[Haskell-cafe] Re: XML transformation difficulties
Adding some code to go along with my last post:main = do [tree] <- runX (readDocument [(a_validate, "0")] " text.xml") [fooDoc ] <- runX (constA tree >>> processChildren isFoo) [expanded] <- runX (constA tree >>> processTopDown (expandNode fooDoc `when` hasName "bar")) [status] <- runX (constA expanded >>> writeDocument [] "-" >>> getErrStatus) (putStrLn . show) status where expandNode :: (ArrowXml a) => XmlTree -> a XmlTree XmlTree expandNode :: (ArrowXml a) => XmlTree -> a XmlTree XmlTree expandNode foos = this -- what here??? isFoo = deep (hasName "foo")The issue is that in 'expandNode', I have an XmlTree (foos) and an arrow whose input is the node to be replaced (also an XmlTree), but I need a String and an arrow whose input is the XmlTree (foos): expandNode' :: (ArrowXml a) => String -> a XmlTree XmlTreeexpandNode' name = processChildren (hasAttrValue "name" name)Somewhat related, if the output of an arrow is a string, can I get access to that string without using 'runX': getAttrValue :: String -> a XmlTree StringAre Arrows the wrong tool for this job?Thanks,GregOn 7/14/06, Greg Fitzgerald < [EMAIL PROTECTED]> wrote:> > I'm trying to think of a way to translate this input, to the output below:> > Input:> >>A > >> B>>>C > >> > >> > > Output:> >>B>A>> > > That is, anywhere there is a 'bar', replace it with the contents of the 'foo' it references. I'm having a difficult time representing this with HXT's Arrow API because the value of the 'ref' attribute is the output of an arrow, but I need it to be just a plain string so that I could use it as an input parameter to the 'hasAttrValue' function. A similar problem, using 'processTopDown', once I traverse to a 'bar' node, I need to then traverse the root again to find the 'foo', but I'm in the context of the 'bar' node, not the root. > > My ears are open to solutions with HaXML or Scrap Your XML-plate, or anything else.> > Thanks,> > Greg> > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comma in the front
Tim Docker wrote: These layouts feel a bit artificial to me. I am quite partial to python's list syntax - a trailing comma is optional. meaning you can write [ a, b, c, ] I'm surprised this approach isn't more widespread - Are there reasons why haskell syntax could not/should not be defined this way? I think this would be confusing because it looks like a syntax error - the last element is missing. The advantage of just having comma as a separator is that it allows syntax errors to be detected, whereas making it optional would mean one less error is detected at compile time. I'll take this opportunity to re-present my hyper-cool syntax proposal for lists ;-) : #[ a b c No need for commas or a closing ] because we could just use layout! Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type of a function?
On 7/14/06, Jenny678 <[EMAIL PROTECTED]> wrote: Hallo Can somebody tell me the type of the following function? func :: ? func f x = (z,y) where (z,y) = f x Let's try to reason about it. Let's start with some approximation to the type of 'func'. It appears to take two parameters so we'll say, func :: a -> b -> c Now we look at the first parameter, f, and see that it can be applied to x to get a tuple. f :: b -> (d, e) Now we can replace the the type variable 'a' with the signature for f. func :: (b -> (d, e)) -> b -> c But we also notice that the type (d, e) which is returned by f is also returned by func. func :: (b -> (d, e)) -> b -> (d, e) So there you have it. That's how you can reason about types. (and hopefully I didn't make a clerical error...) HTH, Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type of a function?
(leave off the line with func :: ??? and the compiler will figure it out for you, if possible---it works in this case) Jared. On 7/14/06, Jared Updike <[EMAIL PROTECTED]> wrote: Did you try putting this in a file, say, t.hsand running ghci t.hs then typing :type func at the GHCi prompt? It should tell you the function type. Jared. On 7/14/06, Jenny678 <[EMAIL PROTECTED]> wrote: > > Hallo > > Can somebody tell me the type of the following function? > > func :: ? > func f x = (z,y) > where (z,y) = f x > > Thanks > -- > View this message in context: http://www.nabble.com/Type-of-a-function--tf1945843.html#a5335607 > Sent from the Haskell - Haskell-Cafe forum at Nabble.com. > > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- http://www.updike.org/~jared/ reverse ")-:" ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type of a function?
