependencies are missing: ghc-paths -any Does anybody know how this issue should be resolved? -- Benjamin L. Russell I forgot to mention that I downloaded Liskell by following the instructions on CFruhwirth's Weblog: Liskell standalone (see http://blog.clemens.endorphin.org/2009/01/liskell-standalone.html). I also posted this question yesterday in a comment there, but nobody has responded yet. Incidentally, the Liskell home page at http://liskell.org/ seems to be down (instead of the original site, an advertisement page, with the error message Sorry, access forbidden, error 403, appears at the bottom of the displayed site). Does anybody know what's up (or down?) with Liskell? -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Translator/Interpreter / Mobile: +011 81 80-3603-6725 Furuike ya, kawazu tobikomu mizu no oto. -- Matsuo Basho^ ___ 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
[Haskell-cafe] Re: MySQL and HDBC?
Sebastian == Sebastian Sylvan syl...@student.chalmers.se writes: Sebastian It doesn't have a MySQL backend. However, it does have an Sebastian ODBC backend which should work fine with MySQL. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql Sincerely, Gour -- Gour | Zagreb, Croatia | GPG key: C6E7162D pgpN67sXfGpTx.pgp Description: PGP signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: how to implement daemon start and stop directives?
2009/1/23 Ertugrul Soeylemez e...@ertes.de: Luke Palmer lrpal...@gmail.com wrote: On Thu, Jan 22, 2009 at 9:04 AM, Ertugrul Soeylemez e...@ertes.de wrote: Sometimes this is inevitable, but I've never seen a case, where IORefs couldn't be replaced by a more elegant State/StateT-based solution. And if you need to do multi-threading, Chans, MVars and semaphores are better anyway. Please define better. For the function in question, what advantages do they give us? It gives you thread-safety for free, which IORefs don't give you. It's the same purpose, the same amount of code, but threadsafe by concept. Of course, for simple variables, IORefs will likely be thread-safe as well, but there is no guarantee here. With MVars you have that guarantee. Just to be clear, I wouldn't dare argue that IORefs can do everything MVars and TVars can do. I'm just perplexed why you say MVars and TVars are better, when an IORef does the trick just fine for our function. What I'm saying is: Even if IORefs suffice, why would you want to use them, if MVar does the job as well and likely better? There is no reason to prefer an IORef over an MVar to signal something to another thread. Well, I usually don't go here, but benchmarks show that IORefs are the fastest of any of the mutable variable primitives, by at least a factor of 2. I wish I remembered where I saw that. Yes, that's a good point, but I think, you can safely disregard that for signalling a thread to quit. By the way, IORefs are by themselves not thread-safe. You need to use a special function, when using it in a multi-threaded manner. You mean atomicModifyIORef? Yes. IORefs *are* thread-safe by themselves: you will not make your program segfault by using them in a multithreaded program. So it all comes down to invariants. IORefs have no synchronization concepts, so code like: x - readIORef ref writeIORef ref (x+1) Is not threadsafe by most standards. That doesn't mean IORefs themselves are not threadsafe, just that you have to be careful how you use them. That's exactly my point. In the above case they do just the same, but need more care. And I will reiterate: *in this case* the use of IORef is fully encapsulated in this function and *is threadsafe!* Which is the basis of my argument: in imperative code, when you can limit the scope of an IORef to a small abstraction that is threadsafe, there is no advantage of TVars or MVars over them. I don't think they deserve bad style sledgehammer. (An instance of use the right tool for the job) daemon :: IO () - IO (IO ()) daemon action = do stopvar - newIORef False let run = do stop - readIORef stopvar if stop then return () else (action run) forkIO run return (writeIORef stopvar True) In this case, it's simply a coincidence that it's safe. It's because you're using an IORef Bool, and because writes to a Bool variable are atomic on virtually all platforms. An Integer may already fail to be thread-safe. You need more care and the payoff is zero. If you still insist on using IORefs, at least use the atomic function above, but that destroys the little performance benefit of using IORefs, unless GHC does some intelligent optimization here, which I doubt. This would be the case if the IORef was unboxed, but it is boxed, so writing an IORef is actually writing a pointer to a thunk; here, Bool and Integer have no difference at all. However, I don't know whether GHC makes pointer writes atomic on all platforms (Java does, so it's at least doable). Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife = sex) http://blog.ertes.de/ ___ 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] MySQL and HDBC?
ok .. thank you. Vasili On Fri, Jan 23, 2009 at 3:23 AM, Duncan Coutts duncan.cou...@worc.ox.ac.ukwrote: On Fri, 2009-01-23 at 08:06 +, Sebastian Sylvan wrote: 2009/1/23 Galchin, Vasili vigalc...@gmail.com Hello, Real World Haskell seems to say that the abstraction layer HDBC doesn't support MySQL. If so, in what sense doesn't HDBC support MySQL?? It doesn't have a MySQL backend. However, it does have an ODBC backend which should work fine with MySQL. This was uploaded to hackage yesterday: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql-0.1 You might like to test it and give feedback to the author. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
On Thu, Jan 22, 2009 at 04:57:56PM -0800, Jonathan Cast wrote: On Tue, 2009-01-20 at 19:14 +, Andrew Coppin wrote: Jonathan Cast wrote: On Mon, 2009-01-19 at 21:04 +, Andrew Coppin wrote: I mean, heck, *I* use Haskell at work - and I'm not even supposed to be coding things! /me feels slightly relieved, if you'll forgive my saying so :) Damn... I didn't think I was *that* bad at using Haskell? o_O You have expressed continued bafflement on any number of subjects that have been repeatedly explained to you, so yes. I call Andrew's bluff. The continued bafflement is just to get attention. He's probably not *that* bad at using Haskell. =) -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Is it possible to model eventually consistent side effects?
Daryoush Mehrtash dmehrt...@gmail.com wrote: I am trying to figure out if there is a way to model cloud computing computations in Haskell. My specific problems is that in cloud computing, as in Amazon WebServices, side effects (writes to storage, simple database, queue) follow the eventually consistent model. Which means even if your writes succeeds, it may or may not be immediately available for reads. Each call to the services may also temporarily experience network errors or end-system unavailability. Virtual machines can mount a local disk that does offers consistent storage, but the storage wont survive a reboot (after a crash). I am trying to find out if I can model computation (and its compositions) where I can hide reliability and consistency issues. And also be able to reason about a computation to make sure no combination of errors and inconsistencies would break my logic. Is it possible? If so is there a good starting point? Haskell implements cheap concurrency, with which you can do these kinds of things. There are a few communication and synchronization concepts. You may be particularly interested in software transactional memory (STM) [1]. However, unfortunately we have not approached network transparency yet, so you cannot use this with cloud computing for now. However, of course nothing stops you from implementing the communication between the hosts yourself. This shouldn't be too difficult. [1] http://haskell.org/haskellwiki/Software_transactional_memory Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife = sex) http://blog.ertes.de/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generic typeclass for converting between types
Hi folks,
I've been thinking today that I frequently need to convert data beween
types:
* Between various numeric types
* Between various calendar types (both within the new calendar
system, and between the old and new)
* Marshalling data back and forth to a database in HDBC
It's hard to remember all the functions to use to do these. I often
resort to a chart I made for numeric conversions.
