[Haskell-cafe] getting fulling qualified names of types from hint/ghc api
Hi, I am using hint's Language.Haskell.Interpreter.typeOf function to dynamically get the type of a haskell expression but, unfortunately for my purposes, I see that it returns unqualified names (I see in Hint.Conversions that it uses the GHC function GHC.getPrintUnqual). Is there any function in the GHC api or any other simple way of getting the fully qualified types? Surprisingly, I could not find any. Thanks, titto -- Dr. Pasqualino Titto Assini http://quid2.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] class-instance
I don't think that's exactly what you want, though. name (2::Int) crashes your program. I think you really want a data type. data Person a b = P a b pid :: Person a b - a pid (P a _) = a pname :: Person a b - b pname (P _ b) = b Now, what it looks like you want is some kind of extensible relation name 1 = john name 2 = julie etc. but there's no simple way to specify this as an open function at the value level. You can make a list of all the people: people = [ P 1 john, P 2 julie ] name :: Int - Maybe String name x = fmap pname $ safeHead $ filter ((== x) . pid) people safeHead [] = Nothing safeHead (x:_) = Just x It's still not really clear what you are trying to do. -- ryan On Mon, Jan 17, 2011 at 9:15 AM, Patrick Browne patrick.bro...@dit.ie wrote: A functional dependency seems to allow me to express my rather strange requirement. class Person i n | i - n where pid :: i name :: i - n instance Person Int String where pid = 1 name(1) = john -- name(pid::Int) will produce john Thanks for your help Pat On 17/01/2011 14:07, Patrick Browne wrote: On 17/01/2011 13:04, Ketil Malde wrote: So other PERSONs would have different types, say: I think the problem is there is just one constant p1 in the class and there needs to be multiple constants in the implementation (one for each person). It seems difficult to specify this using type classes So, some data declaration as you suggest will probably be needed. Thanks, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ 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] Happstack events and web page refreshing
On 17 January 2011 21:50, Jeremy Shaw jer...@n-heptane.com wrote: On Jan 17, 2011, at 2:19 PM, Corentin Dupont wrote: Indeed, I tried with META HTTP-EQUIV=Refresh CONTENT=n ? and it's unusable. It make blink the page, ungrey the stop button for a second and make the fields loose the focus so it's impossible to type in. I'll try with XMLHTTPRequest. Right. Using the jQuery library should make it easier to do ajax requests and modify the DOM on the fly, http://jquery.com/ - jeremy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe A nice variation of polling is long polling: http://en.wikipedia.org/wiki/Push_technology#Long_polling This can easily be accomplished in Haskell by having an MVar per session. Initially an empty MVar is created per new session. When a client makes a request, the server thread that handles the request takes the MVar belonging to the session. This thread will block until the MVar is filled. When the server has an update it will fill all the MVars. This causes all the blocked threads to continue with sending a response to the client notifying it about the update. Like Jeremy said it's a good idea to make these update requests asynchronous. Regards, Bas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Problems installing the haskell platform
Dear Group, Greetings. I've been using haskell for about a year now, though I will readily admit I still don't really know what I am doing when I get error messages. Today I decided to reset my haskell environment from scratch, avoiding the debian packages and going straight to the source. 1. Install ghc from http://www.haskell.org/ghc/download_ghc_6_12_3 Success!! ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.3 cabal update Downloading the latest package list from hackage.haskell.org 2. Next I downloaded the haskell platform and ran cabal install Problems!! cabal: Error: some packages failed to install: haskell-platform-2010.2.0.0 depends on stm-2.1.2.1 which failed to install. stm-2.1.2.1 failed during the building phase. The exception was: ExitFailure 1 3. So I tried installing stm manually, which for some unknown reason worked: runghc Setup.hs install Installing library in /usr/local/lib/stm-2.1.2.1/ghc-6.12.3 Registering stm-2.1.2.1... 4. back to the haskell-platform directory and try again: cabal install Resolving dependencies... Configuring haskell-platform-2010.2.0.0... setup: happy version ==1.18.5 is required but the version found at /usr/local/bin/happy is version 1.18.6 cabal: Error: some packages failed to install: haskell-platform-2010.2.0.0 failed during the configure step. The exception was: ExitFailure 1 5. Crap. Okay, maybe it will work with version 1.18.6 after all, so I edit the .cabal and change happy ==1.18.5 to happy =1.18.5 -- Depending on programs does not work, they are not registered -- We list them to help distro packaging. build-tools: cabal-install ==0.8.2, alex ==2.3.3, happy =1.18.5 -- haddock ==2.7.2 -- valid, but needs ghc-paths 6. Try again cabal install Resolving dependencies... Configuring haskell-platform-2010.2.0.0... Preprocessing library haskell-platform-2010.2.0.0... Building haskell-platform-2010.2.0.0... Registering haskell-platform-2010.2.0.0... Installing library in /usr/local/lib/haskell-platform-2010.2.0.0/ ghc-6.12.3 Registering haskell-platform-2010.2.0.0... 7. Success!! Now even though I'm not sure what I am doing, it looks like it may have worked. I'm worried, though, about the first time user who tries to go through this process. They will probably stop at the first error, when the haskell-platform fails to install. I know some of you will say use the debian packages. Well I did that, but when I tried to install snap, which is not included in the debian packages, I wound up with conflicting dependencies, and could not find a way out. I hope this dialog is helpful to whomever is in charge of the haskell platform, a wonderful resource by the way. Best wishes, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Happstack events and web page refreshing