Did you try putting this in a file, say, t.hsand running ghci t.hs then typing :type func at the GHCi prompt? It should tell you the function type. Jared. On 7/14/06, Jenny678 <[EMAIL PROTECTED]> wrote: Hallo Can somebody tell me the type of the following function? func :: ? func f x = (z,y) where (z,y) = f x Thanks -- View this message in context: http://www.nabble.com/Type-of-a-function--tf1945843.html#a5335607 Sent from the Haskell - Haskell-Cafe forum at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- http://www.updike.org/~jared/ reverse ")-:" ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] XML transformation difficulties
I'm trying to think of a way to translate this input, to the output below:Input: A B C Output: B A That is, anywhere there is a 'bar', replace it with the contents of the 'foo' it references. I'm having a difficult time representing this with HXT's Arrow API because the value of the 'ref' attribute is the output of an arrow, but I need it to be just a plain string so that I could use it as an input parameter to the 'hasAttrValue' function. A similar problem, using 'processTopDown', once I traverse to a 'bar' node, I need to then traverse the root again to find the 'foo', but I'm in the context of the 'bar' node, not the root. My ears are open to solutions with HaXML or Scrap Your XML-plate, or anything else.Thanks,Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Type of a function?
Hallo Can somebody tell me the type of the following function? func :: ? func f x = (z,y) where (z,y) = f x Thanks -- View this message in context: http://www.nabble.com/Type-of-a-function--tf1945843.html#a5335607 Sent from the Haskell - Haskell-Cafe forum at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Windows PowerShell "Monad"
Yep, that's its codename. Now, I'm not much of a Windows person. Is the name just a weird coincidence, or does it have anything to do with monads as we know them? http://en.wikipedia.org/wiki/MSH_(shell) -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Computing lazy and strict list operations at the same time
On Mon, Jun 19, 2006 at 05:50:13PM +0100, Duncan Coutts wrote:
> On Mon, 2006-06-19 at 17:03 +0100, Jon Fairbairn wrote:
> > il [] = error "foo"
> > il [x] = ([], x)
> > il (x:xs) = cof x (il xs)
> > where cof x ~(a,b) = (x:a, b)
> > -- !
>
> From a quick test, it looks like none of our suggested solutions
> actually work in constant space.
>
> main = interact $ \s ->
> case il s of
> (xs, x) -> let l = length xs
>in l `seq` show (l,x)
I was hoping to have enlightenment served to me, but since nobody has
explained this, I took a crack at it. I still can't explain it, but I
got some data that maybe somebody else will understand. My code:
initlast :: [a] -> ([a], a)
initlast [x]= ([], x)
initlast (x:xs) = let (init, last) = initlast xs
in (x:init, {-# SCC "last" #-} last)
lenshow n (_:xs) last = let n1 = n+1 in n1 `seq` lenshow n1 xs last
lenshow n [] last = show (n,last)
main = interact $ \s -> case initlast s of
(xs, x) -> lenshow 0 xs x
lenshow is just "show (length xs, x)", written out so I can tweak it
later. This exhibits the runaway space usage with a large input that
Duncan described. If you throw away "last" in lenshow and just "show
n", it runs in constant space.
It seems that the reference to "last" that I annotated as a cost center
is holding a chain of trivial thunks--trivial because "last" is just
being copied from the result of the recursive call to initlast. I
thought maybe I could get rid of them by returning an unboxed tuple from
initlast, but this turned out to make no difference.
Profiling gave a couple hints. Retainer set profiling (-hr) showed the
retainer set holding all the memory was
{}
I think this confirms that last holding a chain of thunks. I'm still
surprised that ghc doesn't see that they're trivial. It feels like it
should be an easy optimization.
Constructor and type profiling (-hd and -hy) both show the memory held
by stg_ap_1_upd_info. I don't know what that means.
Most frustrating, I can't find any work around: No matter how I tried
to write initlast, it had the same leak when consumed this way. (NB:
functional implementations only need apply.) Granted, I can't think of
any good reason to code in this style, but it's hard for me to accept
that it should be impossible.