It occurs to me that it would be nice to be able to
(convert (5.8::Double))::Int
or
(convert calendarTime)::ZonedTime
So, the first question is: does something like this exist already?
I'm not aware of it, but I'm not sure how to search either.
I'm thinking of something like the below. With a little magic, it's
quite possible to make errors easy to generate in the safe fashion
(for instance, when converting from String to Integer using reads).
{-# LANGUAGE MultiParamTypeClasses #-}
import Control.Monad.Error
type ConvertResult a = Either ConvertError a
class Convertible a b where
safeConvert :: a - ConvertResult b
instance Convertible Int Double where
safeConvert = return . fromIntegral
instance Convertible Double Int where
safeConvert = return . truncate -- could do bounds checking here
instance Convertible Integer Double where
safeConvert = return . fromIntegral
instance Convertible Double Integer where
safeConvert = return . truncate
convert :: Convertible a b = a - b
convert inp = case safeConvert inp of
Left e - error (show e)
Right x - x
-- rudimentary error type for this example
data ConvertError = ConvertError {
sourceValue :: String,
errorMessage :: String
}
deriving (Eq, Read, Show)
instance Error ConvertError where
strMsg x = ConvertError (unknown) x
The other option is to use an approach more like I have in HDBC. In
HDBC, there is a direct need to encapsulate data for transport, so
I've got this:
class (Show a) = SqlType a where
toSql :: a - SqlValue
safeFromSql :: SqlValue - FromSqlResult a
data SqlValue = SqlString String
| SqlByteString B.ByteString
| SqlWord32 Word32
| SqlWord64 Word64
... many more
...
instance SqlType Int32 where
sqlTypeName _ = Int32
toSql = SqlInt32
safeFromSql (SqlString x) = read' x
safeFromSql (SqlByteString x) = (read' . byteString2String) x
safeFromSql (SqlInt32 x) = return x
safeFromSql (SqlInt64 x) = return . fromIntegral $ x
The advantage of this is that if you've got a whole slew of types and
you're going to be converting between all of them (for instance,
numeric types), if you turn on -Wall the compiler will help you know
when your safeFromSql instance doesn't convert everything. The
disadvantage is that the type system doesn't enforce whether or not it
is even possible to convert certain things (for instance, a TimeOfDay
to a Char), and so we have to return a Left for those.
Any thoughts?
-- John
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Re: [Haskell-cafe] Generic typeclass for converting between types
On Fri, Jan 23, 2009 at 3:01 PM, John Goerzen jgoer...@complete.org wrote: Hi folks, I've been thinking today that I frequently need to convert data beween types: * Between various numeric types * Between various calendar types (both within the new calendar system, and between the old and new) * Marshalling data back and forth to a database in HDBC It's hard to remember all the functions to use to do these. I often resort to a chart I made for numeric conversions. Not the type of response you want, but would you publish that chart somewhere, please ;-) /M -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: how to implement daemon start and stop directives?
Eugene Kirpichov ekirpic...@gmail.com wrote: daemon :: IO () - IO (IO ()) daemon action = do stopvar - newIORef False let run = do stop - readIORef stopvar if stop then return () else (action run) forkIO run return (writeIORef stopvar True) In this case, it's simply a coincidence that it's safe. It's because you're using an IORef Bool, and because writes to a Bool variable are atomic on virtually all platforms. An Integer may already fail to be thread-safe. You need more care and the payoff is zero. If you still insist on using IORefs, at least use the atomic function above, but that destroys the little performance benefit of using IORefs, unless GHC does some intelligent optimization here, which I doubt. This would be the case if the IORef was unboxed, but it is boxed, so writing an IORef is actually writing a pointer to a thunk; here, Bool and Integer have no difference at all. However, I don't know whether GHC makes pointer writes atomic on all platforms (Java does, so it's at least doable). It will likely work, but the compiler is not required to make pointer writes atomic, and it would be an enormous performance hit. The pointer write is more likely atomic by platform, not by the code, but even there I would be very careful, because it may be not the case for SMP systems. And you know what happens, when a pointer race condition occurs. The selection between MVar and IORef is essentially a security or performance question. Especially for daemons I would rather choose security. MVar gives you that security with _the same amount of code_. And to prove that IORefs do lead to a pointer race condition and hence are insecure, try the following code: main :: IO () main = do ref - newIORef False forkIO $ forever $ modifyIORef ref not forever $ readIORef ref = print It crashes for me. I'm using GHC 6.10.1 on an Athlon 64 X2 (32 bits system). The error message reads: test: internal error: schedule: invalid what_next field (GHC version 6.10.1 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Greets, Ertugrul. -- nightmare = unsafePerformIO (getWrongWife = sex) http://blog.ertes.de/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generic typeclass for converting between types
Magnus Therning wrote: On Fri, Jan 23, 2009 at 3:01 PM, John Goerzen jgoer...@complete.org wrote: Hi folks, I've been thinking today that I frequently need to convert data beween types: * Between various numeric types * Between various calendar types (both within the new calendar system, and between the old and new) * Marshalling data back and forth to a database in HDBC It's hard to remember all the functions to use to do these. I often resort to a chart I made for numeric conversions. Not the type of response you want, but would you publish that chart somewhere, please ;-) Sure :-) http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.conversion You might also find these useful: Typeclass instances for numeric types: http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.typeclasses Chart of numeric functions: http://book.realworldhaskell.org/read/using-typeclasses.html#numerictypes.funcs -- John /M ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MySQL and HDBC?
On Jan 23, 2009, at 1:23 AM, Duncan Coutts wrote: On Fri, 2009-01-23 at 08:06 +, Sebastian Sylvan wrote: 2009/1/23 Galchin, Vasili vigalc...@gmail.com Hello, Real World Haskell seems to say that the abstraction layer HDBC doesn't support MySQL. If so, in what sense doesn't HDBC support MySQL?? It doesn't have a MySQL backend. However, it does have an ODBC backend which should work fine with MySQL. This was uploaded to hackage yesterday: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql-0.1 You might like to test it and give feedback to the author. I would very much appreciate feedback. I'm still working with John Goerzen (who has been extremely helpful) to iron our some details, see [1] for a list of issues that you should be aware of before you start doing anything too serious. thanks! chris [1] http://www.maubi.net/~waterson/hacks/hdbc-mysql.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: how to implement daemon start and stop directives?