Thanks. There seems to be several technologies to realize this push or polling. Is what you explained feasible on the user's side of happstack-server (I mean, if I can do it myself)? If I take an empty MVar in my ServerPartTs, which are read over client's request, I think that nothing will be sent back to the browser and it will remain blank! Is that to be combined with an HTTP refresh timer on the client side? By sessions, you mean sessions that I create myself for every client connected? Regards, Corentin On Wed, Jan 19, 2011 at 2:21 PM, Bas van Dijk v.dijk@gmail.com wrote: On 17 January 2011 21:50, Jeremy Shaw jer...@n-heptane.com wrote: On Jan 17, 2011, at 2:19 PM, Corentin Dupont wrote: Indeed, I tried with META HTTP-EQUIV=Refresh CONTENT=n ? and it's unusable. It make blink the page, ungrey the stop button for a second and make the fields loose the focus so it's impossible to type in. I'll try with XMLHTTPRequest. Right. Using the jQuery library should make it easier to do ajax requests and modify the DOM on the fly, http://jquery.com/ - jeremy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe A nice variation of polling is long polling: http://en.wikipedia.org/wiki/Push_technology#Long_polling This can easily be accomplished in Haskell by having an MVar per session. Initially an empty MVar is created per new session. When a client makes a request, the server thread that handles the request takes the MVar belonging to the session. This thread will block until the MVar is filled. When the server has an update it will fill all the MVars. This causes all the blocked threads to continue with sending a response to the client notifying it about the update. Like Jeremy said it's a good idea to make these update requests asynchronous. Regards, Bas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Happstack events and web page refreshing
Hi, I've not really followed the thread, but on the client side, for long-polling, you issue an XMLHttpRequest that will receive an answer from the server only when the server has something to push to the client (maybe done with a thread waiting on an MVar). XHR is asynchronous as you have to supply a callback to process the server's response. Usually an XHR request is done from an already loaded page and will update part of that page with the content of the server's response. So no blank page. (So no timer needed either, this is done asynchronously with callbacks.) HTH, Thu 2011/1/19 Corentin Dupont corentin.dup...@gmail.com: Thanks. There seems to be several technologies to realize this push or polling. Is what you explained feasible on the user's side of happstack-server (I mean, if I can do it myself)? If I take an empty MVar in my ServerPartTs, which are read over client's request, I think that nothing will be sent back to the browser and it will remain blank! Is that to be combined with an HTTP refresh timer on the client side? By sessions, you mean sessions that I create myself for every client connected? Regards, Corentin On Wed, Jan 19, 2011 at 2:21 PM, Bas van Dijk v.dijk@gmail.com wrote: On 17 January 2011 21:50, Jeremy Shaw jer...@n-heptane.com wrote: On Jan 17, 2011, at 2:19 PM, Corentin Dupont wrote: Indeed, I tried with META HTTP-EQUIV=Refresh CONTENT=n ? and it's unusable. It make blink the page, ungrey the stop button for a second and make the fields loose the focus so it's impossible to type in. I'll try with XMLHTTPRequest. Right. Using the jQuery library should make it easier to do ajax requests and modify the DOM on the fly, http://jquery.com/ - jeremy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe A nice variation of polling is long polling: http://en.wikipedia.org/wiki/Push_technology#Long_polling This can easily be accomplished in Haskell by having an MVar per session. Initially an empty MVar is created per new session. When a client makes a request, the server thread that handles the request takes the MVar belonging to the session. This thread will block until the MVar is filled. When the server has an update it will fill all the MVars. This causes all the blocked threads to continue with sending a response to the client notifying it about the update. Like Jeremy said it's a good idea to make these update requests asynchronous. Regards, Bas ___ 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] Problems installing the haskell platform
Hi, Am Mittwoch, den 19.01.2011, 13:52 + schrieb Henry Laxen: I know some of you will say use the debian packages. Well I did that, but when I tried to install snap, which is not included in the debian packages, I wound up with conflicting dependencies, and could not find a way out. I hope this dialog is helpful to whomever is in charge of the haskell platform, a wonderful resource by the way. I’m not trying to talk you out of building everything on your own, which is something where you learn a lot. Nevertheless I’m curious which kind of problems you had when trying to use the Debian packages, and if they are specific to the Debian packages? My experience with ghc and lots of package from Debian, cabal-install from Debian and then cabal install’ing (as a user!) everything not in Debian is pretty good so far. 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: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Happstack events and web page refreshing
On 19 January 2011 15:02, Corentin Dupont corentin.dup...@gmail.com wrote:
Is what you explained feasible on the user's side of happstack-server (I
mean, if I can do it myself)?
Yes, you can do it yourself.
If I take an empty MVar in my ServerPartTs, which are read over client's
request,
I think that nothing will be sent back to the browser and it will remain
blank!
Yes, which is the intention.
Here's a short incomplete and non type-checked snapshot of an
application I'm currently developing which uses long-polling:
Somewhere on the client you have the following Javascript which uses
the JQuery library:
update = function(){
$.ajax({url: update, success: onUpdate});
}
onUpdate = function(data){
// Update your page, possibly by making more ajax requests to the server...
update();
}
Now when you call update() the client will asynchronously request the
server for updates. Each time there's an update the onUpdate function
is run which gives you a chance to update your page. onUpdate will
finally call update() again for the next iteration.
