[Haskell-cafe] Detecting numeric overflows
Can someone tell me if there are any primitives, that used to detect machine type overflows, in ghc haskell ? I perfectly understand, that I can build something based on preconditioning of variables, but this will kill any performance, if needed. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC rendering of non-ASCII characters configurable?
On Sun, Jul 29, 2012 at 7:04 PM, Richard Cobbe co...@ccs.neu.edu wrote: I'm working on an application that involves processing a lot of Unicode data, and I'm finding the built-in Show implementation for Char to be really inconvenient. Specifically, it renders all characters at U+0080 and above with decimal escapes: Prelude '\x80' '\128' This is annoying because all of the Unicode charts give the code points in hex, and indeed the charts are split into different PDFs at numbers that are nice and round in hex but not in decimal. So in order to figure out which character I'm looking at, I have to convert back to hex and then look it up in the charts. Is there any way to ask GHC to render super-ASCII characters with their hexadecimal escapes, instead? I'm perfectly happy to write my own custom Show instance, but I don't know how to hook that into ghci's REPL (or, for that matter, the routines that HUnit uses to generate the messages on failed tests, etc.). I'm using GHC 7.4.1 on MacOS 10.7.4. In GHC HEAD there is a new flag -interactive-print that allows to change the function used for printing values in GHCi. It will be in 7.6.1. That won't help with HUnit output, though. BR, Paolo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Capturing the parent element as I parse XML using parsec
Thank you Richard and Antoine. I think I see the pointlessness of my ask. Regards, Kashyap On Mon, Jul 30, 2012 at 4:14 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 29/07/2012, at 6:21 PM, C K Kashyap wrote: I am struggling with an idea though - How can I capture the parent element of each element as I parse? Is it possible or would I have to do a second pass to do the fixup? Why do you *want* the parent element of each element? One of the insanely horrible aspects of the Document Object Model is that every element is nailed in place by pointers everywhere, with the result that you cannot share elements, and even moving an element was painful. I still do a fair bit of SGML/XML process in C using a Document Value Model library that uses hash consing, and it's so much easier it isn't funny. While you are traversing a document tree it is useful to keep track of the path from the root. Given data XML = Element String [(String,String)] [XML] | Text String you do something like traverse :: ([XML] - [a] - a) - ([XML] - String - a) - XML - a traverse f g xml = loop [] xml where loop ancs (Text s) = g ancs s loop ancs e@(Element _ _ ks) = f ancs' (map (loop ancs') ks) where ancs' = e:ancs (This is yet another area where Haskell's non-strictness pays off.) If you do that, then you have the parent information available without it being stored in the tree. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Current state of garbage collection in Haskell
Thank you so much Alexander and Thomas. Regards, Kashyap On Sun, Jul 29, 2012 at 11:59 PM, Thomas Schilling nomin...@googlemail.comwrote: GHC does not provide any form of real-time guarantees (and support for them is not planned). That said, it's not as bad as it sounds: - Collecting the first (young) generation is fast and you can control the size of that first generation via runtime system (RTS) options. - The older generation is collected rarely and can be collected in parallel. - You can explicitly invoke the GC via System.Mem.performGC In a multi-threaded / multi-core program collecting the first generation still requires stopping all application threads even though only one thread (CPU) will perform GC (and having other threads help out usually doesn't work out due to locality issues). This can be particularly expensive if the OS decides to deschedule an OS thread, as then the GHC RTS has to wait for the OS. You can avoid that particular problem by properly configuring the OS via (linux boot isolcpus=... and taskset(8)). The GHC team has been working on a independent *local* GC, but it's unlikely to make it into the main branch at this time. It turned out to be very difficult to implement, with not large enough gains. Building a fully-concurrent GC is (AFAICT) even harder. I don't know how long the pause times for your 500MB live heap would be. Generally, you want your heap to be about twice the size of your live data. Other than that it depends heavily on the characteristics of you heap objects. E.g., if it's mostly arrays of unboxed non-pointer data, then it'll be very quick to collect (since the GC doesn't have to do anything with the contents of these arrays). If it's mostly many small objects with pointers to other objects, GC will be very expensive and bound by the latency of your RAM. So, I suggest you run some tests with realistic heaps. Regarding keeping up, Simon Marlow is the main person working on GHC's GC (often collaborating with others) and he keeps a list of papers on his homepage: http://research.microsoft.com/en-us/people/simonmar/ If you have further questions about GHC's GC, you can ask them on the glasgow-haskell-us...@haskell.org mailing list (but please consult the GHC user's guide section on RTS options first). HTH On 29 July 2012 08:52, C K Kashyap ckkash...@gmail.com wrote: Hi, I was looking at a video that talks about GC pauses. That got me curious about the current state of GC in Haskell - say ghc 7.4.1. Would it suffer from lengthy pauses when we talk about memory in the range of 500M +? What would be a good way to keep abreast with the progress on haskell GC? Regards, Kashyap ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Push the envelope. Watch it bend. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC rendering of non-ASCII characters configurable?