Finally, here is a (silly) version that doesn't leak:
initlast :: [a] -> ([a], [a])
initlast [x]= ([], [x])
initlast (x:xs) = let (init, last) = initlast xs
in (x:init, undefined:last)
lenshow n (_:xs) (_:ls) = let n1 = n+1 in n1 `seq` lenshow n1 xs ls
lenshow n [] [last] = show (n,last)
This is the first case I recall in which adding more constructors makes
a space leak go away, because it gives me something to force (vis
"_:ls). With the original implementation, there was no way to "partly
force" last, one thunk at a time. Have others used this technique?
Andrew
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Re: [Haskell-cafe] The History of Haskell
Hello Simon, Friday, July 14, 2006, 7:21:26 PM, you wrote: > The History of Haskell how about naming it "Haskell: lazy programmer's language" ? :) -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ldap-haskell questions
On Thu, 13 Jul 2006, Ferenc Wagner wrote: ... > Second, I find no trace of SSL/TLS routines. Is that really > left out, or do I overlook something? OpenLDAP supports an option LDAP_OPT_X_TLS -- ldap_set_option Nothing LDAP_OPT_X_TLS LDAP_OPT_X_TLS_DEMAND ... ldapconnection <- ldap_initialize url I assume you will find something like that there (the above is actually from my own LDAP interface, but since I haven't seen any other response to this question ...) Have no idea about static binaries, even what that means. When I compile with ghc, I get reasonably static libraries inasmuch as there aren't any GHC shared objects, but of course libc is shared, as well as anything specified on the link command that turns out to be a shared library. If for example I have both a libldap.so and libldap.a and I want to link to the latter (as indeed I would), then it's up to me to say so on the link command, libldap.a instead of -lldap. Pardon me if that's obvious! Donn Cave, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] can't WashNGo (abstract tables)
Hi, trying to put WashNGo-2.9 to a nontrivial prototyping job gave some very compelling results so far, but also got me stumped on occasions. I'd be grateful for some guidance on the following points, concerning abstract tables mainly. * selectionDisplay: looks like displayFun (fourth arg) mustn't fail for null arguments, as it's invoked with them after leaving the page it appears on. An error or a feature? Is it documented? * trying to get around the "Strings only" restriction of AT's I threw in some show-read pairs, but got the impression that the channel is not Char-clean: Base64.encode . UTF8.encode helped. What's the spec? * should a click on an AT radio button result in submission of the form? It does not, which is OK with me. But the following code fails in a rather strange way: > data Record = P String Person | R String Role | Unknown String deriving > (Read,Show) > > unAtSafe :: String -> Record > unAtSafe = read . fst . UTF8.decode . Base64.decode > > edit sg = case record of > P dn p-> editPerson dn p > R dn r-> editRole dn r > Unknown s -> standardQuery "No dice" $ CGI.div $ -- provokes error > in AbstractSelector.hs:115 >do p $ text s > goBack > where record = unAtSafe $ head $ unAR $ value sg > > editPerson :: String -> Person -> CGI () > editPerson dn p = > standardQuery "Edit person" $ CGI.div $ > do inputs <- personTable p >actionPar "Modify" (createPerson dn) inputs In this case Person and Role contain String fields only. The abstract table contains Records, and the above function 'edit' dispatches on the type of the selected Record. The first two cases work, the Unknown case trips on an 'error' in the WashNGo code as indicated. It's in the valueSelectionGroup function, if selectionValue is Nothing. How could I fix this? * Lastly, how can I go back some queries? goBack above is basically a 'submit0 (return ())' which continues the main 'forever' loop. But what if I need to go back in the middle of the current loop, as if the user pressed Back several times? -- Thanks, Feri. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] The History of Haskell