Now on the server side you need something like this:
main :: IO ()
main = do
sessionStore - newSessionStore
forkIO (updateLoop sessionStore)
simpleHTTP conf $ msum
[ dir update (update sessionStore), ... ]
update :: SessionStore - ServerPartT IO Response
update sessionStore = do
mvar - getSessionData sessionStore
liftIO $ takeMVar mvar
return someResponse
getSessionData will check if there's an active session and if so will
return the associated MVar. If there's no active session a new one
will be created which will have an empty MVar associated with it.
updateLoop is a loop that creates a new update and then notifies the clients:
updateLoop :: SessionStore - IO ()
updateLoop store = forever $ do
makeNewUpdate
notifySessions store
notifySessions :: SessionStore - IO ()
notifySessions store = do
sessionData - getAllSessionData store
mapM_ (\mv - tryPutMVar mv ()) sessionData
Note that it's not necessary to have a separate thread for creating
updates and notifying clients. You can just as well do this in one of
your request handlers.
I know the example is incomplete but I hope I got the essentials across.
Is that to be combined with an HTTP refresh timer on the client side?
No, as Thu already explain, no HTTP refresh timer is needed.
By sessions, you mean sessions that I create myself for every client
connected?
Yes. However, I don't think sessions are required for long-polling. I
just used them in my example because I used them in a similar way in
an application I'm currently developing.
Regards,
Bas
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Re: [Haskell-cafe] web-routes and forms
Thanks Jeremy.
I had it to work now ;)
Corentin
On Tue, Jan 18, 2011 at 6:01 PM, Jeremy Shaw jer...@n-heptane.com wrote:
Hello,
trhsx will be installed in ~/.cabal/bin, so you will need to add that
to your PATH.
In order to use the demo code I provided you would need the latest
happstack from darcs because it contains a few differences in the API.
The code can be made to work with what is on hackage though.
The submit issue is actually a bug in digestive-functors-blaze. The
return type should be, Form m i e BlazeFormHtml (). jaspervdj is going
to patch it and upload a new version.
- jeremy
On Thu, Jan 13, 2011 at 2:40 PM, Corentin Dupont
corentin.dup...@gmail.com wrote:
Hello,
I'm using the combination happstack + digestive-functors + web-routes +
blazeHTML.
I'm not finding any examples on the net...
I've tried to adapt your example (thanks):
type NomicForm a = HappstackForm IO String BlazeFormHtml a
demoForm :: NomicForm (Text, Text)
demoForm =
(,) $ ((TDB.label greeting: ++ inputNonEmpty Nothing) * br)
* ((TDB.label noun: ++ inputNonEmpty Nothing) * br)
* (submit submit)
where
br :: NomicForm ()
br = view H.br
-- make sure the fields are not blank, show errors in line if they
are
inputNonEmpty :: Maybe Text - NomicForm Text
inputNonEmpty v =
(inputText v `validate` (TD.check You can not leave this field
blank. (not . T.null)) ++ errors)
But I've got a problem on submit and inputText. I don't see how they are
compatible with HappstackForm.
NomicForm a reduces to:
Form (ServerPartT IO) Input String BlazeFormHtml a
whereas the type of submit is:
submit :: Monad m
= String-- ^ Text on the submit
button
- Form m String e BlazeFormHtml () -- ^ Submit button
Maybe I miss some instance?
BTW, I also tried to execute your exemple, but I can't install some
packages.
cabal install digestive-functors-hsp
cabal: Unknown build tool trhsx
Whereas trhsx is in my PATH (under linux).
You said I need the latest happstack from darcs, why?
Cheers,
Corentin
On Sun, Jan 9, 2011 at 8:36 PM, Jeremy Shaw jer...@n-heptane.com
wrote:
Hello,
newRule also needs to have the type, RoutedNomicServer. The
transformation of RoutedNomicServer into NomicServer is done in the
handleSite function. Something like this:
nomicSpec :: ServerHandle - Site Route (ServerPartT IO Response)
nomicSpec sh =
Site { handleSite = \f url - unRouteT (nomicSite sh url)
f
...
main =
do ...
simpleHTTP nullConf $ siteImpl (nomicSpec sh)
Or something like that -- it's hard to tell exactly what is going on
in your app based on the snippets you provided.
Also, I highly recommend using digestive functors instead of formlets.
It is the successor to formlets. Same core idea, better implementation
and actively maintained.
I have attached a quick demo of using:
happstack+digestive-functors+web-routes+HSP
To use it you will need the latest happstack from darcs plus:
hsp
web-routes
web-routes-hsp
web-routes-happstack
web-routes-mtl
digestive-functors
digestive-functors-hsp
I plan to clean up this example and document it better in the crash
course for the upcoming release. Clearly things like the FormInput
instance and the formPart function belong a library.
let me know if you have more questions.
- jeremy
On Sat, Jan 8, 2011 at 6:44 PM, Corentin Dupont
corentin.dup...@gmail.com wrote:
Hello,
I have difficulties mixing web-routes and forms:
I have put routes in all my site, except for forms which remains with
the
type ServerPartT IO Response.
How to make them work together?
