Re: [Haskell-cafe] Non-termination due to context
OK, I'll try to get to the SYB3 paper at some point. For now I'll just
add to my knowledge that UndecidableInstances allows you to create
non-terminating dictionaries in addition to the well-known risk of
making the type checker loop.
Thanks!
/ Emil
Simon Peyton-Jones skrev:
It's a feature!
You have
* B is a superclass of A
* Eq is a superclass of B
So every A dictionary has a B dictionary inside it, and every B dictionary has
an Eq dictionary inside it.
Now, your instance declaration
instance (A a, Eq a) = B a
says if you give me an A dictionary and an Eq dictionary, I'll make you a B
dictionary.
Now, 'test' needs a (B Int) dictionary. To get one, we need an (A Int) dictionary and an (Eq Int) dictionary. But
when solving these sub-problems, GHC assumes that you
have in hand a solution to the original problem, this case (B Int)
Why? Read the SYB3 paper.
OK so now you see the problem: we can solve the (A Int) and (Eq Int)
sub-problems by selection from the (B Int) dictionary.
Still, I confess that I have not fully grokked the relationship between the
SYB3-style recursion stuff and the question of superclasses. So I will think
about your example some more, thank you.
Meanwhile, it's clear that you are on thin ice.
Simon
| -Original Message-
| From: haskell-cafe-boun...@haskell.org
[mailto:haskell-cafe-boun...@haskell.org] On
| Behalf Of Emil Axelsson
| Sent: 22 January 2010 11:25
| To: Haskell Cafe
| Subject: [Haskell-cafe] Non-termination due to context
|
| Hello all!
|
| Consider the following program:
|
| {-# LANGUAGE FlexibleInstances, OverlappingInstances, UndecidableInstances #-}
|
| class B a = A a
|
| instance A Int
|
| class Eq a = B a
|
| instance (A a, Eq a) = B a
|
| eq :: B a = a - a - Bool
| eq = (==)
|
| test = 1 `eq` (2::Int)
|
| (This is a condensed version of a much larger program that I've been
| debugging.)
|
| It compiles just fine, but `test` doesn't terminate (GHCi 6.10.4). If I
| change the context `B a` to `Eq a` for the function `eq`, it terminates.
|
| Although I don't know all the details of the class system, it seems
| unintuitive that I can make a program non-terminating just by changing
| the context of a function (regardless of UndecidableInstances etc.).
|
| Is this a bug or a feature?
|
| / Emil
|
| ___
| 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] Failing to install hxt: tagsoup versioning
Specifying one exact version is too restrictive, you'll end up with everybody having umpteen versions of almost all packages installed. I don't see this as a problem. Disk space is extremely cheap, but our time is precious. Aplications on Mac OS X provide necessary libraries in their .app folder. That's some waste but it relieves you from dependency hell. Minor version bumps which leave the API unchanged shouldn't break anything, small additions to the API should rarely break things, so if people adhere to http://www.haskell.org/haskellwiki/Package_versioning_policy , Indeed. Neil, in this case i think, tagsoup's version shouldn't have changed from 0.6 to minor 0.8 while exported definitions such as optLookupEntity were removed. What do you think? I'd be glad if we could find a general solution. Currently everybody using ghc 6.10.4 and trying to install hxt will go through this hassle. Which would be almost everybody.___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ghc: unrecognised flags: -I
Lyndon Maydwell schrieb: For example, when I cabal install -v storable-complex I get the following: /usr/bin/ghc -package-name storable-complex-0.2.1 --make -hide-all-packages -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -I -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package base-3.0.3.1 -O Foreign.Storable.Complex ghc: unrecognised flags: -I For me this works (and does not have that single -I): /home/mac-bkb/bin/ghc -package-name storable-complex-0.2.1 --make -hide-all-packages -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package base-3.0.3.1 -O Foreign.Storable.Complex Has anyone encountered this before, or more realistically, can anyone give me some advice on how to narrow this problem down further? Sorry, no idea. I'm running cabal version 1.6.0.1, ghc 6.10.4 on OS X 10.5. I've got Cabal-1.6.0.3, ghc 6.10.4 on OS X 10.5 (Intel) cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library Christian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Failing to install hxt: tagsoup versioning
Minor version bumps which leave the API unchanged shouldn't break anything, http://www.haskell.org/haskellwiki/Package_versioning_policy Neil, in this case i think, tagsoup's version shouldn't have changed from 0.6 to minor 0.8 while exported definitions such as optLookupEntity were removed. Changing version number 0.6 - 0.8 is a major release increment, not a minor one, according the package versioning policy linked to. Regards, Malcolm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Failing to install hxt: tagsoup versioning
Am Montag 25 Januar 2010 14:31:07 schrieb Malcolm Wallace: Minor version bumps which leave the API unchanged shouldn't break anything, http://www.haskell.org/haskellwiki/Package_versioning_policy Neil, in this case i think, tagsoup's version shouldn't have changed from 0.6 to minor 0.8 while exported definitions such as optLookupEntity were removed. Changing version number 0.6 - 0.8 is a major release increment, not a minor one, according the package versioning policy linked to. Which I found rather irritating initially. I had 'learned' that version numbers go Major.Minor.Patchlevel[.optionalLineNoise] and then coming to terms with Major.major.Minor[.optionalStuff] took a couple of days. Regards, Malcolm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Naive booleans and numbers - type-checking fails
As other people have said, you need higher rank types for this.