Friends, Phil Wadler, John Hughes, Paul Hudak and I have been writing a paper about the The History of Haskell We've submitted an earlier draft to the History Of Programming Languages conference (HOPL'07), and it's been accepted. We have to submit a more-or-less final draft by 1 September. This message is to invite you to read it, tell us what you think, and help us improve it. Here it is, along with a Wiki page for you to write comments: http://haskell.org/haskellwiki/History_of_Haskell Enjoy! Simon PS: If you followed a link earlier today, and have a version dated Feb, discard it. The one you want is dated July 14th. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Partial Tuple application
Henning Thielemann schrieb: > I have seen $f(\cdot)$ instead of $f$ really often, as well as > $f(\cdot-k)$ for \x -> f(x-k). yes, but for first order functions only. Then parens can be seen as part of the identifier (as I showed for tuples before). Christian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
On Fri, Jul 14, 2006 at 02:28:20PM +0100, David House wrote: > On 14/07/06, David Roundy <[EMAIL PROTECTED]> wrote: > >Anyhow, just thought I'd mention that > >this isn't useful only for "ordinary" cyclic objects like dates. > > Correct. Which is why Chris Kuklewicz included instances for, e.g., Int :) Ah, that's what I get for just skimming the thread! :) > I think this would be a great class to have in the standard libs. It > would be even better if we could derive it. I agree, it sounds very nice (although I've obviously not even looked at the implementation). -- David Roundy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
David House wrote: On 14/07/06, David Roundy <[EMAIL PROTECTED]> wrote: Anyhow, just thought I'd mention that this isn't useful only for "ordinary" cyclic objects like dates. Correct. Which is why Chris Kuklewicz included instances for, e.g., Int :) And the new version takes everything to Integer so there is never any internal overflow. I think this would be a great class to have in the standard libs. It would be even better if we could derive it. All the existing instances are just three words: instance Cyclic Foo You only need more if you have something strange, or you want to avoid projecting to Integer. -- Chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
On 14/07/06, Johan Holmquist <[EMAIL PROTECTED]> wrote: > You mean [Monday, Tuesday ... Sunday, Monday] ? Actually not. No repetitions. That seems like a very bad idea. '..' normally means 'inclusive', breaking those semantics would be a very weird thing to do, and breaks the principle of least surprise. -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
On 14/07/06, David Roundy <[EMAIL PROTECTED]> wrote: Anyhow, just thought I'd mention that this isn't useful only for "ordinary" cyclic objects like dates. Correct. Which is why Chris Kuklewicz included instances for, e.g., Int :) I think this would be a great class to have in the standard libs. It would be even better if we could derive it. -- -David House, [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
Interestingly, your Cyclic class idea may have practical purposes beyond enumeration. Integers modulo some number are also cyclical, and can come in very handy. In fact, raw unsigned ints are modulo 2^32 (or something like that), so they really ought (under one interpretation) to be members of Cyclic rather than Bounded. It would certainly be more efficient, since you wouldn't need to do any overflow checking, and the cpu implements the Cyclic operations, but not the Bounded operations. Anyhow, just thought I'd mention that this isn't useful only for "ordinary" cyclic objects like dates. -- David Roundy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Building Data.Time on WinXP GHC6.4.1
Alistair Bayley wrote: Has anyone built Ashley's Data.Time package on Windows? I ask because timestuff.c fails to compile for me because it refers to fields (tm_zone, tm_gmtoff) in struct tm which don't seem to exist on Windows (at least, mingw). Yes, I made it work on Windows recently. The fixes should be in the darcs repo at http://darcs.haskell.org/packages/time, did you get it from there? Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is Day and Month bounded?