I have:
type NomicServer = ServerPartT IO
type RoutedNomicServer = RouteT PlayerCommand NomicServer
newRule :: ServerHandle - NomicServer Response
newRule sh = do
methodM POST -- only accept a post method
mbEntry - getData -- get the data
case mbEntry of
Nothing - error $ error: newRule
Just (NewRule name text code pn) - do
html - nomicPageComm pn sh (submitRule name text code pn))
ok $ toResponse html
nomicPageComm :: PlayerNumber - ServerHandle - Comm () -
RoutedNomicServer Html
nomicPageComm pn sh comm =
(..)
launchWebServer :: ServerHandle - IO ()
launchWebServer sh = do
putStrLn Starting web server...\nTo connect, drive your browser to
\http://localhost:8000/Login\ http://localhost:8000/Login%5C
d - liftIO getDataDir
simpleHTTP nullConf $ mconcat [dir postLogin $ postLogin,
fileServe [] d,
dir Login $ ok $ toResponse $
loginPage,
dir NewRule $
[Haskell-cafe] pla...@community.haskell.org and Mailman Images
Just two small comments: The e-mail address pla...@community.haskell.org mentioned at http://planet.haskell.org/ bounces (connect to community.haskell.org[72.249.126.23]: Connection timed out). Also, registering on this mailing list to report the problem, I realized that the mailman images at pages such as http://www.haskell.org/mailman/subscribe/haskell-cafe are broken too. Something you might want to take a look at! Warm regards, Jon Kristensen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Adding a builder to the bytestring package?
Most people who work with binary data have had to construct bytestrings at some point. The most common solution is to use a Builder, a monoid representing how to construct a bytestring. There are currently three packages (that I know of) which include builder implementations: binary, cereal, and blaze-builder. However, all of these libraries have additional dependencies beyond just bytestring. All three depend on array and containers, and blaze-builder additionally depends on text (and thus deepseq). Since the current implementation of GHC uses static linking, every additional dependency adds to the final size of a binary. Obviously the Builder concept is very useful, as it has been implemented at least three times. How about adding it to the bytestring package itself? We could have a module Data.ByteString.Builder, with functions (at minimum): toByteString :: Builder - Data.ByteString.ByteString toLazyByteString :: Builder - Data.ByteString.Lazy.ByteString fromByteString :: Data.ByteString.ByteString - Builder fromLazyByteString :: Data.ByteString.Lazy.ByteString - Builder empty :: Builder append :: Builder - Builder - Builder Plus whatever implementation details might be useful to expose. Existing libraries could then add their extra features (word - builder for binary and cereal, UTF/HTTP for blaze-builder) on top of the existing types. Is this something the community is interested in? Is there any work currently aimed at this goal? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems installing the haskell platform
Dear Joachim, Well, I got away with a lot of stuff, but as I said, when I tried to install snap via cabal-install, I could not break through the dependency jungle. I believe the stumbling block was trying to install hint. All of the debian packages installed just fine, but there is no debian package for snap, which I really wanted to try out. Best wishes, Henry Laxen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Community.haskell.org is down
http://www.downforeveryoneorjustme.com/http://community.haskell.org Currently community is down for me. I remember some infrastructure/website strike team was set up, but I couldn't find where to get in contact with them? Thanks, Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Community.haskell.org is down
CCing commit...@haskell.org Here's the wiki page: http://haskell.org/haskellwiki/Haskell.org_committee And a neat twitter account, which can be viewed as RSS: http://twitter.com/#!/haskellorg Antoine On Wed, Jan 19, 2011 at 1:07 PM, Neil Mitchell ndmitch...@gmail.com wrote: http://www.downforeveryoneorjustme.com/http://community.haskell.org Currently community is down for me. I remember some infrastructure/website strike team was set up, but I couldn't find where to get in contact with them? Thanks, Neil ___ 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] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 6:51 PM, John Millikin jmilli...@gmail.com wrote: Most people who work with binary data have had to construct bytestrings at some point. The most common solution is to use a Builder, a monoid representing how to construct a bytestring. There are currently three packages (that I know of) which include builder implementations: binary, cereal, and blaze-builder. However, all of these libraries have additional dependencies beyond just bytestring. All three depend on array and containers, and blaze-builder additionally depends on text (and thus deepseq). Since the current implementation of GHC uses static linking, every additional dependency adds to the final size of a binary. Obviously the Builder concept is very useful, as it has been implemented at least three times. How about adding it to the bytestring package itself? We could have a module Data.ByteString.Builder, with functions (at minimum): toByteString :: Builder - Data.ByteString.ByteString toLazyByteString :: Builder - Data.ByteString.Lazy.ByteString fromByteString :: Data.ByteString.ByteString - Builder fromLazyByteString :: Data.ByteString.Lazy.ByteString - Builder empty :: Builder append :: Builder - Builder - Builder Plus whatever implementation details might be useful to expose. Existing libraries could then add their extra features (word - builder for binary and cereal, UTF/HTTP for blaze-builder) on top of the existing types. Is this something the community is interested in? Is there any work currently aimed at this goal? I think both Duncan and I agree that we should move Data.Binary.Builder (which doesn't have any extra dependencies) to bytestring. I've already added Data.Text.Lazy.Builder to text. Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 9:32 PM, Johan Tibell johan.tib...@gmail.com wrote: On Wed, Jan 19, 2011 at 6:51 PM, John Millikin jmilli...@gmail.com wrote: Most people who work with binary data have had to construct bytestrings at some point. The most common solution is to use a Builder, a monoid representing how to construct a bytestring. There are currently three packages (that I know of) which include builder implementations: binary, cereal, and blaze-builder. However, all of these libraries have additional dependencies beyond just bytestring. All three depend on array and containers, and blaze-builder additionally depends on text (and thus deepseq). Since the current implementation of GHC uses static linking, every additional dependency adds to the final size of a binary. Obviously the Builder concept is very useful, as it has been implemented at least three times. How about adding it to the bytestring package itself? We could have a module Data.ByteString.Builder, with functions (at minimum): toByteString :: Builder - Data.ByteString.ByteString toLazyByteString :: Builder - Data.ByteString.Lazy.ByteString fromByteString :: Data.ByteString.ByteString - Builder fromLazyByteString :: Data.ByteString.Lazy.ByteString - Builder empty :: Builder append :: Builder - Builder - Builder Plus whatever implementation details might be useful to expose. Existing libraries could then add their extra features (word - builder for binary and cereal, UTF/HTTP for blaze-builder) on top of the existing types. Is this something the community is interested in? Is there any work currently aimed at this goal? I think both Duncan and I agree that we should move Data.Binary.Builder (which doesn't have any extra dependencies) to bytestring. I've already added Data.Text.Lazy.Builder to text. Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Isn't Simon Meier working on migrating his code from blaze-builder into binary? I agree with John that it would make more sense to go in bytestring. Assuming that happens, would the builder from text end up being based on it? Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 8:37 PM, Michael Snoyman mich...@snoyman.com wrote: Isn't Simon Meier working on migrating his code from blaze-builder into binary? So I heard (although not directly from Simon). I think it would be nice to port the blaze-builder implementation to binary, but to keep binaries current interface (for now). I agree with John that it would make more sense to go in bytestring. Assuming that happens, would the builder from text end up being based on it? ByteString and Text don't share an underlying data structure at the moment (one uses pinned ForeignPtrs and one unpinned ByteArray#s) so they can use the same builder efficiently. Some day perhaps. Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: Applicative = Monad: Call for consensus
Hi Tyson (So OT, I'm switching to cafe.) On 19 Jan 2011, at 18:24, Tyson Whitehead wrote: On January 17, 2011 16:20:22 Conor McBride wrote: Ahem : ) The unfortunate pain you pay for this additional power is manually having to specify the application ($ and *) and merging (join). If the compiler could figure this all out for you based on the underlying types, wow! To achieve such a thing, one would need to ensure a slightly more deliberate separation of value and computation in the presentation of types. In Haskell, we use, e.g., [Int], for * pure computations of lists of integers * nondeterministic computations of integers [..] I'm pretty sure I know what pure computations of lists of integers is, but I'm not so sure about nondeterministic computations of integers. If it is not too much of an effort, could you clarify with a quick example? Viewing [] as a monad, you can view [1,2] as a nondeterministic integer with possible values 1 and 2. Lifting operations to this monad gives you all possible combinations computation, so, as SHE would have it, (| [1,2] + [3,4] |) = pure (+) * [1,2] * [3,4] = [4,5,5,6] It's not hard to come up with examples where lifted and unlifted both make sense. With a bit of help from Twitter, we have (courtesy of Andy Gimblett) ([under,over] ++ [state,wear]) = [under,over,state,wear] but (|[under,over] ++ [state,wear]|) = [understate,underwear,overstate,overwear] and (courtesy of Dan Piponi) ([plan,tan] ++ [gent,king]) = [plan,tan,gent,king] but (|[plan,tan] ++ [gent,king]|) = [plangent,planking,tangent,tanking] In each case, the former has (++) acting on lists of strings as pure values, while the latter has (++) acting on strings as values given in []-computations. The type [String] determines a domain, it does not decompose uniquely to a notion of computation and a notion of value. We currently resolve this ambiguity by using one syntax for pure computations with [String] values and a different syntax for [] computations with String values. Just as we use newtypes to put a different spin on types which are denotationally the same, it might be worth considering a clearer (but renegotiable) separation of the computation and value aspects of types, in order to allow a syntax in which functions are typed as if they act on *values*, but lifted to whatever notion of computation is ambient. After types for representation, let us have types for explanation. All the best Conor ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 12:04, Johan Tibell johan.tib...@gmail.com wrote: On Wed, Jan 19, 2011 at 8:37 PM, Michael Snoyman mich...@snoyman.com wrote: Isn't Simon Meier working on migrating his code from blaze-builder into binary? So I heard (although not directly from Simon). I think it would be nice to port the blaze-builder implementation to binary, but to keep binaries current interface (for now). From the perspective of a library user, if there's to be code shuffling, I'd much rather have it be one-time (blaze-builder - bytestring) than having multiple merges/ports going on. Especially since building bytestrings is a much more generic operation than binary serialisation. Regarding the interface, I think that as long as the same *basic* operations are available, it's fine to have extra operations as well. Several of blaze-builder's special-case functions (toByteStringIO , fromWriteList) allow more efficient operation than the generic interface. I agree with John that it would make more sense to go in bytestring. Assuming that happens, would the builder from text end up being based on it? ByteString and Text don't share an underlying data structure at the moment (one uses pinned ForeignPtrs and one unpinned ByteArray#s) so they can use the same builder efficiently. Some day perhaps. Can any of the blaze-builder optimizations be translated to the Text builder? When I benchmark it against binary and cereal, blaze-builder is approximately 2-3 times faster for most use cases. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 10:04 PM, Johan Tibell johan.tib...@gmail.com wrote: On Wed, Jan 19, 2011 at 8:37 PM, Michael Snoyman mich...@snoyman.com wrote: Isn't Simon Meier working on migrating his code from blaze-builder into binary? So I heard (although not directly from Simon). I think it would be nice to port the blaze-builder implementation to binary, but to keep binaries current interface (for now). What's the advantage to moving in into binary as opposed to bytestring? I agree with John that it would make more sense to go in bytestring. Assuming that happens, would the builder from text end up being based on it? ByteString and Text don't share an underlying data structure at the moment (one uses pinned ForeignPtrs and one unpinned ByteArray#s) so they can use the same builder efficiently. Some day perhaps. I hadn't realized, I had just assumed it was using ByteString underneath. I'll pay more attention next time, thanks for informing me :). Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 9:51 PM, John Millikin jmilli...@gmail.com wrote: Can any of the blaze-builder optimizations be translated to the Text builder? When I benchmark it against binary and cereal, blaze-builder is approximately 2-3 times faster for most use cases. Yes, but I haven't had time to do so yet. I even wrote a blog post about not having time to do it. ;) http://blog.johantibell.com/2011/01/haskell-library-improvements-id-like-to.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 10:30 PM, Michael Snoyman mich...@snoyman.com wrote: What's the advantage to moving in into binary as opposed to bytestring? To test that the implementation can indeed be ported to that interface. We could of course skip that step if we want to. Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] source line annotations
One of the first things I did when starting a larger haskell project
was to write a preprocessor that would replace certain tokens with
(token_srcpos (filename, func_name, lineno)) and then write x and
x_srcpos versions of the various 'throw' and logging functions. This
is extremely handy to have, and since logs are part of my app's UI,
it's part of the app itself.