Ertugrul Soeylemez wrote: And to prove that IORefs do lead to a pointer race condition and hence are insecure, try the following code: main :: IO () main = do ref - newIORef False forkIO $ forever $ modifyIORef ref not forever $ readIORef ref = print It crashes for me. I'm using GHC 6.10.1 on an Athlon 64 X2 (32 bits system). The error message reads: test: internal error: schedule: invalid what_next field (GHC version 6.10.1 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Interesting. Looks like a bug in the single-threaded runtime to me: I can reproduce this crash only when compiling without the `-threaded' flag. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell mode for Emacs question
looks like a neath editor. too bad on windows you don't get anti aliased fonts (I guess it uses an old J2SE or something?) at first sight I was not able to find Haskell support in the freely downloadable version. is this available in the commercial version? On Thu, Jan 22, 2009 at 6:50 PM, John A. De Goes j...@n-brain.net wrote: Not that you're looking to switch editors, but if you want something a little more hassle-free: http://www.n-brain.net/unashots/Haskell/ErrorHighlighting.png Regards, John On Jan 22, 2009, at 10:17 AM, Peter Verswyvelen wrote: I have a silly problem. I'm using Emacs with the Haskell mode extension on Windows I have a source file in say c:/foo/src/main.hs main.hs is importing some other modules in that same src directory When I invoke GHCi from within Emacs, the first thing it does is :cd c:/foo and then :load src/main.hs But of course GHCi won't find the imported modules now, since the current directory is wrong. If I type in GHCi :cd src :load main.hs then it compiles fine. Does anyone have an idea why Emacs or the Haskell mode is switching to the parent directory of src instead of src itself, and how to fix this? Thanks a lot, Peter ___ 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] Haskell mode for Emacs question
No, Haskell support in the free version too. However, you'll have to add the tools to compile .hs files in the Tools menu. You can pipe the output to directly highlight errors/warnings in the document (which is what's done in the screenshot below), because the editor understands what GHC errors/warnings look like. Regards, John http://www.n-brain.net|877-376-2724 x 101 On Jan 23, 2009, at 10:45 AM, Peter Verswyvelen wrote: looks like a neath editor. too bad on windows you don't get anti aliased fonts (I guess it uses an old J2SE or something?) at first sight I was not able to find Haskell support in the freely downloadable version. is this available in the commercial version? On Thu, Jan 22, 2009 at 6:50 PM, John A. De Goes j...@n-brain.net wrote: Not that you're looking to switch editors, but if you want something a little more hassle-free: http://www.n-brain.net/unashots/Haskell/ErrorHighlighting.png Regards, John On Jan 22, 2009, at 10:17 AM, Peter Verswyvelen wrote: I have a silly problem. I'm using Emacs with the Haskell mode extension on Windows I have a source file in say c:/foo/src/main.hs main.hs is importing some other modules in that same src directory When I invoke GHCi from within Emacs, the first thing it does is :cd c:/foo and then :load src/main.hs But of course GHCi won't find the imported modules now, since the current directory is wrong. If I type in GHCi :cd src :load main.hs then it compiles fine. Does anyone have an idea why Emacs or the Haskell mode is switching to the parent directory of src instead of src itself, and how to fix this? Thanks a lot, Peter ___ 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] MySQL and HDBC?
On Fri, 2009-01-23 at 03:54 -0600, Galchin, Vasili wrote: On Fri, Jan 23, 2009 at 3:23 AM, Duncan Coutts duncan.cou...@worc.ox.ac.uk wrote: This was uploaded to hackage yesterday: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-mysql-0.1 You might like to test it and give feedback to the author. ooops ... cabal install HDBC-mysql doesn't work ?? Vasili Vasili, it's really no good telling me this, especially since you did not cc the author or haskell-cafe. I am not the author of the package. And even if I were you have provided no details on what goes wrong. I'm sure it was not your intention, but to some people your response would sound demanding and unhelpful, possibly even rude. Make sure that when you do contact the author that you send them something that they will find helpful. So when you email the author make sure you provide full details about what you did and about the environment you are using, eg platform, ghc version, where your mysql libraries are installed, what version they are etc. I would guess that the most likely problem is that you do not have the mysql development package installed. It should provide the mysql_config program. If that is not the problem then contact the package author with the full details. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] mapM_ - Monoid.Monad.map
I always considered the monad functions with names ending on '_' a concession to the IO monad. Would you need them for any other monad than IO? For self-written monads you would certainly use a monoid instead of monadic action, all returning (), but IO is a monad. (You could however wrap (newtype Output = Output (IO ())) and define a Monoid instance on Output.) However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ and it is also safer since it connects the monadic result types of the atomic and the sequenced actions. There was a recent discussion on the topic: http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapM_ - Monoid.Monad.map
On 23 Jan 2009, at 21:50, Henning Thielemann wrote: I always considered the monad functions with names ending on '_' a concession to the IO monad. Would you need them for any other monad than IO? For self-written monads you would certainly use a monoid instead of monadic action, all returning (), but IO is a monad. (You could however wrap (newtype Output = Output (IO ())) and define a Monoid instance on Output.) However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ and it is also safer since it connects the monadic result types of the atomic and the sequenced actions. There was a recent discussion on the topic: http://neilmitchell.blogspot.com/2008/12/mapm-mapm-and-monadic-statements.html Of note btw, these don't need Monad at all... sequence :: Applicative f = [f a] - f [a] sequence = foldr (liftA2 (:)) (pure []) mapA :: (Traversable t, Applicative f) = (a - f b) - t a - f (t b) mapA = (fmap . fmap) sequence fmap Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generalizing a filter-making function
Novice question here. Sorry if the post is wordy.
In the following code (which doesn't actually compile as-is), I'm trying
to generalize these 'make*Filter' functions into a single 'makeFilter'
function. However, I can't get the types to work right.
Foo is a tuple type on which a large number of accessor functions are
defined. All of them have type Foo - Int, Foo - String, or Foo
- [a] (so far).
I tried defining 'Query' using exsistential types instead, but had
difficulty with 'escaped type variables' when I tried to write a
generalized 'makeFilter' function.
The general point of makeFilter is to take as parameters a Query, a
value, a comparison function, and then return a function (Foo - Bool).
This returned function takes as its argument an object of type Foo,
applies the function 'q' to it, compares that value to 'val', and
finally returns a Bool.
Later on in the program, a list of these filter functions is to be used
with a list of Foo objects, to determine which Foo objects satisfy all
of the filters.
Advice would be greatly appreciated.
--- code ---
data Query a = Query { query_identifier :: String, query_func :: (Foo - a) }
makeIntFilter :: Query Int - Int - (Int - Int - Bool)
- (Foo - Bool)
makeIntFilter q val cmp = (\k - val `cmp` (query_func q $ k))
makeStringFilter :: Query String - String - (String - String - Bool)
- (Foo - Bool)
makeStringFilter q val cmp = (\k - val `cmp` (query_func q $ k))
-- ??? broken, and the 'cmp' argument is thrown away, which seems wrong
makeMemberFilter :: Eq a = Query [a] - a - (a - a - a)
- (Foo - Bool)
makeMemberFilter q val cmp =(\k - val `elem` (query_func q $ k))
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Re: [Haskell-cafe] MySQL and HDBC?
