- Haskell-Cafe - Haskell.org

Iteratee rest and monad instance
by Tilo Wiklund 12 Jun '10

12 Jun '10

I've mostly been studying the Iteratee IO slides (with notes), so these things may already have been noted elsewhere (though I have not been able to find them with my cursory search). Also excuse my probably somewhat confused terminology, I hope I get the point across anyway. In both the slides and the current hackage package "finished" iteratees carry with them (in the constructor) the rest of the stream. I may miss something obvious here, but this seems to lead to some fishy details in the monad instance of iteratees presented in the slides. Firstly the iteratee given by return/pure has to carry with it a rest before a stream comes into the picture, though I guess one could argue that (Chunk "") is some kind of identity but this does not seem to be enforced in any way. Furthermore in the bind operator the rest of the iteratee from the bound (RHS) function seems to be ignored at some times and not at others (depending on the rest of the bound LHS value). I am probably missing something obvious or something relating to optimisation/server software but defining iteratees as "Iteratee s a = Cont (s -> Either (s, a) (Iteratee s a))" seems to lead to a more natural monad instance, and does not suffer from (what seems to me to be an issue) having to make predictions about the stream it will be passed. My question is thus, why the "rest" stream has to be carried by an Iteratee constructor, rather then be part of the value of the continuation function.

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GATD and pattern matching
by Dupont Corentin 12 Jun '10

12 Jun '10

Hello, I am making a little GATD: > {-# LANGUAGE GADTs#-} > data Obs a where > Equal :: Obs a -> Obs a -> Obs Bool > Plus :: (Num a) => Obs a -> Obs a -> Obs a (etc..) > instance Show t => Show (Obs t) where > show (Equal a b) = (show a) ++ " Equal " ++ (show b) > show (Plus a b) = (show a) ++ " Plus " ++ (show b) > instance Eq t => Eq (Obs t) where > a `Equal` b == c `Equal` d = (a == c) && (b == d) > a `Plus` b == c `Plus` d = (a == c) && (b == d) The Equal constructor causes me problems, while the Plus is fine: test3.lhs:8:26: Could not deduce (Show a) from the context (t ~ Bool) arising from a use of `show' at test3.lhs:8:26-31 Possible fix: add (Show a) to the context of the constructor `Equal' In the first argument of `(++)', namely `(show a)' In the expression: (show a) ++ " Equal " ++ (show b) In the definition of `show': show (Equal a b) = (show a) ++ " Equal " ++ (show b) I guess this is because GADT refines type with pattern matching. Since Equal return type is Obs Bool and not Obs a, it cannot bind the type variable a to belong to Show. I don't see how to precise the type in the pattern match. I have another problem: test3.lhs:12:41: Couldn't match expected type `a' against inferred type `a1' `a' is a rigid type variable bound by the constructor `Equal' at test3.lhs:12:8 `a1' is a rigid type variable bound by the constructor `Equal' at test3.lhs:12:23 Expected type: Obs a Inferred type: Obs a1 In the second argument of `(==)', namely `c' In the first argument of `(&&)', namely `(a == c)' Cheers, Corentin

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Re: [Haskell-cafe] Problems with threading?
by Isaac Gouy 11 Jun '10

11 Jun '10

--- On Thu, 6/10/10, Louis Wasserman <wasserman.louis(a)gmail.com> wrote: > Date: Thursday, June 10, 2010, 1:32 AM > Yeah, Control.Parallel would be nice to have. Heck, ideally I could get > the whole Haskell Platform, which would be a reasonable comparison to > the huge Java and C++ libraries accessible to those languages. And having the whole Haskell Platform still isn't enough to compile those regex-dna programs.