On Sun, Jul 29, 2012 at 8:04 PM, Richard Cobbe co...@ccs.neu.edu wrote: This is annoying because all of the Unicode charts give the code points in hex, and indeed the charts are split into different PDFs at numbers that are nice and round in hex but not in decimal. So in order to figure out which character I'm looking at, I have to convert back to hex and then look it up in the charts. My reading of the Haskell 98 report is that the Show instance for Char *could* use hex escapes, so this is a compiler choice. If there isn't a good reason for this choice, perhaps GHC could change? --Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Logging pure code
On Mon, Jul 30, 2012 at 1:54 AM, Thomas Schilling nomin...@googlemail.com wrote: On 27 July 2012 14:52, Marco Túlio Gontijo e Silva marcotmar...@gmail.com wrote: thread blocked indefinitely in an MVar operation IIRC, that means that a thread is blocked on an MVar and the MVar is only reachable by that thread. This sounds right. STM has noticed that there's a thread waiting for something that will never be available. It seems it have found a deadlock. I once tried mixing IO and STM actions (IO inside STM) and ended up rewriting the code to seperate the two. Before the rewrite, I got some of the strangest and most unpredictable errors I've ever seen in a Haskell program. I reckon this could be related. Here's the thread from back then: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098453.html -- Johan Brinch ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Wed, Jul 25, 2012 at 12:22 PM, Ross Paterson r...@soi.city.ac.uk wrote: As I understand it, the plan is to modify the following packages in hackage in-situ to remove the test sections (which contain the troublesome conditionals): HUnit-1.2.5.0 bloomfilter-1.2.6.10 codemonitor-0.1 codemonitor-0.2 fixhs-0.1.4 leksah-server-0.12.0.3 leksah-server-0.12.0.4 leksah-server-0.12.0.5 pqc-0.5 pqc-0.5.1 Does anyone object? No objections, but some impatience. ;-) Cheers, /Niklas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Mon, Jul 30, 2012 at 01:46:24PM +0100, Niklas Broberg wrote: On Wed, Jul 25, 2012 at 12:22 PM, Ross Paterson r...@soi.city.ac.uk wrote: As I understand it, the plan is to modify the following packages in hackage in-situ to remove the test sections (which contain the troublesome conditionals): HUnit-1.2.5.0 bloomfilter-1.2.6.10 codemonitor-0.1 codemonitor-0.2 fixhs-0.1.4 leksah-server-0.12.0.3 leksah-server-0.12.0.4 leksah-server-0.12.0.5 pqc-0.5 pqc-0.5.1 Does anyone object? No objections, but some impatience. ;-) OK, done. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC rendering of non-ASCII characters configurable?