> Would not "liftM2 (,) [0..] [Sunday .. Saturday]" do the trick? You mean liftM2 (,) [0..] [January .. December] ? (I chose Months in order to get a sensible example.) Ah, yes. My mistake. I assume that the Bounded class exists in order to stay independent from the particular type. With minBound and maxBound you do not rely on whether the counting starts on Sunday or Monday if you only want to do something for all days. Now I see. Sure, your code can be used for any bounded type while mine is specific to Days (or Months). Well, to achieve the same with cyclic enums I would need yet another type class to get bounds which do not put any constraints on exceptions for succ and pred. That would be rather stupid, I think. Would be better with a dedicated cyclic enum class that do not have the succ and pred constraints for bounded instances. > Sure there are subtleties, like what should [Monday .. Monday] return, either: > [Monday] or > [Monday, Tuesday ... Sunday] > but to settle for one would be ok to me. You mean [Monday, Tuesday ... Sunday, Monday] ? Actually not. No repetitions. I also think that there must be a possibility to obtain an empty list. That would be tricky. Not sure why an empty list must be obtained. /Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
On Fri, 14 Jul 2006, Johan Holmquist wrote: > from Henning Thielemann: > > Since the days are cycling, what is more natural about your result > > compared to my one? > > I would prefer the one without any repetitions. > > > I assume the Bounded instance exists in order to allow loops like > > liftM2 (,) [0..] [minBound .. (maxBound::System.Time.Month)] > > Would not "liftM2 (,) [0..] [Sunday .. Saturday]" do the trick? You mean liftM2 (,) [0..] [January .. December] ? (I chose Months in order to get a sensible example.) I assume that the Bounded class exists in order to stay independent from the particular type. With minBound and maxBound you do not rely on whether the counting starts on Sunday or Monday if you only want to do something for all days. > Sure there are subtleties, like what should [Monday .. Monday] return, either: > [Monday] or > [Monday, Tuesday ... Sunday] > but to settle for one would be ok to me. You mean [Monday, Tuesday ... Sunday, Monday] ? I also think that there must be a possibility to obtain an empty list. Now I get it with [Monday .. Sunday] but I assume that you prefer [Monday, Tuesday ... Sunday] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Partial Tuple application
On Fri, 14 Jul 2006, Christian Maeder wrote: > Henning Thielemann schrieb: > >> in haskell-prime list there was a proposal to use '?' for such things: > >> > >> (1,2,?) > >> > >> only problem is what it's hard to define exactly where the lambda > >> should arise: > > the placeholder '?' becomes part of a new implicit identifier (that > exceeds Haskell's identifier syntax). If we have: > > (,,) :: a -> b -> c -> (a, b, c) > > we get a bunch of new identifiers: > > (?,,) :: b -> c -> a -> (a, b, c) > (,?,) :: a -> c -> b -> (a, b, c) > (,,?) :: a -> b -> c -> (a, b, c) > > And maybe also: > (?,?,) :: c -> a -> b -> (a, b, c) > > and so on. For infix ops this looks natural to me. I see. It would require that ? belongs to tuples exclusively and that it will never be used elsewhere, say for lists like in [1,2,?]. Because then the expression [(1,?)] becomes ambigous. Like with every kind of syntactic sugar, I fear that the next thing that people request, are expressions like (1,2,3+?) for \x -> (1,2,3+x) > > Yes that's a really evil problem that I already encountered in > > mathematics. Some mathematicians like to write f(·) or even f(·-k) which > > exhibits exactly the ambiguity you mention. Such placeholders are a really > > bad idea. > > "f(?)" should not make sense in Haskell, since the parens do not belong > to the identifier "f". (It's also not necessary: \ x -> f(x) = f) Necessary or not, applied mathematicians don't care about redundancy. :-) I have seen $f(\cdot)$ instead of $f$ really often, as well as $f(\cdot-k)$ for \x -> f(x-k). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is Day and Month bounded?