I found out later that jhc has a pragma that basically does the same
thing, only built in. My preprocessor works but has some problems.
Namely, replacing non-qualified tokens requires parsing the source
because you want to replace only function calls, not declarations,
import / export lists, etc. I get around this because I always import
qualified, but still I need to do some hacking to get qualified tokens
('lex' mysteriously doesn't understand them... aren't they in haskell
2010 now?) and not screw up on odd corners like backslash string
continuations. Then I start wanting to e.g. replace 'throw' in the
same module it's defined in, which really lets the dragons out because
now I'm trying to replace unqualified names.
Another problem is that the names to replace are hardcoded in the
preprocessor. Not a problem for me, but would be for a general tool.
My preprocessor works well, but occasionally I do have to go in and
fix yet another odd corner that came up. Initially I thought I would
only simplistically replace tokens and avoid using a full syntax
parser, but to really do it correctly a full parser is needed. And of
course this is specific to my style (qualified imports) and project.
To do this in full generality really requires the compiler's help.
But as I use it now, it's been extremely useful, especially for
exceptions which may not carry a unique or easily greppable msg. Or
any msg at all.
Are there any more sophisticated tools out there that do this?
Interest in extending ghc to support SRCLOC_ANNOTATE? Reasons why I
don't actually want this after all?
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Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Wed, Jan 19, 2011 at 14:06, Johan Tibell johan.tib...@gmail.com wrote: On Wed, Jan 19, 2011 at 10:30 PM, Michael Snoyman mich...@snoyman.com wrote: What's the advantage to moving in into binary as opposed to bytestring? To test that the implementation can indeed be ported to that interface. We could of course skip that step if we want to. blaze-builder already implements the binary builder interface, minus the putWord* functions. I think those would be trivial to reimplement on top of Write. Since it sounds like everyone agrees with / has already thought of moving Builder into bytestring, I'll start poking at a patch. Who is the current patch-reviewer for binary and bytestring? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding a builder to the bytestring package?
On Thu, Jan 20, 2011 at 12:16 AM, John Millikin jmilli...@gmail.com wrote: blaze-builder already implements the binary builder interface, minus the putWord* functions. I think those would be trivial to reimplement on top of Write. Since it sounds like everyone agrees with / has already thought of moving Builder into bytestring, I'll start poking at a patch. Who is the current patch-reviewer for binary and bytestring? I'd suggest addressing the patch to Don Stewart, Duncan Coutts, and Lennart Kolmodin. Johan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] source line annotations
On 19 January 2011 22:43, Evan Laforge qdun...@gmail.com wrote: Reasons why I don't actually want this after all? Are you aware of the magic assert function? http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-Exception-Base.html#v:assert The compiler spots it and replaces it with a function that raises an exception that contains the source location if the assert fails. It is deeply evil, but you can write a library function like this (untested): getSourceLoc :: (Bool - a - a) - String getSourceLoc my_assert = unsafePerformIO (evaluate (my_assert False (error Impossible)) `catch` (\(AssertionFailed s) - return s)) Now your consumers can write: getSourceLoc assert :: String And you will have a value that describes where you are. You can then use this to implement logging, throw errors etc. The annoying part is that you have to explicitly pass in assert from the call site to make sure that you get the right source location reported. Cheers, Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] source line annotations
Now your consumers can write: getSourceLoc assert :: String And you will have a value that describes where you are. You can then use this to implement logging, throw errors etc. The annoying part is that you have to explicitly pass in assert from the call site to make sure that you get the right source location reported. Yes, in fact the 'loch' package does this. Initially I thought that having a dummy argument on every single log call and throw (quite a few of those) would be too much hassle, but if I have to keep fixing the preprocessor I might start reconsidering. Another thing is that performance in logging is pretty critical. When I profile, logging functions wind up in the expensive list if I'm not careful. I don't know bad an unsafePerformIO plus a catch for every log msg is going to be, but it's not going to be as fast as doing the work at compile time. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building lambdabot
On Tue, Jan 18, 2011 at 3:41 PM, Max Bolingbroke batterseapo...@hotmail.com wrote: That sounds like a good thing to do. Also, oo you know if there's any reason that the most recent lambdabot is not pushed to Hackage? That might make things even easier for others who wish to install it. It certainly confused me! Cheers, Max It had a lot of issues which meant it wouldn't build anywhere, where at least the Hackage version worked at some point. I spent this evening working on fixing issues (rewriting the show package to use QuickCheck 2 rather than QuickCheck 1 was not fun!) and have uploaded a lambdabot and show that compile for me on GHC 6.12.1 -- gwern http://www.gwern.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell Weekly News: Issue 165 - January 19, 2011