On Fri, 2009-01-23 at 03:54 -0600, Galchin, Vasili wrote: ooops ... cabal install HDBC-mysql doesn't work ?? for what it's worth, calbal install hdbc-mysql worked on my office's pc. ( kubuntu 8.10 ) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapM_ - Monoid.Monad.map
Hi, Am Freitag, den 23.01.2009, 21:50 +0100 schrieb Henning Thielemann: However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ will it? This is based on a naive, not well-founded understanding of haskell evaluation, but looking at instance Monoid () where mempty= () _ `mappend` _ = () mconcat _ = () I’d assume that evaluating mapM_ (putStrLn) lotsOfLargeStrings with your proposed mapM_ will leave a thunk equivalent to () `mappend` () `mappend` () `mappend`... in memory until the mapM_ has completely finished, where each () is actually an unevalutated thunk that still has a reference to one of the elements in the lotsOfLargeStrings list. Greetings, Joachim -- Joachim nomeata Breitner mail: m...@joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nome...@joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nome...@debian.org signature.asc Description: Dies ist ein digital signierter Nachrichtenteil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
On Monday 19 January 2009 23:26:09 Sittampalam, Ganesh wrote: We (Credit Suisse) have Haskell developers in both London and NY, although the page only listed NY (I've now corrected it). If CS had Haskell positions in Wroclaw, Poland I'd apply for it! Best, Bartek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapM_ - Monoid.Monad.map
On Friday 23 January 2009 3:50:18 pm Henning Thielemann wrote: I always considered the monad functions with names ending on '_' a concession to the IO monad. Would you need them for any other monad than IO? For self-written monads you would certainly use a monoid instead of monadic action, all returning (), but IO is a monad. (You could however wrap (newtype Output = Output (IO ())) and define a Monoid instance on Output.) However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ and it is also safer since it connects the monadic result types of the atomic and the sequenced actions. There was a recent discussion on the topic: What is your proposed definition of the new mapM_? For instance: mapM_ f = foldr (liftM2 mappend) (return mempty) Has exactly the same problem as mapM for the situations you'd actually want to use it (roughly, it's not monadically 'tail recursive'). You could instead have: mapM_ f = foldr () (return mempty) which does have the right behavior, but it doesn't naturally have the type you give above, and always returns mempty, which isn't much more useful than always returning (). mapM_ is also useful in ST (perhaps not surprising) and strict State. It should be useful in other monads in which = implies a strict sequencing of evaluation, as it's the difference between: nonTail (x:xs) = do a - foo x b - nonTail xs -- overflows for long lists return $ f a b and: tail (x:xs) = do foo x tail xs Even if f doesn't even look at its arguments, it leads to stack overflows for long lists in certain monads. You can, of course, write a 'tail recursive' mapM by using an Endo m accumulator (to preseve ordering), and that might work out well enough, but I haven't thought much about it. In any case, I have a hard time believing IO is the *only* monad where you might want to write a loop purely for its side effects, but maybe I'm off the mark. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapM_ - Monoid.Monad.map
On Fri, 2009-01-23 at 21:30 +, Joachim Breitner wrote: Hi, Am Freitag, den 23.01.2009, 21:50 +0100 schrieb Henning Thielemann: However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ will it? This is based on a naive, not well-founded understanding of haskell evaluation, but looking at instance Monoid () where mempty= () _ `mappend` _ = () mconcat _ = () I’d assume that evaluating mapM_ (putStrLn) lotsOfLargeStrings with your proposed mapM_ will leave a thunk equivalent to () `mappend` () `mappend` () `mappend`... in memory until the mapM_ has completely finished, where each () is actually an unevalutated thunk that still has a reference to one of the elements in the lotsOfLargeStrings list. Perhaps this is why the Monoid instance for () in GHC's source has the comment should this be strict? :) signature.asc Description: PGP signature rcmAttmntahOV2C Description: micalg/pgp-sha1 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to define an operation in terms of itself (but of different type)?
Hello Haskellers! It's probably a simple question but I can't find a proper solution... Suppose we have a class Vector which overloads (+) operation. I'd like to represent a Matrix data type as a set of vectors: data Matrix3 = M3 !Vector3 !Vector3 !Vector3 In this case (+) for matrices could be implemented as: (M3 r11 r12 r13) + (M3 r21 r22 r23) = M3 (r11 + r21) (r12 + r22) (r13 + r23) But GHC says about ambiguous occurrence. I don't understand why because it's should be pretty clear that rows are vectors and (+) has to be vector type as well. I could use Vector.+ instead but it doesn't look good. Probably I'm missing something. Any ideas? Cheers, Oleksandr. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
There are quite a few people with working Haskell knowledge in Wroclaw. I'm sure many of my colleagues (from University of Wroclaw: http://www.ii.uni.wroc.pl/ http://www.ii.uni.wroc.pl/cms/) know Haskell and I think would enjoy working with it. All best Christopher Skrzętnicki On Fri, Jan 23, 2009 at 23:31, Bartosz Wójcik bar...@sudety.it wrote: On Monday 19 January 2009 23:26:09 Sittampalam, Ganesh wrote: We (Credit Suisse) have Haskell developers in both London and NY, although the page only listed NY (I've now corrected it). If CS had Haskell positions in Wroclaw, Poland I'd apply for it! Best, Bartek ___ 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] How to define an operation in terms of itself (but of different type)?
Seems like all telepath are on vacation, so you would have to show the code. On 24 Jan 2009, at 01:15, Olex P wrote: Hello Haskellers! It's probably a simple question but I can't find a proper solution... Suppose we have a class Vector which overloads (+) operation. I'd like to represent a Matrix data type as a set of vectors: data Matrix3 = M3 !Vector3 !Vector3 !Vector3 In this case (+) for matrices could be implemented as: (M3 r11 r12 r13) + (M3 r21 r22 r23) = M3 (r11 + r21) (r12 + r22) (r13 + r23) But GHC says about ambiguous occurrence. I don't understand why because it's should be pretty clear that rows are vectors and (+) has to be vector type as well. I could use Vector.+ instead but it doesn't look good. Probably I'm missing something. Any ideas? Cheers, Oleksandr. ___ 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] How to define an operation in terms of itself (but of different type)?
On Sat, 2009-01-24 at 01:30 +0300, Miguel Mitrofanov wrote: Seems like all telepath are on vacation, Now, now, you didn't let enough time elapse to know that for sure. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generalizing a filter-making function
makeFilter :: (b - b - Bool) - (a - b) - b - a - Bool
makeFilter (==) proj expected = (expected ==) . proj
makeEqFilter :: Eq b = (a - b) - b - a - Bool
makeEqFilter = makeFilter (==)
Then you have a foo:
data Foo = Foo { fooA :: String, fooB :: Int }
foos = [Foo a 1, Foo b 2]
filter (makeEqFilter fooA 1) foos
and so on.
Though this is not really buying you all that much over
filter ((1 ==) . fooA) foos
Or for storing
data Query a = Query String (a - Bool)
let myQuery = Query Test if fst is 1 ((1 ==) . fst)
...
filter myQuery foos
Does this help?
-Ross
On Jan 23, 2009, at 4:20 PM, Dominic Espinosa wrote:
Novice question here. Sorry if the post is wordy.