Yes, it would be possible to make ones own class and instantiate from that. I was just thinking Day and Month should be cyclic per default. from Henning Thielemann: Since the days are cycling, what is more natural about your result compared to my one? I would prefer the one without any repetitions. I assume the Bounded instance exists in order to allow loops like liftM2 (,) [0..] [minBound .. (maxBound::System.Time.Month)] Would not "liftM2 (,) [0..] [Sunday .. Saturday]" do the trick? Sure there are subtleties, like what should [Monday .. Monday] return, either: [Monday] or [Monday, Tuesday ... Sunday] but to settle for one would be ok to me. from Malcolm Wallace: But how do you feel about the way it breaks some algebraic laws? fromEnum x < fromEnum (succ x) fromEnum x > fromEnum (pred x) I would take the irresponsible and easy path and just accept it. As you said, it is not stated that any such relations must hold. Probably the cleanest approach is the one suggested by Chris Kuklewicz: Make the cyclings explicit in their own class and escape all surprises involved in sudden cyclic Enums. /Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Partial Tuple application (Was: Comma in the front)
Henning Thielemann schrieb: >> in haskell-prime list there was a proposal to use '?' for such things: >> >> (1,2,?) >> >> only problem is what it's hard to define exactly where the lambda >> should arise: the placeholder '?' becomes part of a new implicit identifier (that exceeds Haskell's identifier syntax). If we have: (,,) :: a -> b -> c -> (a, b, c) we get a bunch of new identifiers: (?,,) :: b -> c -> a -> (a, b, c) (,?,) :: a -> c -> b -> (a, b, c) (,,?) :: a -> b -> c -> (a, b, c) And maybe also: (?,?,) :: c -> a -> b -> (a, b, c) and so on. For infix ops this looks natural to me. >> >> (1,2,\x->x) I see no reason for this interpretation. >> (\x -> (1,2,x)) That should exactly be the implicit definition of my above implicit identifiers. > Yes that's a really evil problem that I already encountered in > mathematics. Some mathematicians like to write f(·) or even f(·-k) which > exhibits exactly the ambiguity you mention. Such placeholders are a really > bad idea. "f(?)" should not make sense in Haskell, since the parens do not belong to the identifier "f". (It's also not necessary: \ x -> f(x) = f) Cheers Christian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
Okay...final version attached.
This one fixes the toCycle bugs and changes from Int to Integer so overflow is
no longer an issue.
The result of cycleFromThenTo fits what I would expect, but you are free to drop
this or adapt it.
cycleFrom and cycleFromTo and cycleFromThen are easy, since there is no
difference between ascending and descending. Note that the returned list is
never null.
cycleFromThenTo can be either ascending or descending depending on the first two
arguments, and it considers the first occurrence of the third argument in that
direction of the cycle starting from the initial argument:
*Cycle> cycleFromThenTo Monday Wednesday Tuesday
[Monday]
instead of
*Cycle> cycleFromThenTo Monday Wednesday Tuesday
[Monday,Wednesday,Friday,Sunday,Tuesday]
This agrees with things like [1,3 .. 2] returning [1] and [3,1 .. 2] returning
[3].
{- By Chris Kuklewicz <[EMAIL PROTECTED]>
3 Clause BSD license, copyright 2006
-}
module Cycle (Cyclic(..)) where
import System.Time
import Data.Word
import Data.Int
default ()
class (Eq c,Enum c, Bounded c) => Cyclic c where
cyclePeriod :: c -> Integer
cyclePeriod _ = fromCycle (maxBound :: c)
- fromCycle (minBound :: c) + 1
succCycle :: c -> c
succCycle c | c == maxBound = minBound
| otherwise = succ c
predCycle :: c-> c
predCycle c | c == minBound = maxBound
| otherwise = pred c
fromCycle :: c -> Integer
fromCycle = toInteger . fromEnum
toCycle :: Integer -> c
toCycle = toEnum
. fromInteger
. (+ (fromCycle (minBound::c)))
. (`mod` (cyclePeriod (undefined::c)))
. (subtract (fromCycle (minBound::c)))
cycleFrom :: c -> [c]
cycleFrom x = map toCycle [fromCycle x ..]
cycleFromTo :: c -> c -> [c]
cycleFromTo x y = let xi = fromCycle x
yi = fromCycle y
zi = if xi > yi then yi + cyclePeriod (undefined::c)
else yi
in map toCycle [xi .. zi]
cycleFromThen :: c -> c -> [c]
cycleFromThen x y = let xi = fromCycle x
yi = fromCycle y
in map toCycle [xi, yi ..]
cycleFromThenTo :: c -> c -> c -> [c]
cycleFromThenTo x y z = let
c = cyclePeriod (undefined::c)
xi = fromCycle x; yi = fromCycle y; zi = fromCycle z
zi' = if xi <= yi
then if xi <= zi
then zi
else zi + c
else if zi <= xi
then zi
else zi - c
in map toCycle [xi, yi .. zi']
instance Cyclic ()
instance Cyclic Bool
instance Cyclic Ordering
instance Cyclic Int
instance Cyclic Char
instance Cyclic Day -- Imported from System.Time
instance Cyclic Month -- Imported from System.Time
instance Cyclic Int8-- Imported from GHC.Int
instance Cyclic Int64 -- Imported from GHC.Int
instance Cyclic Int32 -- Imported from GHC.Int
instance Cyclic Int16 -- Imported from GHC.Int
instance Cyclic Word8 -- Imported from GHC.Word
instance Cyclic Word64 -- Imported from GHC.Word
instance Cyclic Word32 -- Imported from GHC.Word
instance Cyclic Word16 -- Imported from GHC.Word
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Re: [Haskell-cafe] Re: Why is Day and Month bounded?