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Code review of text-xml-generic
by Oscar Finnsson 11 Jun '10

11 Jun '10

Hi, I've been working on a (de)serialization package from/to XML (using SYB) and wondering if anyone feels like giving me a quick code review before I upload it to hackagedb. The source code can be found at http://github.com/finnsson/Text.XML.Generic It is *heavily* inspired by Text.JSON.Generic. The three files DataEx.hs (slightly modified version of Olegs code), ExGeneric.hs and ExTestGeneric.hs are not part of the package yet since I haven't got the deserialization to work for existentials yet (but I haven't given up!). I plan to upload the package to hackagedb by the end of this weekend so any comments before (and after) are more than welcome. -- Oscar

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Encoding the encoding type of a string into its type
by Christopher Done 11 Jun '10

11 Jun '10

There are a lot of issues with string encoding type mismatches. Especially "automatic" conversions. This mailing list gets enough posts about encoding confusions. Would it make sense to make the string depend on its encoding type? E.g. a String UTF16 cannot be used with putStrLn :: String UTF8, it has to be used with putStrLn :: String UTF16. Provided the fundamental functions that read and write strings are type safe, there'll be no mix-ups? I'll think about this more later. Just putting the question out there so that I remember when I get home.

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Re: [Haskell] Woes on MacOS 10.6 - linking issues
by Pepe Iborra 11 Jun '10

11 Jun '10

(moving to the cafe) There is an incompatibility between the version of iconv in Mac Os and the one included in MacPorts. As the RTS of the build of ghc in the Haskell Platform is linked against Mac Os iconv, you cannot use it with the build of gtk2hs from MacPorts. This is a known issue already commented elsewhere, in case you haven't found it already: http://www.mail-archive.com/haskell-cafe@haskell.org/msg69139.html http://stackoverflow.com/questions/2726248/ghc-6-12-and-macports Cheers, On Fri, Jun 11, 2010 at 4:53 AM, Brett Giles <brett.giles(a)ucalgary.ca> wrote: > Hi Folks > > I seem to have Gtk2HS 0.11 installed, but not quite working. Interestingly, I can run a demo, such as the hello/World.hs example, directly in ghci. However, when I try to do a ghc --make on any code containing gtk2hs I get a link error like this: > > Undefined symbols: > "_iconv_close", referenced from: > _hs_iconv_close in libHSbase-4.2.0.0.a(iconv.o) > (maybe you meant: _hs_iconv_close) > "_iconv", referenced from: > _hs_iconv in libHSbase-4.2.0.0.a(iconv.o) > (maybe you meant: _hs_iconv_open, _hs_iconv , _hs_iconv_close ) > "_iconv_open", referenced from: > _hs_iconv_open in libHSbase-4.2.0.0.a(iconv.o) > (maybe you meant: _hs_iconv_open) > ld: symbol(s) not found > > > I do have libiconv installed as a universal library via macports. gtk, glade etc., are also universal installed via macports. > > I downloaded the OSX Haskell Platform package and am running ghc 6.12.1 > > Other programs seem to be having some issues as well though, For instance, a command line program seems to compile fine, but when it runs I get the message: > > $ emlqpl (<<<--- My "successfully" compiled program - batch only, no gtk items) > dyld: Library not loaded: /opt/local/lib/libgtk-quartz-2.0.0.dylib > Referenced from: /usr/local/bin/emlqpl > Reason: image not found > Trace/BPT trap > > > > Does anyone have any suggestions? > > Brett Giles > Grad Student, Formal Methods, Category Theory, > University of Calgary > brett.giles(a)ucalgary.ca > > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > -- -- José Iborra http://www.dsic.upv.es/~jiborra -- UPV Valencia Telf. (+34) 96 387 00 00 (ext) 83529 -- Camino de Vera s/n. 46022 Valencia (Spain)

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Re: package naming policy (was: Please check your dependencies on fgl)
by John Lato 11 Jun '10