In the following code (which doesn't actually compile as-is), I'm
trying
to generalize these 'make*Filter' functions into a single 'makeFilter'
function. However, I can't get the types to work right.
Foo is a tuple type on which a large number of accessor functions are
defined. All of them have type Foo - Int, Foo - String, or Foo
- [a] (so far).
I tried defining 'Query' using exsistential types instead, but had
difficulty with 'escaped type variables' when I tried to write a
generalized 'makeFilter' function.
The general point of makeFilter is to take as parameters a Query, a
value, a comparison function, and then return a function (Foo -
Bool).
This returned function takes as its argument an object of type Foo,
applies the function 'q' to it, compares that value to 'val', and
finally returns a Bool.
Later on in the program, a list of these filter functions is to be
used
with a list of Foo objects, to determine which Foo objects satisfy all
of the filters.
Advice would be greatly appreciated.
--- code ---
data Query a = Query { query_identifier :: String, query_func ::
(Foo - a) }
makeIntFilter :: Query Int - Int - (Int - Int - Bool)
- (Foo - Bool)
makeIntFilter q val cmp = (\k - val `cmp` (query_func q $ k))
makeStringFilter :: Query String - String - (String - String -
Bool)
- (Foo - Bool)
makeStringFilter q val cmp = (\k - val `cmp` (query_func q $ k))
-- ??? broken, and the 'cmp' argument is thrown away, which seems
wrong
makeMemberFilter :: Eq a = Query [a] - a - (a - a - a)
- (Foo - Bool)
makeMemberFilter q val cmp =(\k - val `elem` (query_func q $ k))
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Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
On 24 Jan 2009, at 01:35, Jonathan Cast wrote: On Sat, 2009-01-24 at 01:30 +0300, Miguel Mitrofanov wrote: Seems like all telepath are on vacation, Now, now, you didn't let enough time elapse to know that for sure. They're always on vacation when you need one. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
2009/1/23 Olex P hoknam...@gmail.com Hello Haskellers! It's probably a simple question but I can't find a proper solution... Suppose we have a class Vector which overloads (+) operation. I'd like to represent a Matrix data type as a set of vectors: data Matrix3 = M3 !Vector3 !Vector3 !Vector3 In this case (+) for matrices could be implemented as: (M3 r11 r12 r13) + (M3 r21 r22 r23) = M3 (r11 + r21) (r12 + r22) (r13 + r23) But GHC says about ambiguous occurrence. I don't understand why because it's should be pretty clear that rows are vectors and (+) has to be vector type as well. I could use Vector.+ instead but it doesn't look good. Probably I'm missing something. Any ideas? I'm am seeing an image. It's a... a Vector class, with an operation (+). Oh, he's with another one though. It's the Num class. Oh they're fighting over the (+). Vector is trying to take it from Num, but Num isn't letting go. Why doesn't Vector see that there is a whole pile of operations neglected in the corner, like (^+^), (.+.). He just really wants that (+), huh... Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
Well if telepaths on vacation... class Vector v where (^+^) :: v - v - v data Vector3 = V3 !Double !Double !Double instance Vector Vector3 where (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2) class Matrix m where (^+^) :: m - m - m data Matrix3 = M3 !Vector3 !Vector3 !Vector3 instance Matrix Matrix3 where (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21) (r12 ^+^ r22) (r13 ^+^ r23) Hope this is better :) So yeah... r11 ^+^ r21 just doesn't work. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
2009/1/23 Olex P hoknam...@gmail.com: Well if telepaths on vacation... class Vector v where (^+^) :: v - v - v data Vector3 = V3 !Double !Double !Double instance Vector Vector3 where (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2) class Matrix m where (^+^) :: m - m - m data Matrix3 = M3 !Vector3 !Vector3 !Vector3 instance Matrix Matrix3 where (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21) (r12 ^+^ r22) (r13 ^+^ r23) Hope this is better :) So yeah... r11 ^+^ r21 just doesn't work. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Luke's story about Num and a supposed Vector class is repeated here between Vector and Matrix classes. Either you make a class Add instanciated by both Vector and Matrix, or you use different name for each of them. Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
2009/1/23 Olex P hoknam...@gmail.com: class Vector v where (^+^) :: v - v - v data Vector3 = V3 !Double !Double !Double instance Vector Vector3 where (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2) class Matrix m where (^+^) :: m - m - m data Matrix3 = M3 !Vector3 !Vector3 !Vector3 instance Matrix Matrix3 where (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21) (r12 ^+^ r22) (r13 ^+^ r23) Hope this is better :) So yeah... r11 ^+^ r21 just doesn't work. It works fine if you make Matrix3 an instance of Vector. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapM_ - Monoid.Monad.map
On Fri, 2009-01-23 at 13:39 -0800, George Pollard wrote: On Fri, 2009-01-23 at 21:30 +, Joachim Breitner wrote: Hi, Am Freitag, den 23.01.2009, 21:50 +0100 schrieb Henning Thielemann: However our recent Monoid discussion made me think about mapM_, sequence_, and friends. I think they could be useful for many monads if they would have the type: mapM_ :: (Monoid b) = (a - m b) - [a] - m b I expect that the Monoid instance of () would yield the same efficiency as todays mapM_ will it? This is based on a naive, not well-founded understanding of haskell evaluation, but looking at instance Monoid () where mempty= () _ `mappend` _ = () mconcat _ = () I’d assume that evaluating mapM_ (putStrLn) lotsOfLargeStrings with your proposed mapM_ will leave a thunk equivalent to () `mappend` () `mappend` () `mappend`... in memory until the mapM_ has completely finished, where each () is actually an unevalutated thunk that still has a reference to one of the elements in the lotsOfLargeStrings list. Perhaps this is why the Monoid instance for () in GHC's source has the comment should this be strict? :) It's easy to calculate the answer. mempty `mappend` undefined = undefined (left identity monoid law) The above definition doesn't meet this, similarly for the right identity monoid law. That only leaves one definition, () `mappend` () = () which does indeed satisfy the monoid laws. So the answer to the question is Yes. Another example of making things as lazy as possible going astray. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
2009/1/24 Dan Piponi dpip...@gmail.com: 2009/1/23 Olex P hoknam...@gmail.com: class Vector v where (^+^) :: v - v - v data Vector3 = V3 !Double !Double !Double instance Vector Vector3 where (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2) class Matrix m where (^+^) :: m - m - m data Matrix3 = M3 !Vector3 !Vector3 !Vector3 instance Matrix Matrix3 where (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21) (r12 ^+^ r22) (r13 ^+^ r23) Hope this is better :) So yeah... r11 ^+^ r21 just doesn't work. It works fine if you make Matrix3 an instance of Vector. -- Dan To paraphrase a bit, your Vector class simply means : a type is a Vector if it provides an operation having a - a - a has its type (and called ^+^). The Matrix class says exactly the same thing, so don't bother repeat that... and simply make Vector3 and Matrix3 instances of Vector (which could then be renamed in Add or Plus or something). Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
On Fri, 2009-01-23 at 22:58 +, Olex P wrote: Well if telepaths on vacation... class Vector v where (^+^) :: v - v - v data Vector3 = V3 !Double !Double !Double instance Vector Vector3 where (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2) class Matrix m where (^+^) :: m - m - m data Matrix3 = M3 !Vector3 !Vector3 !Vector3 instance Matrix Matrix3 where (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21) (r12 ^+^ r22) (r13 ^+^ r23) Hope this is better :) So yeah... r11 ^+^ r21 just doesn't work. I think you mis-understand how classes work in Haskell. They aren't like classes in OO languages. They're a little bit like interfaces in OO languages, but here we see another difference: in OO, you know an object has a certain class, so you use that to find out what a method name means. In Haskell, it's the other way around: you know a method belongs to a certain class, so you know its argument (or result) must come from an instance of that class. You can't re-use method names for multiple classes, though - you have to either use different names (e.g., div vs. (/) in Prelude) or use a common base class. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Laws and partial values (was: [Haskell-cafe] mapM_ - Monoid.Monad.map)
On Fri, 23 Jan 2009, Derek Elkins wrote: mempty `mappend` undefined = undefined (left identity monoid law) The above definition doesn't meet this, similarly for the right identity monoid law. That only leaves one definition, () `mappend` () = () which does indeed satisfy the monoid laws. So the answer to the question is Yes. Another example of making things as lazy as possible going astray. I'd like to argue that laws, such as monoid laws, do not apply to partial values. But I haven't thought my position through yet. -- Russell O'Connor http://r6.ca/ ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.'' ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Error in HTTP 4.004 + CouchDB?