Lets start with just pairs to get you headed in the right direction:
{-# LANGUAGE RankNTypes #-}
(As an aside, almost all of my real programs require at least rank-2
types, so I usually turn on RankNTypes on principle)
mpair :: a - b - (a - b - c) - c
mpair f s k = k f s
mfst p = p (\x y - x)
msnd p = p (\x y - y)
Here is where it gets tricky. GHC infers these types:
mfst :: ((a - b - a) - c) - c
msnd :: ((a - b - b) - c) - c
But these aren't the right types; if you try to use both mfst and msnd
on the same pair, you will unify a and b which is almost certainly
wrong; it says the first and second element of the pair are the same
type.
Now, lets look at the type signature for the partially-applied mpair;
given x :: A, and y :: B, we have
mpair x y :: (A - B - c) - c
Notice this: a pair is a polymorphic function! Lets make functions
that take pairs specify that explicitly:
mfst :: (forall c. (a - b - c) - c) - a
msnd :: (forall c. (a - b - c) - c) - b
Here we tell the typechecker that mfst and msnd are required to be
passed polymorphic functions; this makes us free to determine the
result type when we call p, and the same pair can be passed to both
of these functions successfully. The placement of the parentheses
around the forall is very important, because that's how we specify
where the polymorphism is required.
Similar tricks generally need to be used when defining church
numerals, if you are headed in that direction. These sort of objects
which work in untyped lambda calculus are just not expressible in the
simply typed lambda calculus, even with rank-1 polymorphism. Once you
move to full System F, a lot more becomes possible.
-- ryan
2010/1/24 Dušan Kolář ko...@fit.vutbr.cz:
Dear cafe,
I'm trying to prepare a naive definition of booleans, numbers and some
helper functions such a way, so that definition in lambda-calculus can be
used in a straightforward way in Haskell. I was caught in trouble quite soon
during change of lambda-expressions to Haskell - defining prefn as a helper
for prev. When using Haskell-ish if-then-else then there is no problem (the
code commented out), while using defined if-then-else (mif), the
type-checking fails, but just for this case! Other simple tests are OK for
the mif. Do I need some extra option for type-checker, or is it a principal
failure (infinite type is reported) - I'm running ghci 6.10.4.
mtrue x y = x
mfalse x y = y
m0 f n = n
m1 f n = f n
m2 f n = f (f n)
msucc x g m = x g (g m)
iszero m = m (\_ - mfalse) mtrue
mif c t f = c t f
mpair f s = \e - e f s
mfst p = p mtrue
msnd p = p mfalse
-- mprefn f p = if mex True False then mth else mel
mprefn f p = mif mex mth mel
where
mex = mfst p
mth = mpair mfalse (msnd p)
mel = mpair mfalse (f (msnd p))
Please, change of definitions is not a solution, I'm trying to follow
available resources, so using something else is not an option. :-(
Thanks for any help
Dusan
___
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] HList darcs repo missing?
Hello all, I was looking for the HList darcs repo at: http://darcs.haskell.org/HList/ but it seems to be missing. Has it been moved somewhere else? Thanks, Jake Wheat ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Failing to install hxt: tagsoup versioning
Hi Minor version bumps which leave the API unchanged shouldn't break anything, small additions to the API should rarely break things, so if people adhere to http://www.haskell.org/haskellwiki/Package_versioning_policy , Neil, in this case i think, tagsoup's version shouldn't have changed from 0.6 to minor 0.8 while exported definitions such as optLookupEntity were removed. As Malcolm says, the rule is that 0.6 - 0.8 I can do absolutely anything. If it's any consolation, I'm intending 0.9 to be perfectly compatible with 0.8. If there's anything I can do to make the hxt thing a bit easier I'm happy to do so. The change to remove optLookupEntity was necessary to support HTML 5 correctly, so I didn't really have a choice. In general I do try and keep the API as stable as possible. Thanks, Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Adopting hpodder?