On 30 July 2012 04:04, Richard Cobbe co...@ccs.neu.edu wrote: I'm working on an application that involves processing a lot of Unicode data, and I'm finding the built-in Show implementation for Char to be really inconvenient. Specifically, it renders all characters at U+0080 and above with decimal escapes: Prelude '\x80' '\128' This is annoying because all of the Unicode charts give the code points in hex, and indeed the charts are split into different PDFs at numbers that are nice and round in hex but not in decimal. So in order to figure out which character I'm looking at, I have to convert back to hex and then look it up in the charts. Can I ask what you're doing here? Are you printing individual characters or entire chunks of text? putStrLn and similar IO-based functions (at least for me) will un-escape characters if that helps. Otherwise, are you using Text or String? Is there any way to ask GHC to render super-ASCII characters with their hexadecimal escapes, instead? I'm perfectly happy to write my own custom Show instance, but I don't know how to hook that into ghci's REPL (or, for that matter, the routines that HUnit uses to generate the messages on failed tests, etc.). I'm using GHC 7.4.1 on MacOS 10.7.4. Thanks, Richard ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com http://IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] AI - machine learning
miro miroslav.karpis at gmail.com writes: Hi All, recently I started to take a look at haskell, especially at AI. I can see some email addresses of interested people there but not so much of other activity behind. Does it exist some mailing group especially for AI? Basically I'm interested in trying some machine learning algorithms. Start with reinforcement learning and value-based), and go towards AGI (Artificial General Intelligence). Does anybody know about some already existing haskell approaches, or is there anybody working on this? Cheers, m. Hi Miro For the past month or so I've been working through some of the algorithms in Artificial Intelligence: A Modern Approach by Russell and Norvig, and implementing them in Haskell. The code is available on github (https://github.com/chris-taylor/aima-haskell), you may be interested in taking a look. I haven't written any code for reinforcement learning yet, though I have implemented value iteration and policy iteration for MDPs. Chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] AI - machine learning
On Mon, Jul 30, 2012 at 4:39 PM, Chris Taylor crntay...@gmail.com wrote: miro miroslav.karpis at gmail.com writes: Hi All, recently I started to take a look at haskell, especially at AI. I can see some email addresses of interested people there but not so much of other activity behind. Does it exist some mailing group especially for AI? Basically I'm interested in trying some machine learning algorithms. Start with reinforcement learning and value-based), and go towards AGI (Artificial General Intelligence). Does anybody know about some already existing haskell approaches, or is there anybody working on this? Cheers, m. Hi Miro For the past month or so I've been working through some of the algorithms in Artificial Intelligence: A Modern Approach by Russell and Norvig, and implementing them in Haskell. The code is available on github (https://github.com/chris-taylor/aima-haskell), you may be interested in taking a look. I haven't written any code for reinforcement learning yet, though I have implemented value iteration and policy iteration for MDPs. Hi, I have an implementation of maze solving via genetic algorithms in Haskell at https://github.com/mihaimaruseac/HsMaze It was a homework with some added extensions but I guess that it is worth a look and some feedabck if possible :) Thanks -- MM ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Polyvariadic composition
Hello,
I have accidentally written my version of polyvariadic composition
combinator, `mcomp`. It differs from Oleg’s version (
http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp ) in three
aspects: a) it is simpler, b) it works without enumerating basic cases
(all existing types, in other words), and c) it needs more type extensions.