Welcome to issue 165 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 9 to 15, 2011. Announcements Brent Yorgey [2]announced the release of issue 17 of The Monad.Reader, containing the following three articles: List Leads Off with the Letter Lambda by Douglas M. Auclair, The InterleaveT Abstraction: Alternative with Flexible Ordering by Neil Brown, and The Reader Monad and Abstraction Elimination by Petr Pudlak. Andy Stewart [3]sent an invite to anyone interested in joining the Manatee team. Manatee is a Haskell integrated environment written in Haskell. Follow the link to see some videos posted for a better idea of what the tool is. Niklas Broberg [4]announced the release of version 1.10.1 of haskell-src-extensions. Huibiao Zhu [5]sent a call for papers to the 13th International Conference on Formal Engineering Methods to be held in Durham, UK on October 25-28 2011. Read on to see what was hot in the Haskell community around the net... Quotes of the Week * ion: ddarius: Yeah, abstract concepts absolutely hate being anthropomorphized. * gwern: there are no beginnings or ends to the circular list; but a cons cell thunked in Amador... * tac-tics: If you wish to create the universe from scratch, you must first invent the continuation. * sproingie: hm. Â i can't remember how to use @remember * NOTE: Just say @remember :) I'm grepping the logs to find these things * Kaidelong: Kaidelong take 6 bananas lambdabot banana Kaidelong clearly lambdabot is keeping the bananas to itself and lying about it * hpc: functor and applicative are easy, since you can remember them as those things what look like function application, sort of Top Reddit Stories * RFC: migrating GHC development to git Domain: haskell.org, Score: 51, Comments: 66 On Reddit: http://www.reddit.com/r/haskell/comments/ezgvs/rfc_migrating_ghc_development_to_git/ Original: http://www.haskell.org/pipermail/glasgow-haskell-users/2011-January/019752.html * [CCC] On how Haskell’s Arrows are NOT just function compositions with a fancier name Domain: ro-che.info, Score: 38, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/f3e06/ccc_on_how_haskells_arrows_are_not_just_function/ Original: http://ro-che.info/ccc/12.html * foldl vs foldl', and why you should always use the latter Domain: reddit.com, Score: 35, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/f25iv/foldl_vs_foldl_and_why_you_should_always_use_the/ Original: http://www.reddit.com/r/haskell/comments/f150x/whats_wrong_with_my_haskell_noob_question/c1ck6ey * ~Haskell 2011 Domain: haskell.org, Score: 32, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/ezmzh/haskell_2011/ Original: http://www.haskell.org/pipermail/haskell/2011-January/022497.html * The Monad.Reader: Issue 17 Domain: themonadreader.wordpress.com, Score: 30, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/ezft9/the_monadreader_issue_17/ Original: http://themonadreader.wordpress.com/2011/01/09/issue-17/ * Painless NP-complete problems: an embedded DSL for SMT solving Domain: donsbot.wordpress.com, Score: 27, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/f3fp6/painless_npcomplete_problems_an_embedded_dsl_for/ Original: http://donsbot.wordpress.com/2011/01/16/painless-np-complete-problems-an-embedded-dsl-for-smt-solving/ * Oregon Programming Languages Summer School: Types, Semantics, and Verification Domain: lists.seas.upenn.edu, Score: 22, Comments: 20 On Reddit: http://www.reddit.com/r/haskell/comments/f15rm/oregon_programming_languages_summer_school_types/ Original: http://lists.seas.upenn.edu/pipermail/types-announce/2011/002241.html * Announcing Web Application Interface 0.3.0 (now with more speed) Domain: docs.yesodweb.com, Score: 20, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/f0e6f/announcing_web_application_interface_030_now_with/ Original: http://docs.yesodweb.com/blog/announcing-wai-0-3/ * A bran new functional programming (Haskell inter alia) jobs site (corrected link) Domain: functionaljobs.com, Score: 19, Comments: 11 On Reddit: http://www.reddit.com/r/haskell/comments/ezqst/a_bran_new_functional_programming_haskell_inter/ Original: http://functionaljobs.com/ * [PDF] Orthogonal Serialization For Haskell Domain: mathematik.uni-marburg.de, Score: 17, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f1o8t/pdf_orthogonal_serialization_for_haskell/ Original: http://www.mathematik.uni-marburg.de/~berthold/papers/mainIFL10-withCopyright.pdf Top StackOverflow Questions * Monads as adjunctions votes: 15, answers: 3
[Haskell-cafe] Call for Submissions: Special Poetry and Fiction Edition of the Monad.Reader
Submissions are now being accepted for a special POETRY AND FICTION edition of the Monad.Reader. Poems about Haskell, short stories about Haskell, poems about poems about Haskell, elaborate monad tutorial allegories, Haskell/Dinosaur Comics crossovers: all fair game! If you want a bit of inspiration, check out the links at the end of this message [1,2,3]. But you are a creative bunch, I'm sure you can come up with all sorts of fun ideas and formats. The submission deadline will be **Friday, February 18** but feel free to send submissions at any time between now and then. Questions That May Be Asked Frequently (QTMBAF) --- Q. Are you serious? A. Absolutely. Q. The deadline is only four weeks away! It will take me at least five weeks to write my incredible masterpiece! A. Are you serious? Q. Will you take every submission or will there be some sort of selection process? A. I hope to publish every submission, although not without going through a process of editing and suggested revisions. Ultimately, it depends on the number of submissions. Q. Can I submit the same poem multiple times to increase my chances? A. I have written a sophisticated, 5000-line Haskell program to detect this and alter your submission in a subtle yet highly embarrassing (for you) way. Q. Seriously, this is a dumb idea. A. Please try to phrase your question in the form of a question. Q. Seriously, is this a dumb idea? A. No, I'm pretty sure it is awesome. Submission Details -- Submissions should be sent to (byorgey at cis.upenn.edu). Simple text submissions are fine for short submissions such as poetry. For longer submissions, LaTeX is appreciated. See the guidelines here to get started: http://themonadreader.wordpress.com/contributing/ If submitting in LaTeX is a hardship, text submissions will be accepted. Submissions in Microsoft Word format will be deleted and then overwritten fifty times with random bits. [1] http://haskell.org/haskellwiki/Humor/Poetry [2] http://haskell.org/haskellwiki/Haiku [3] http://www.cis.upenn.edu/~byorgey/haskell-alphabet/index.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell time line?