11 Jun '10

> From: Don Stewart <dons(a)galois.com> > > ivan.miljenovic: >> Thomas Bereknyei are currently re-writing fgl (just about completely >> from scratch) and we plan to make an initial release to get feedback >> on the API in the next few weeks. >> >> However, I'm sending this email out now to warn people that I highly >> doubt any code that was written for the current version of fgl >> (http://hackage.haskell.org/package/fgl-5.4.2.2) will work with the >> new version. > > How about you give the library a different name then -- so as not to > break all those programs? > > A complete rewrite with a new maintainer: fgl-awesome I would like to argue against this practice, i.e. re-naming new, incompatible versions of existing packages. I think it's bad for the following reasons: 1. It makes development by new users more difficult by fracturing the package-space (the "Which version of QuickCheck should I use?" problem). Since this is already an acknowledged issue, I think it's better that developers not add to it. 2. It discourages adoption of the latest version despite any benefits the later version may provide. This also leads to greater incompatibility between dependent packages. 3. For packages in the platform, I believe this will create uncertainty about which package(s) should be included with new major releases. 4. It adds to the maintainer workload as the same person or team will often be responsible for both packages. I do agree that there are legitimate reasons why users may decide to remain with older versions, however I think that in nearly all cases the proper solution is to follow the PVP and for users to include upper dependency bounds in .cabal files. In particular, for the very common case of compatibility with older code, an upper dependency bound seems like the correct approach. IMHO changing a package name should be for when developers intend to continue development (not just maintenance releases) along both branches. My 2 cents, anyway. John

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Re: [Haskell-cafe] How to "Show" an Operation?
by Martin Drautzburg 11 Jun '10

11 Jun '10

On Monday, 7. June 2010 23:28:08 Evan Laforge wrote: > I just meant you could add instances: > > instance Functor (Named a) where fmap f named = named { val_of = f > (val_of named) } > instance Applicative (Named a) where ... likewise, but maybe not a > great fit unless you have a "no name" for 'pure' So far so good. However my "Named" things are all functions and I don't see I ever want to map over any of them. But what I'd like to do is use them like ordinary functions as in: f::Named (Int->Int) f x Is there a way to do this, other than writing apply::Named Int ->Int apply n x = (val_of n) x -- Martin

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Help with Bird problem 3.3.3
by Peter Hilal 11 Jun '10

11 Jun '10

I'm working my way through Bird's _Introduction to Functional Programming Using Haskell_. I'd appreciate any help with problem 3.3.3, which is: "Division of natural numbers can be specified by the condition (n * m) / n = m for all positive n and all m. Construct a program for division and prove that it meets the specification." The required construction relies on the following definitions: data Nat = Zero| Succ Nat (+) :: Nat -> Nat m + Zero = m m + Succ n = Succ (m + n) (*) :: Nat -> Nat m * Zero = Zero m * Succ n = m * n + m Proceeding as Bird does in Sec. 3.2.2, where he derives the definition of "-" from the specification "(m + n) - n = m", I've so far gotten the first case, in which m matches the pattern "Zero", simply by (i) substituting Zero for m in the specification, (ii) substituting Succ n for n in the specification (solely because n is constrained to be positive), and (iii) reducing by applying the first equation of "*": Case Zero: (Succ n * Zero) / Succ n = Zero ≡ {first equation of "*"} Zero / Succ n = Zero Easy enough, and completely intuitive, since we expect Zero divided by any non-Zero finite number to be Zero. The next case, in which m matches the pattern "Succ m", is where I get stuck, and I really have no intuition about what the definition is supposed to be. My first step is to substitute "Succ m" for "m" in the given specification, and to substitute Succ n for n in the specification (for the same reason, as above, that n is constrained to be positive), and then to use the definition of "*" to reduce the equation: Case Succ m: Succ n * Succ m / Succ n = Succ m ≡ {second equation of "*"} (Succ n * m + Succ n) / Succ n = Succ m Where do I go from here? Thank you so much.

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Thread scheduling
by Andrew Coppin 11 Jun '10

11 Jun '10

Control.Concurrent provides the threadDelay function, which allows you to make the current thread sleep until T=now+X. However, I can't find any way of making the current thread sleep until T=X. In other words, I want to specify an absolute wakeup time, not a relative one. What's even more frustrating is that, reading the source code, it appears that the internal RTS primitive *is* actually using an absolute time, but this isn't exposed anywhere that I can get at. Is there any danger that this could be fixed at some point? (I might also interject something about Data.Time, which seems to hardly let me do anything, but strictly that's a seperate topic...)

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