* Alex Ott alex...@gmail.com [2009-01-22 20:32:26 +0100]: PUT http://127.0.0.1:5984/test1/Users_ott_tmp_1_tst HTTP/1.1 Content-Type: ... ... Host: 127.0.0.1:5984 ... Note that this is a valid HTTP request, according to my reading of RFC2616. -- mithrandi, i Ainil en-Balandor, a faer Ambar signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Laws and partial values (was: [Haskell-cafe] mapM_ - Monoid.Monad.map)
On Fri, Jan 23, 2009 at 6:10 PM, rocon...@theorem.ca wrote: On Fri, 23 Jan 2009, Derek Elkins wrote: mempty `mappend` undefined = undefined (left identity monoid law) The above definition doesn't meet this, similarly for the right identity monoid law. That only leaves one definition, () `mappend` () = () which does indeed satisfy the monoid laws. So the answer to the question is Yes. Another example of making things as lazy as possible going astray. I'd like to argue that laws, such as monoid laws, do not apply to partial values. But I haven't thought my position through yet. Please try to change your mind. You know how annoying it is when you are doing math, and you want to divide, but first you have to add the provision that the denominator isn't zero. Saying that monoid laws do not apply to partial values, while easing the implementation a bit, add similar provisions to reasoning. For example, it is possible to prove that foldr mappend mempty (x:xs) = foldr1 mappend (x:xs). Which means that anywhere in the source where we see the former, we can clean it up to the latter. However, if monad laws don't apply to partial values, then we first have to prove that none of the (x:xs) are _|_, perhaps even that no substrings are _|_. This is a much more involved transformation, so much so that you probably just wouldn't do it if you want to be correct. Bottoms are part of Haskell's semantics; theorems and laws have to apply to them to. You can pretend they don't exist, but then you have to be okay with never using an infinite data structure. I.e. if your programs would run just as well in Haskell as they would in a call-by-value language, then you don't have to worry about bottoms. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Current research on overlapping/closed type families?
Provided all the overlapping instances are supplied together, as you suggest, I
think what you say makes perfect sense and does not threaten soundness.
But we have not yet implemented the idea yet. First priority is to get type
families working properly, and in conjunction with type classes. Then we can
move on to adding features.
Simon
| -Original Message-
| From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-
| boun...@haskell.org] On Behalf Of Ryan Ingram
| Sent: 19 January 2009 23:24
| To: Haskell Cafe
| Subject: [Haskell-cafe] Current research on overlapping/closed type families?
|
| What's the status of overlapping/closed type families? I'm interested
| in something like the following, which can currently be implemented in
| GHC with Oleg-magic using functional dependencies, but cannot, to my
| knowledge, be implemented with type families:
|
| data HTrue = HTrue
| data HFalse = HFalse
|
| type family IsFunction f
|
| {- not legal in GHC6.10 -}
| type instances
|IsFunction (a - b) = HTrue
|IsFunction a = HFalse
|
| -- ryan
| ___
| Haskell-Cafe mailing list
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Re: [Haskell-cafe] Current research on overlapping/closed type families?
Of course. I was just wondering if anyone was looking into the
implications of said features :)
For example, with this instance, if a is a type variable, you can't
reduce IsFunction a to HFalse, because a might be b - c for
some b, c.
Whereas in the current formulation, you can treat the declaration as a
rewrite rule without considering how variables are instantiated. Of
course, this makes IsFunction far less useful; if you were using it
to, for example, define automatic currying and uncurrying of
n-argument functions, it matters whether the result is higher-order or
not, which is somewhat unsatisfying.
-- ryan
On Fri, Jan 23, 2009 at 6:06 PM, Simon Peyton-Jones
simo...@microsoft.com wrote:
Provided all the overlapping instances are supplied together, as you suggest,
I think what you say makes perfect sense and does not threaten soundness.
But we have not yet implemented the idea yet. First priority is to get type
families working properly, and in conjunction with type classes. Then we can
move on to adding features.
Simon
| -Original Message-
| From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-
| boun...@haskell.org] On Behalf Of Ryan Ingram
| Sent: 19 January 2009 23:24
| To: Haskell Cafe
| Subject: [Haskell-cafe] Current research on overlapping/closed type
families?
|
| What's the status of overlapping/closed type families? I'm interested
| in something like the following, which can currently be implemented in
| GHC with Oleg-magic using functional dependencies, but cannot, to my
| knowledge, be implemented with type families:
|
| data HTrue = HTrue
| data HFalse = HFalse
|
| type family IsFunction f
|
| {- not legal in GHC6.10 -}
| type instances
|IsFunction (a - b) = HTrue
|IsFunction a = HFalse
|
| -- ryan
| ___
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Re: [Haskell-cafe] Error in HTTP 4.004 + CouchDB?
On Sat, 24 Jan 2009 14:18:24 Tristan Seligmann wrote: * Alex Ott alex...@gmail.com [2009-01-22 20:32:26 +0100]: PUT http://127.0.0.1:5984/test1/Users_ott_tmp_1_tst HTTP/1.1 Content-Type: ... ... Host: 127.0.0.1:5984 ... Note that this is a valid HTTP request, according to my reading of RFC2616. Depends what you mean by valid. Yes it's a valid request to receive, no an implementation is not compliant with RFC2616 if it issues a request like this to an origin server. Have a look at section 5.1.2. An implementation is REQUIRED to issue requests like this to proxies, an implementation MUST handle receiving requests like this, an implementation MUST use the abs_path form in requests to origin servers and gateways, ie PUT /test1/Users_ott_tmp_1_tst HTTP/1.1 Host: 127.0.0.1:5984 Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Current research on overlapping/closed type families?