The Haskell language is some twenty years old. Is there a time line for what features were in the language's initial release and what has been added along the way? Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell time line?
On Wed, Jan 19, 2011 at 06:49:49PM -0800, michael rice wrote: The Haskell language is some twenty years old. Is there a time line for what features were in the language's initial release and what has been added along the way? See A History of Haskell: Being Lazy with Class by Hudak, Hughes, and Jones. It's a few years out of date now and doesn't cover absolutely everything, but it's an excellent place to start. -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell time line?
I found a copy. Thanks! Michael --- On Wed, 1/19/11, Brent Yorgey byor...@seas.upenn.edu wrote: From: Brent Yorgey byor...@seas.upenn.edu Subject: Re: [Haskell-cafe] Haskell time line? To: Haskell Cafe haskell-cafe@haskell.org Date: Wednesday, January 19, 2011, 10:03 PM On Wed, Jan 19, 2011 at 06:49:49PM -0800, michael rice wrote: The Haskell language is some twenty years old. Is there a time line for what features were in the language's initial release and what has been added along the way? See A History of Haskell: Being Lazy with Class by Hudak, Hughes, and Jones. It's a few years out of date now and doesn't cover absolutely everything, but it's an excellent place to start. -Brent ___ 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] Wondering about networking programming in Haskell.
Hi, Originally, my idea for networking programming is like: read some bytes, judge if this is enough for a packet (defined in certain protocol), if not, read more until the packet is complete, then parse it into a message. Now we have lazy in Haskell. Can I do it as: buffer - hGetContents handle (message1, leftData1) - parse1 buffer (message2, leftData2) - parse2 leftData2 ... -- 竹密岂妨流水过 山高哪阻野云飞 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Formal Program verification in Meta-Haskell
Hi, I have no understanding of template haskell and thus this message is uninformed speculation. Would it not be possible to write a function verifier in TH that, unlike QuickCheck which provides bounds on a function being probably approximately correct, is given a list over properties which are proven over a quasi-quoted syntax tree? For example: multiply :: Num a = a - a - a multiply_group :: Proof Bool multiply_group = group multiply 1 group :: (a - a - a) - a - Proof Bool group f ident = [proof| f, [associative, identity 1]] where `associative` and `ident` are proof-constructing functions that act over the `f` syntax tree. (Maybe some invokation of an Isabelle-like theorem prover or an SMT-like solver as part of the 'proof' module). Then we might even be able to list properties with class declarations. The properties get verified at compile time. class Monoid a where mzero :: a msum :: a - a - a property a [associative msum, identity mzero msum] Any thoughts? Vivian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A question about monad based on http://en.wikipedia.org/wiki/Monad_(category_theory)
On 1/17/11 10:46 PM, C K Kashyap wrote: Hi, I was going through http://en.wikipedia.org/wiki/Monad_(category_theory) - under Formal Definition I notice that monad is a Functor T:C - C My question is - when we think of Maybe as a functor T:C - C should we think that C here refers to Hakell types? Not quite. A functor is a mapping between categories. Well, then what's a mapping between categories? It's a pair of maps, one mapping objects to objects and the other mapping arrows to arrows, such that they preserve the category structure (i.e., obey the functor laws). For categories like the ones for lambda calculi, the objects are types and the arrows are functions. Thus, the C there is referring to some category (namely a category modeling Haskell's type system). If you wish, you can think of categories as being like Either Object Arrow, for a particular class of objects and a particular class of arrows. So we'd have, T : Either Ob Hom - Either Ob Hom T (Left ob) = ... T (Right hom) = ... Though nobody does that and we prefer our notation to slide over the fact that functors and categories have two parts, and so we actually write: T : C - C T ob = ... T hom = ... So we might have that T Int = Bool, or T Int = [Int], or whatever. But we don't usually give types for the components of a functor, since we care more about the functor as a whole (e.g., we don't give types to individual lines of a function definition either, we only give a type for the whole function). As in, (Int and Maybe Int are objects in C) and (Int - Int and Maybe Int - Maybe Int are arrows in C) and T is an arrow between them. Is that right? For some reason, I was imagining them as two different categories C and D. Generally speaking functors do go between different categories, but there's no reason they can't be on the same category (e.g., the identity functor :) Conventional monads are always endofunctors though, since otherwise we couldn't talk about the natural transformations like return and join (we don't have arrows between objects in different categories). There are some generalizations of monads that relax this however. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] class-instance
On 19/01/2011 10:41, Ryan Ingram wrote: It's still not really clear what you are trying to do. I am trying to see what how this requirement can be represented using just the normal instance-implements-class relation for a comparison with a specification language approach. If there is no simple way to do this using type classes then I am obviously mis-using the technique. Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