Hmmm... I think I should have said: toCycle :: Int -> c toCycle = toEnum . (+ (fromEnum (minBound::c))) . (`mod` (cyclePeriod (undefined::c))) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
Try this: module Cycle (Cyclic(..)) where import System.Time import Data.Word import Data.Int class (Eq c,Enum c, Bounded c) => Cyclic c where cyclePeriod :: c -> Int cyclePeriod _ = fromEnum (maxBound :: c) - fromEnum (minBound :: c) + 1 succCycle :: c -> c succCycle c | c == maxBound = minBound | otherwise = succ c predCycle :: c-> c predCycle c | c == minBound = maxBound | otherwise = pred c fromCycle :: c -> Int fromCycle = fromEnum toCycle :: Int -> c toCycle = toEnum . (`mod` (cyclePeriod (undefined::c))) cycleFrom :: c -> [c] cycleFrom x = map toCycle [fromCycle x ..] cycleFromTo :: c -> c -> [c] cycleFromTo x y = let xi = fromCycle x yi = fromCycle y zi = if xi > yi then yi + cyclePeriod (undefined::c) else yi in map toCycle [xi .. zi] cycleFromThen :: c -> c -> [c] cycleFromThen x y = let xi = fromCycle x yi = fromCycle y in map toCycle [xi, yi ..] cycleFromThenTo :: c -> c -> c -> [c] cycleFromThenTo x y z = let c = cyclePeriod (undefined::c) xi = fromCycle x; yi = fromCycle y; zi = fromCycle z zi' = if xi <= yi then if yi <= zi then zi else zi + c else if zi <= yi then zi else zi - c in map toCycle [xi, yi .. zi'] instance Cyclic Day instance Cyclic Month instance Cyclic Bool instance Cyclic () instance Cyclic Ordering instance Cyclic Int instance Cyclic Char instance Cyclic Int8-- Imported from GHC.Int instance Cyclic Int64 -- Imported from GHC.Int instance Cyclic Int32 -- Imported from GHC.Int instance Cyclic Int16 -- Imported from GHC.Int instance Cyclic Word8 -- Imported from GHC.Word instance Cyclic Word64 -- Imported from GHC.Word instance Cyclic Word32 -- Imported from GHC.Word instance Cyclic Word16 -- Imported from GHC.Word ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
"Johan Holmquist" <[EMAIL PROTECTED]> wrote: > If Day (and Month) where NOT instances of Bounded, the following would > be possible: > > [Saturday .. Tuesday] > => should return [Saturday, Sunday, Monday, Tuesday] > => but returns [] This does seem like a reasonable argument to me. Some enumerations are semantically cyclic, rather than linear. But how do you feel about the way it breaks some algebraic laws? fromEnum x < fromEnum (succ x) fromEnum x > fromEnum (pred x) Of course, such laws are not explicitly stated anywhere in the definition of the classes, so one would be foolish to rely on them in any case. Regards, Malcolm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Partial Tuple application (Was: Comma in the front)
On Fri, 14 Jul 2006, Bulat Ziganshin wrote: > Hello Tomasz, > > Friday, July 14, 2006, 9:31:32 AM, you wrote: > > >> There might be issues with tuples though, for example (1,2,) would be > >> the (,) tuple and not the (,,) tuple, which is a bit weird. > > > Besides, it might be a bit more natural if (1,2,) was a shorthand for > (\x ->> (1,2,x)) > > in haskell-prime list there was a proposal to use '?' for such things: > > (1,2,?) > > only problem is what it's hard to define exactly where the lambda > should arise: > > (1,2,\x->x) > (\x -> (1,2,x)) Yes that's a really evil problem that I already encountered in mathematics. Some mathematicians like to write f(�) or even f(�-k) which exhibits exactly the ambiguity you mention. Such placeholders are a really bad idea. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Why is Day and Month bounded?