Would this then also eventually work?
data Zero
data Succ a = Succ a
type family IsFunction f
type instances
IsFunction (a - b) = Succ (IsFunction b)
IsFunction c= Zero
Simon Peyton-Jones wrote:
Provided all the overlapping instances are supplied together, as you suggest, I
think what you say makes perfect sense and does not threaten soundness.
But we have not yet implemented the idea yet. First priority is to get type
families working properly, and in conjunction with type classes. Then we can
move on to adding features.
Simon
| -Original Message-
| From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-
| boun...@haskell.org] On Behalf Of Ryan Ingram
| Sent: 19 January 2009 23:24
| To: Haskell Cafe
| Subject: [Haskell-cafe] Current research on overlapping/closed type families?
|
| What's the status of overlapping/closed type families? I'm interested
| in something like the following, which can currently be implemented in
| GHC with Oleg-magic using functional dependencies, but cannot, to my
| knowledge, be implemented with type families:
|
| data HTrue = HTrue
| data HFalse = HFalse
|
| type family IsFunction f
|
| {- not legal in GHC6.10 -}
| type instances
|IsFunction (a - b) = HTrue
|IsFunction a = HFalse
|
| -- ryan
| ___
| Haskell-Cafe mailing list
| Haskell-Cafe@haskell.org
| http://www.haskell.org/mailman/listinfo/haskell-cafe
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Re: Laws and partial values (was: [Haskell-cafe] mapM_ - Monoid.Monad.map)
Thanks for letting me reflect on this. I assume that my final program (my final value) is always a total value. Anything else is an error. Therefore, if we required relaxed monoid laws of the form x `mappend` mzero = x then we could safely substitute (x `mappend` mzero) by x without changing the final value (I think this true). But the reverse substituion ,replacing x with (x `mappend` mzero), might not be sound. Now, I can see why you would prefer that the laws hold for partial values. On Fri, 23 Jan 2009, Luke Palmer wrote: On Fri, Jan 23, 2009 at 6:10 PM, rocon...@theorem.ca wrote: On Fri, 23 Jan 2009, Derek Elkins wrote: mempty `mappend` undefined = undefined (left identity monoid law) The above definition doesn't meet this, similarly for the right identity monoid law. That only leaves one definition, () `mappend` () = () which does indeed satisfy the monoid laws. So the answer to the question is Yes. Another example of making things as lazy as possible going astray. I'd like to argue that laws, such as monoid laws, do not apply to partial values. But I haven't thought my position through yet. Please try to change your mind. You know how annoying it is when you are doing math, and you want to divide, but first you have to add the provision that the denominator isn't zero. Saying that monoid laws do not apply to partial values, while easing the implementation a bit, add similar provisions to reasoning. For example, it is possible to prove that foldr mappend mempty (x:xs) = foldr1 mappend (x:xs). Which means that anywhere in the source where we see the former, we can clean it up to the latter. However, if monad laws don't apply to partial values, then we first have to prove that none of the (x:xs) are _|_, perhaps even that no substrings are _|_. This is a much more involved transformation, so much so that you probably just wouldn't do it if you want to be correct. Bottoms are part of Haskell's semantics; theorems and laws have to apply to them to. You can pretend they don't exist, but then you have to be okay with never using an infinite data structure. I.e. if your programs would run just as well in Haskell as they would in a call-by-value language, then you don't have to worry about bottoms. Luke -- Russell O'Connor http://r6.ca/ ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generalizing a filter-making function
Thanks, that does help. I see I was unnecessarily passing a function
parameter (as a newcomer to Haskell, I tend to forget how to properly
use composition).
The reason to do these maneuvers is that the particular accessor
function to use is being parsed from user input. The main problem is
along the lines of: given a user input string such as s 5, t 'baz',
return a list of all Foos f such that (s f 5) and ('baz' `elem` $ t f).
Someone else tipped me off about Parsec's expression parsing support, so
I need to look at that as well. The expressions to be parsed are very
minimal, however, so I'm not sure it's entirely necessary.
Thanks again.
On Fri, Jan 23, 2009 at 05:39:07PM -0500, Ross Mellgren wrote:
makeFilter :: (b - b - Bool) - (a - b) - b - a - Bool
makeFilter (==) proj expected = (expected ==) . proj
makeEqFilter :: Eq b = (a - b) - b - a - Bool
makeEqFilter = makeFilter (==)
Then you have a foo:
data Foo = Foo { fooA :: String, fooB :: Int }
foos = [Foo a 1, Foo b 2]
filter (makeEqFilter fooA 1) foos
and so on.
Though this is not really buying you all that much over
filter ((1 ==) . fooA) foos
Or for storing
data Query a = Query String (a - Bool)
let myQuery = Query Test if fst is 1 ((1 ==) . fst)
...
filter myQuery foos
Does this help?
-Ross
On Jan 23, 2009, at 4:20 PM, Dominic Espinosa wrote:
Novice question here. Sorry if the post is wordy.
In the following code (which doesn't actually compile as-is), I'm
trying
to generalize these 'make*Filter' functions into a single 'makeFilter'
function. However, I can't get the types to work right.
Foo is a tuple type on which a large number of accessor functions are
defined. All of them have type Foo - Int, Foo - String, or Foo
- [a] (so far).
I tried defining 'Query' using exsistential types instead, but had
difficulty with 'escaped type variables' when I tried to write a
generalized 'makeFilter' function.
The general point of makeFilter is to take as parameters a Query, a
value, a comparison function, and then return a function (Foo -
Bool).
This returned function takes as its argument an object of type Foo,
applies the function 'q' to it, compares that value to 'val', and
finally returns a Bool.
Later on in the program, a list of these filter functions is to be
used
with a list of Foo objects, to determine which Foo objects satisfy all
of the filters.
Advice would be greatly appreciated.
--- code ---
data Query a = Query { query_identifier :: String, query_func ::
(Foo - a) }
makeIntFilter :: Query Int - Int - (Int - Int - Bool)
- (Foo - Bool)
makeIntFilter q val cmp = (\k - val `cmp` (query_func q $ k))
makeStringFilter :: Query String - String - (String - String -
Bool)
- (Foo - Bool)
makeStringFilter q val cmp = (\k - val `cmp` (query_func q $ k))
-- ??? broken, and the 'cmp' argument is thrown away, which seems
wrong
makeMemberFilter :: Eq a = Query [a] - a - (a - a - a)
- (Foo - Bool)
makeMemberFilter q val cmp =(\k - val `elem` (query_func q $ k))
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Re: [Haskell-cafe] how to implement daemon start and stop directives?