{-# LANGUAGE
MultiParamTypeClasses
, FunctionalDependencies
, FlexibleInstances
, UndecidableInstances
, TypeFamilies , OverlappingInstances
#-}
class Mcomp a ar b br | a br - b where
mcomp :: a - (ar - br) - b
instance (a ~ ar, b ~ br) = Mcomp a ar b br where
mcomp a f = f a
instance (Mcomp a ar b br) = Mcomp (x - a) ar (x - b) br where
mcomp a f = \x - mcomp (a x) f
My question is: why doesn’t it work when I replace
instance (a ~ ar, b ~ br) = Mcomp a ar b br
with
instance Mcomp a a b b
? I thought that equal letters mean equal types…
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Re: [Haskell-cafe] Detecting numeric overflows
Евгений Пермяков permea...@gmail.com писал в своём письме Mon, 30 Jul 2012 09:47:48 +0300: Can someone tell me if there are any primitives, that used to detect machine type overflows, in ghc haskell ? I perfectly understand, that I can build something based on preconditioning of variables, but this will kill any performance, if needed. In GHC.Prim — primitives addIntC# and subIntC#: addIntC# :: Int# - Int# - (#Int#, Int##) Add with carry. First member of result is (wrapped) sum; second member is 0 iff no overflow occured. subIntC# :: Int# - Int# - (#Int#, Int##) Subtract with carry. First member of result is (wrapped) difference; second member is 0 iff no overflow occured. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polyvariadic composition
Works here.
GHC 7.4.2
On Jul 30, 2012, at 11:32 PM, Artyom Kazak artyom.ka...@gmail.com wrote:
Hello,
I have accidentally written my version of polyvariadic composition
combinator, `mcomp`. It differs from Oleg’s version (
http://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp ) in three
aspects: a) it is simpler, b) it works without enumerating basic cases (all
existing types, in other words), and c) it needs more type extensions.
{-# LANGUAGE
MultiParamTypeClasses
, FunctionalDependencies
, FlexibleInstances
, UndecidableInstances
, TypeFamilies , OverlappingInstances
#-}
class Mcomp a ar b br | a br - b where
mcomp :: a - (ar - br) - b
instance (a ~ ar, b ~ br) = Mcomp a ar b br where
mcomp a f = f a
instance (Mcomp a ar b br) = Mcomp (x - a) ar (x - b) br where
mcomp a f = \x - mcomp (a x) f
My question is: why doesn’t it work when I replace
instance (a ~ ar, b ~ br) = Mcomp a ar b br
with
instance Mcomp a a b b
? I thought that equal letters mean equal types…
___
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___
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Re: [Haskell-cafe] Detecting numeric overflows
On 07/31/2012 12:04 AM, Artyom Kazak wrote: Евгений Пермяков permea...@gmail.com писал в своём письме Mon, 30 Jul 2012 09:47:48 +0300: Can someone tell me if there are any primitives, that used to detect machine type overflows, in ghc haskell ? I perfectly understand, that I can build something based on preconditioning of variables, but this will kill any performance, if needed. In GHC.Prim — primitives addIntC# and subIntC#: addIntC# :: Int# - Int# - (#Int#, Int##) Add with carry. First member of result is (wrapped) sum; second member is 0 iff no overflow occured. subIntC# :: Int# - Int# - (#Int#, Int##) Subtract with carry. First member of result is (wrapped) difference; second member is 0 iff no overflow occured. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Still no way to detect overflow in *. Strangely enough, I found some relevant descriptions in *.pp in dev branch, so I expect them in 7.6.1. They applies to native-size Word and Int only. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] why does a foldr not have a space leak effect?