On Fri, 14 Jul 2006, Johan Holmquist wrote: > I'll answer my own post to elaborate: > > If Day (and Month) where NOT instances of Bounded, the following would > be possible: > > [Monday .. Sunday] > => should return [Monday, Tuesday ... Saturday, Sunday] > => but returns [] Why not [Monday, Tuesday ... Saturday, Sunday, Monday, Tuesday ... Saturday, Sunday] ? Since the days are cycling, what is more natural about your result compared to my one? I assume the Bounded instance exists in order to allow loops like liftM2 (,) [0..] [minBound .. (maxBound::System.Time.Month)] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Why is Day and Month bounded?
I'll answer my own post to elaborate: If Day (and Month) where NOT instances of Bounded, the following would be possible: [Monday .. Sunday] => should return [Monday, Tuesday ... Saturday, Sunday] => but returns [] [Saturday .. Tuesday] => should return [Saturday, Sunday, Monday, Tuesday] => but returns [] succ Saturday => should return Sunday => but yields an exception It would feel natural to be able to get the desired results above. The Haskell98 report stipulates: --->8--- For any type that is an instance of class Bounded as well as Enum, the following should hold: The calls succ maxBound and pred minBound should result in a runtime error. --->8--- So, the standard library System.Time respects that and yields exception for succ Saturday. But if Day was not an instance of Bounded should it not be legal for succ Saturday to return Sunday? Now I cannot see the advantage of letting Day (and Month) be instances of Bounded. The only thing we get out of that is access to functions maxBound and minBound, but those do not seem useful to me for either Day or Month. /Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: Takusen 0.5
Oleg and I are pleased to announce the release of a new version of Takusen (it's been a while; so long that we don't remember the last version number we used). The most significant code change is a new internal design (courtesy of Oleg) which gives better separation of concerns like statement preparation, binding, and result-set processing. Also of note is a conversion to darcs, and hosting at haskell.org (thanks to Simon Marlow): darcs get http://darcs.haskell.org/takusen ... which also has the nice property that the Haddock documentation is browseable online: http://darcs.haskell.org/takusen/doc/html/index.html A comprehensive description of API usage can be found in the documentation for module Database.Enumerator: http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html We hope that the use of darcs will encourage contributions... Other changes: - support for multiple result sets returned from functions (PostgreSQL only). Oracle support for this is in the pipeline, which will allow use of nested cursors, and processing RefCursors returned as output parameters from procedure calls. Future plans: - Cabalisation - use of Data.Time instead of System.Time - ODBC and MS Sql Server backends For those of you unfamiliar with Takusen, here is our HCAR blurb: Takusen is a library for accessing DBMS's. It is a low-level library like HSQL, in the sense that it is used to issue SQL statements. Takusen's "unique-selling-point" is a design for processing query results using a left-fold enumerator. For queries the user creates an iteratee function, which is fed rows one-at-a-time from the result-set. We also support processing query results using a cursor interface, if you require finer-grained control. Currently we fully support Oracle, Sqlite, and PostgreSQL. Alistair ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Building Data.Time on WinXP GHC6.4.1
Has anyone built Ashley's Data.Time package on Windows? I ask because timestuff.c fails to compile for me because it refers to fields (tm_zone, tm_gmtoff) in struct tm which don't seem to exist on Windows (at least, mingw). Alistair ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Comma in the front
Hello Tomasz, Friday, July 14, 2006, 9:31:32 AM, you wrote: >> There might be issues with tuples though, for example (1,2,) would be >> the (,) tuple and not the (,,) tuple, which is a bit weird. > Besides, it might be a bit more natural if (1,2,) was a shorthand for (\x ->> (1,2,x)) in haskell-prime list there was a proposal to use '?' for such things: (1,2,?) only problem is what it's hard to define exactly where the lambda should arise: (1,2,\x->x) (\x -> (1,2,x)) or even at some more outer level. aside of this, it would be very handy addition to various existing partial application styles: f a `f` a a `f` (*2) (2*) (*) -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