Hi folks, I wrote hpodder a little while back. I no longer listen to podcasts at all (blame my Kindle for that). Is anyone here interested in adopting it and giving it some needed care? Thanks, -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HList darcs repo missing?
On Mon, Jan 25, 2010 at 3:47 PM, Jake Wheat jakewheatm...@googlemail.com wrote: Hello all, I was looking for the HList darcs repo at: http://darcs.haskell.org/HList/ but it seems to be missing. Has it been moved somewhere else? Thanks, Jake Wheat It was there as of 15 September 2009 when I sent Oleg my last patch. Maybe some of the server changes since messed around with it? -- gwern ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: ftdi-0.1
Hello, I just released a very early version of my 'ftdi' library [1]. It is a small layer on top of the usb package [2] that enables you to communicate with FTDI [3] devices. Its design is based on the C-library libftdi [4], but it implemented completely in Haskell. Most functionality is untested. I only have access to an FT2232C chip, so testing other chips is not possible at the moment. Furthermore, the only thing I do with that chip is perform bulk reads and the occasional small (1 byte) bulk write. The good news is that it is quite good a bulk reads. It outperformed both libftdi-0.16 and the official driver [5] by about 40% for my use case. Although to be fair, both C libraries used the old libusb-0.1 library. This library is ultimately linked with libusb-1.0.* which is a complete rewrite. To play with the library either cabal install ftdi or darcs get http://code.haskell.org/~roelvandijk/code/ftdi Regards, Roel van Dijk 1 - http://hackage.haskell.org/package/ftdi 2 - http://ftdichip.com/ 3 - http://hackage.haskell.org/package/usb 4 - http://www.intra2net.com/en/developer/libftdi/ 5 - http://www.ftdichip.com/Drivers/D2XX.htm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HList darcs repo missing?
On Mon, Jan 25, 2010 at 05:27:57PM -0500, Gwern Branwen wrote: It was there as of 15 September 2009 when I sent Oleg my last patch. Maybe some of the server changes since messed around with it? Most things that could be moved to community.haskell.org weren't moved across to the new machine: http://www.haskell.org/pipermail/haskell/2010-January/021861.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HList darcs repo missing?
2010/1/25 Ross Paterson r...@soi.city.ac.uk: Most things that could be moved to community.haskell.org weren't moved across to the new machine: http://www.haskell.org/pipermail/haskell/2010-January/021861.html Thanks, I found what seems to be the latest version here (last update 14th Jan 2010): http://old-darcs.well-typed.com/HList/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Codensity improvement of free monads
Hello haskell-cafe, I have just read Asymptotic Improvement of Computations over Free Monads by Janis Voigtlander, since I have been working with free monads a lot recently and don't want to get hit by their quadratic performance when I start to care about that. But after reading the paper, I still don't really understand how improve actually improves anything. Can anyone provide a good explanation for where the work is saved? Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Codensity improvement of free monads
On Mon, Jan 25, 2010 at 7:37 PM, Luke Palmer lrpal...@gmail.com wrote: Hello haskell-cafe, I have just read Asymptotic Improvement of Computations over Free Monads by Janis Voigtlander, since I have been working with free monads a lot recently and don't want to get hit by their quadratic performance when I start to care about that. But after reading the paper, I still don't really understand how improve actually improves anything. Can anyone provide a good explanation for where the work is saved? With a free monad, the structure keeps growing via substitution. This requires you on each bind to re-traverse the common root of the free monad, which isn't changing in each successive version. Lets say the size of this accumulated detritus increases by one each time. Then you will be traversing over 1, 2, 3, 4, ... items to get to the values that you are substituting as you keep binding your free monadic computation. The area near the root of your structure keeps getting walked over and over, but there isn't any work do to there, the only thing bind can do is substitution, and all the substitution is down by the leaves. On the other hand, when you have CPS transformed it, at each layer you only have to climb over the 1 item you just added to get to where you apply the next computation. This only gets better when you are dealing with free monads that provide multiple points for extension: i.e. that grow in a treelike fashion like: data Bin a = Bin a a data Free f a = Return a | Free (f (Free f a)) data Codensity f a = Codensity (forall r. (a - f r) - f r) If you build the free monad Free Bin, then you will wind up having to walk all the way down to the leaves on each bind, well, modulo demand, that is. But since it is a free monad, the 'body' of the tree will never change. Just the data you are substituting at the leaves. Codensity (Free Bin) lets you only generate the body of the tree once, while your substitutions just get pushed down to the leaves in one pass. An interesting exercise is to work out why Codensity can change the asymptotics of certain operations over Free Bin, but Density doesn't change the asymptotics of anything non-trivial over Cofree Bin for the better. -Edward Kmett Luke ___ 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] ANN: The Monad.Reader Issue 15
I am very pleased to announce that Issue 15 of The Monad.Reader is now available for your reading pleasure [1]. Issue 15 consists of the following four articles: * The hp2any project by Gergely Patai * Adventures in Three Monads by Edward Z. Yang * The Operational Monad Tutorial by Heinrich Apfelmus * Implementing STM in pure Haskell by Andrew Coppin Feel free to browse the source files. You can check out the entire repository using darcs: darcs get http://code.haskell.org/~byorgey/TMR/Issue15 If you'd like to write something for Issue 16, please get in touch. The deadline will likely be sometime in mid-late April; more details will be forthcoming. -Brent [1] http://themonadreader.files.wordpress.com/2010/01/issue15.pdf ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: could we get a Data instance for Data.Text.Text?
On Sun, Jan 24, 2010 at 5:49 AM, Neil Mitchell ndmitch...@gmail.com wrote: Hi, The problem with Data for Text isn't that we have to write a new instance, but that you could argue that proper handling of Text with Data would not be using a type class, but have special knowledge baked in to Data. That's far worse than the Serialise problem mentioned above, and no one other than the Data authors could solve it. Of course, I don't believe that, but it is a possible interpretation. Right.. that is the problem with Text. Do you think the correct thing to do for gunfold and toConstr is to convert the Text to a String and then call the gufold and toConstr for String? Or something else? The Serialise problem is a serious one. I can't think of any good solutions, but I recommend you give knowledge of your serialise class to Derive (http://community.haskell.org/~ndm/derive/) and then at least the instances can be auto-generated. Writing lots of boilerplate and regularly ripping it up is annoying, setting up something to generate it for you reduces the pain. We currently use template haskell to generate the Serialize instances in most cases (though some data types have more optimized encodings that were written by hand). However, you must supply the Version and Migration instances by hand (they are super classes of Serialize). I am all for splitting the Serialize stuff out of happstack .. it is not really happstack specific. Though I suspect pulling it out is not entirely trivial either. I think the existing code depends on syb-with-class. - jeremy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] imaging textbooks?
Hey all Can anyone recommend a good textbook on computer vision or image processing? I don't care if it favors Haskell or not, I'm just trying to find a good text. -Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] imaging textbooks?
I have just started reading this so I don't know how good it is yet but the draft is freely available: http://research.microsoft.com/en-us/um/people/szeliski/Book/ Best Keith 2010/1/25 Dan Mead d.w.m...@gmail.com: Hey all Can anyone recommend a good textbook on computer vision or image processing? I don't care if it favors Haskell or not, I'm just trying to find a good text. -Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- keithsheppard.name ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] scheduling an alarm
I cobbled the following together from bits and pieces I found on mailing lists. Seems straightforward, but the code I've included just sits there. The awaitSignal seems to be causing a problem, in that if I remove it (and don't call forever recursively) then I get my ALARM 5 sec later. Otherwise nothing. The problem is that if I don't include awaitSignal, then, of course, forever is called just as fast as the CPU will go :-) Brian import System.Posix.Signals import System.IO import Control.Concurrent alarm = do putStrLn ALARM hFlush stdout return () forever = do putStrLn foo scheduleAlarm 5 awaitSignal Nothing yield forever main = do installHandler realTimeAlarm (Catch alarm) Nothing forever ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] imaging textbooks?