On 2009 Jan 22, at 1:36, Belka wrote: Could somebody please share some experience on how to implement daemon start and stop directives. In theory I need something like this: 1. my_daemon start - starts my app with an infinite loop of serving inside. 2. my_daemon stop - puts in some TVar a value signalizing, that stop is given - infinite loop brakes. 3. ...it on Linux platform. Thanks in advance! If you want a normal daemon, you want to look at System.Posix.Process to create a proper daemon (forkProcess, createSession; don't forget to close stdOutput (and, errr, Haskell library types: stdin and stderr are where? not System.Posix.IO with stdOutput) and reopen them on / dev/null, at least if they're ttys as determined by System.Posix.Terminal.queryTerminal). You also want to write out a pidfile (/var/run/programname.pid) for use by startproc / killproc, and you want to install a handler for sigTERM (System.Posix.Signals.installHandler) which sets the exit flag (TVar, MVar, IORef, Chan, take your pick). Ideally you also catch sigHUP and reload your state. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] how to implement daemon start and stop directives?
On 2009 Jan 22, at 3:11, Belka wrote: Actually, I'm more interested in technical details how to communicate from shell with background process - how to send commands to it. Currently looking into POSIX libraries hope to find answers there... Also, maybe a FIFO-pipe-file would solve my problem. Imagine writing a command in it, while one of daemon's thread is locked-while-awaits for anything to come out from the other side of the pipe... You can try it; make sure to open the FIFO for read+write to avoid deadlocks, and I think there are some weird behaviors caused by the GHC runtime's non-blocking IO. It will be a lot easier to use a socket (AF_INET or AF_UNIX depending on whether you want to support remote access or not). -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generalizing a filter-making function
Parsec is an awesome piece of software. Throw away anything you might
know from writing parsers in something else -- writing parsers with
parsec is so succinct you can use it most anytime without feeling like
you're resorting to it.
-Ross
On Jan 23, 2009, at 11:26 PM, Dominic Espinosa wrote:
Thanks, that does help. I see I was unnecessarily passing a function
parameter (as a newcomer to Haskell, I tend to forget how to properly
use composition).
The reason to do these maneuvers is that the particular accessor
function to use is being parsed from user input. The main problem is
along the lines of: given a user input string such as s 5, t
'baz',
return a list of all Foos f such that (s f 5) and ('baz' `elem` $
t f).
Someone else tipped me off about Parsec's expression parsing
support, so
I need to look at that as well. The expressions to be parsed are very
minimal, however, so I'm not sure it's entirely necessary.
Thanks again.
On Fri, Jan 23, 2009 at 05:39:07PM -0500, Ross Mellgren wrote:
makeFilter :: (b - b - Bool) - (a - b) - b - a - Bool
makeFilter (==) proj expected = (expected ==) . proj
makeEqFilter :: Eq b = (a - b) - b - a - Bool
makeEqFilter = makeFilter (==)
Then you have a foo:
data Foo = Foo { fooA :: String, fooB :: Int }
foos = [Foo a 1, Foo b 2]
filter (makeEqFilter fooA 1) foos
and so on.
Though this is not really buying you all that much over
filter ((1 ==) . fooA) foos
Or for storing
data Query a = Query String (a - Bool)
let myQuery = Query Test if fst is 1 ((1 ==) . fst)
...
filter myQuery foos
Does this help?
-Ross
On Jan 23, 2009, at 4:20 PM, Dominic Espinosa wrote:
Novice question here. Sorry if the post is wordy.
In the following code (which doesn't actually compile as-is), I'm
trying
to generalize these 'make*Filter' functions into a single
'makeFilter'
function. However, I can't get the types to work right.
Foo is a tuple type on which a large number of accessor functions
are
defined. All of them have type Foo - Int, Foo - String, or Foo
- [a] (so far).
I tried defining 'Query' using exsistential types instead, but had
difficulty with 'escaped type variables' when I tried to write a
generalized 'makeFilter' function.
The general point of makeFilter is to take as parameters a Query, a
value, a comparison function, and then return a function (Foo -
Bool).
This returned function takes as its argument an object of type Foo,
applies the function 'q' to it, compares that value to 'val', and
finally returns a Bool.
Later on in the program, a list of these filter functions is to be
used
with a list of Foo objects, to determine which Foo objects satisfy
all
of the filters.
Advice would be greatly appreciated.
--- code ---
data Query a = Query { query_identifier :: String, query_func ::
(Foo - a) }
makeIntFilter :: Query Int - Int - (Int - Int - Bool)
- (Foo - Bool)
makeIntFilter q val cmp = (\k - val `cmp` (query_func q $ k))
makeStringFilter :: Query String - String - (String - String -
Bool)
- (Foo - Bool)
makeStringFilter q val cmp = (\k - val `cmp` (query_func q $ k))
-- ??? broken, and the 'cmp' argument is thrown away, which seems
wrong
makeMemberFilter :: Eq a = Query [a] - a - (a - a - a)
- (Foo - Bool)
makeMemberFilter q val cmp =(\k - val `elem` (query_func q $ k))
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Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
On 2009 Jan 23, at 17:58, Olex P wrote: class Vector v where (^+^) :: v - v - v class Matrix m where (^+^) :: m - m - m You can't reuse the same operator in different classes. Vector owns (^+^), so Matrix can't use it itself. You could say instance Matrix m = Vector m where (^+^) = ... allowing Matrix to inherit Vector's operator (or turn it around the other way, make it Matrix then make Vector a Matrix), but that's only linguistically possible, not necessarily mathematically sane. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
2009/1/23 Brandon S. Allbery KF8NH allb...@ece.cmu.edu On 2009 Jan 23, at 17:58, Olex P wrote: class Vector v where (^+^) :: v - v - v class Matrix m where (^+^) :: m - m - m You can't reuse the same operator in different classes. Vector owns (^+^), so Matrix can't use it itself. You could say instance Matrix m = Vector m where (^+^) = ... No you can't! Stop thinking you can do that! It would be sane to do: class Vector m = Matrix m where -- matrix ops that don't make sense on vector Thus anything that implements Matrix must first implement Vector. Which is sane because matrices are square vectors with some additional structure, in some sense. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Laws and partial values (was: [Haskell-cafe] mapM_ - Monoid.Monad.map)
On 24 Jan 2009, at 02:33, Luke Palmer wrote: On Fri, Jan 23, 2009 at 6:10 PM, rocon...@theorem.ca wrote: On Fri, 23 Jan 2009, Derek Elkins wrote: mempty `mappend` undefined = undefined (left identity monoid law) The above definition doesn't meet this, similarly for the right identity monoid law. That only leaves one definition, () `mappend` () = () which does indeed satisfy the monoid laws. So the answer to the question is Yes. Another example of making things as lazy as possible going astray. I'd like to argue that laws, such as monoid laws, do not apply to partial values. But I haven't thought my position through yet. Please try to change your mind. I'd actually argue that this is just the wrong way of formulating my statement. Please correct my possibly ill informed maths, if Im doin it rong though... Isn't the point of bottom that it's the least defined value. Someone above made the assertion that for left identity to hold, _|_ `mappend` () must be _|_. But, as there is only one value in the Unit type, all values we have no information about must surely be that value, so this is akin to saying () `mappend` () must be (), which our definition gives us. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