Handbook of Computer Vision Algorithms in Image Algebra I started to read it felt it was a solid text but just don't have the time. Thomas http://www.amazon.com/Handbook-Computer-Vision-Algorithms-Algebra/dp/0849300754/ref=sr_1_1?ie=UTF8s=booksqid=1264487502sr=8-1 2010/1/25 Keith Sheppard keiths...@gmail.com: I have just started reading this so I don't know how good it is yet but the draft is freely available: http://research.microsoft.com/en-us/um/people/szeliski/Book/ Best Keith 2010/1/25 Dan Mead d.w.m...@gmail.com: Hey all Can anyone recommend a good textbook on computer vision or image processing? I don't care if it favors Haskell or not, I'm just trying to find a good text. -Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- keithsheppard.name ___ 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] scheduling an alarm
1) Don't use System.Posix.Signals It isn't necessary and makes your code less portable 2) The POSIX SIGALRM is used/caught by the RTS and that is why you are seeing strange behavior. 3) Consider using Haskell exceptions from Control.Concurrent (throwTo). Not sure what you want to do but you can always myThreadId = \tid - forkIO $ threadDelay someDelayTime (throwTo tid someExceptionVal) Thomas On Mon, Jan 25, 2010 at 10:25 PM, Brian Denheyer bri...@aracnet.com wrote: I cobbled the following together from bits and pieces I found on mailing lists. Seems straightforward, but the code I've included just sits there. The awaitSignal seems to be causing a problem, in that if I remove it (and don't call forever recursively) then I get my ALARM 5 sec later. Otherwise nothing. The problem is that if I don't include awaitSignal, then, of course, forever is called just as fast as the CPU will go :-) Brian import System.Posix.Signals import System.IO import Control.Concurrent alarm = do putStrLn ALARM hFlush stdout return () forever = do putStrLn foo scheduleAlarm 5 awaitSignal Nothing yield forever main = do installHandler realTimeAlarm (Catch alarm) Nothing forever ___ 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] Re: could we get a Data instance for Data.Text.Text?
Hi Jeremy, As Neil Mitchell said before, if you really don't want to expose the internals of Text (by just using a derived instance) then you have no other alternative than to use String conversion. If you've been using it already and performance is not a big problem, then I guess it's ok. Regarding the Serialize issue, maybe I am not understanding the problem correctly: isn't that just another generic function? There are generic implementations of binary get and put for at least two generic programming libraries in Hackage [1, 2], and writing one for SYB shouldn't be hard either, I think. Then you could have a trivial way of generating instances of Serialize, namely something like instance Serialize MyType where getCopy = gget putCopy = gput and you could provide Template Haskell code for generating these. Or even just do instance (Data a) = Serialize a where ... if you are willing to use OverlappingInstances and UndecidableInstances... Cheers, Pedro [1] http://hackage.haskell.org/packages/archive/regular-extras/0.1.2/doc/html/Generics-Regular-Functions-Binary.html [2] http://hackage.haskell.org/packages/archive/multirec-binary/0.0.1/doc/html/Generics-MultiRec-Binary.html On Tue, Jan 26, 2010 at 03:16, Jeremy Shaw jer...@n-heptane.com wrote: On Sun, Jan 24, 2010 at 5:49 AM, Neil Mitchell ndmitch...@gmail.comwrote: Hi, The problem with Data for Text isn't that we have to write a new instance, but that you could argue that proper handling of Text with Data would not be using a type class, but have special knowledge baked in to Data. That's far worse than the Serialise problem mentioned above, and no one other than the Data authors could solve it. Of course, I don't believe that, but it is a possible interpretation. Right.. that is the problem with Text. Do you think the correct thing to do for gunfold and toConstr is to convert the Text to a String and then call the gufold and toConstr for String? Or something else? The Serialise problem is a serious one. I can't think of any good solutions, but I recommend you give knowledge of your serialise class to Derive (http://community.haskell.org/~ndm/derive/http://community.haskell.org/%7Endm/derive/) and then at least the instances can be auto-generated. Writing lots of boilerplate and regularly ripping it up is annoying, setting up something to generate it for you reduces the pain. We currently use template haskell to generate the Serialize instances in most cases (though some data types have more optimized encodings that were written by hand). However, you must supply the Version and Migration instances by hand (they are super classes of Serialize). I am all for splitting the Serialize stuff out of happstack .. it is not really happstack specific. Though I suspect pulling it out is not entirely trivial either. I think the existing code depends on syb-with-class. - jeremy ___ 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] list mail
Sorry, for some reasons, I had to change my E-mail, so please don't send the list mail anymore, and I will get it with a new mail. Best wishes. _ Windows Live: Make it easier for your friends to see what you’re up to on Facebook. http://www.microsoft.com/middleeast/windows/windowslive/see-it-in-action/social-network-basics.aspx?ocid=PID23461::T:WLMTAGL:ON:WL:en-xm:SI_SB_2:092009___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