The difference is that foldl *must* produce the entire list of thunks, even if f is lazy in its first argument. There's no foldl that can perform better given a sufficiently-lazy f; given head = foldr go fail where go x y = x fail = error head: empty list head [a,b,c,d] = foldr go fail [a,b,c,d] = go a (foldr go fail [b,c,d]) = a you might think you can define last = foldl go fail where go x y = y fail = error last: empty list but this fails to be sufficiently lazy: last [a,b,c,d] = foldl go fail [a,b,c,d] = foldl go (go fail a) [b,c,d] = foldl go (go (go fail a) b) [c,d] = foldl go (go (go (go fail a) b) c) [d] = foldl go (go (go (go (go fail a) b) c) d) [] = go (go (go (go fail a) b) c) d = d which allocates lots of extra space for thunks, which may even take more memory than the original list. Whereas if last uses foldl': last [a,b,c,d] = foldl' go fail [a,b,c,d] = let z = go fail a in seq z $ foldl' go z [b,c,d] = foldl' go a [b,c,d] = let z = go a b in seq z $ foldl' go z [c,d] = foldl' go b [c,d] = ... = let z = go c d in seq z $ foldl' go z [] = foldl' go d [] = d -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl hjgt...@chello.nl wrote: I am willing to do administrator tasks. 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external links' - which is all of the spam. This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with buy o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, Nike Air Jordan) Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
Can we have at least 5 consonants? There are enough people with names such as Srbský in eastern European In fact, the Czechs can make use of as many as 9 consonants in a row! http://ld.johanesville.net/perlicky/03- jazykova-nej-a-jine-hricky On a side note, image based CAPACHA's can cause problems for blind people. -- Původní zpráva -- Od: Henk-Jan van Tuyl hjgt...@chello.nl Datum: 30. 7. 2012 Předmět: Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl hjgt...@chello.nl wrote: I am willing to do administrator tasks. 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external links' - which is all of the spam. This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with buy o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, Nike Air Jordan) Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/(http://Van.Tuyl.eu/) http://members.chello.nl/hjgtuyl/tourdemonad.html (http://members.chello.nl/hjgtuyl/tourdemonad.html) Haskell programming -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe (http://www.haskell.org/mailman/listinfo/haskell-cafe)___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
On Mon, Jul 30, 2012 at 2:35 PM, Henk-Jan van Tuyl hjgt...@chello.nlwrote: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) This is a nice idea, but I think it will end up moving spam onto the mailing lists. There is hardly any policy in place to keep people out of the mailing lists. Mailing list spam is attractive to spammers, since it all gets mirrored to archive sites all over the place. Not to volunteer others, but how feasible would it be to require credentials from Haskellers.org? - Let new users only change pages, not create new pages This is good for stopping the creation of walled gardens full of spam. But it won't stop vandalism spam, where somebody goes to a page that isn't accessed much and changes it. Does anybody have statistics about how often pages are edited/added? If the numbers aren't too big, I'd volunteer to moderate insofar as scanning new edits/adds for spam. Maybe this role should just forward articles with spam on them to a real moderator to roll-back. We could even have a report spam button on each page, and if enough users click on it (for a given revision), the revision gets forwarded to a moderator. - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with buy o longer than 20 characters o with more than 4 consonants in a row I don't see this providing any security against spam, and I'm thinking it will take longer to implement than it will take for a spammer to fix his scripts in response. - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, Nike Air Jordan) Same. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
On 31 July 2012 05:35, Henk-Jan van Tuyl hjgt...@chello.nl wrote: ... with more than one x or q This would exclude legitimate Chinese (pinyin) usernames for not much gain. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
On 07/30/2012 05:35 PM, Henk-Jan van Tuyl wrote: On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl hjgt...@chello.nl wrote: I am willing to do administrator tasks. 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external links' - which is all of the spam. This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: There are two (easy) things that will make a huge dent in the automated stuff. 1. Add a fake field, hidden through CSS, labeled something like You must leave this field blank to submit the form (for non-visual browsers). Put it on every page with a submit button. If it isn't empty, don't process the submission. You can give it a /name/ that sounds tempting, though. 2. Force previews. If the bots are targeted at your wiki software and you modify it to preview all submissions, the bots will stop working. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Polyvariadic composition
My completely off-the-cuff guess is that
a a b b
isn't considered more or less specific than
(x - a) ar (x - b) br
since they both apply some constraint on the types. For example, it's not
immediately clear that the first instance can't be used for (x - a) (x -
a) (x - b) (x - b)
Whereas when you say
a ar b br
the type
(x - a) ar (x - b) br
is strictly more specific, so the overlapping instance can be chosen.
Remember instance selection is done entirely via the instance head, so
instance X a a
is not the same as
instance (a ~ b) = X a b
The first case supplies an instance for any two equal types, and the second
case supplies an instance for *any two types*, then throws an error if the
compiler can't prove that the two types are equal.
For example, without overlapping instances, you can write
class X a b where foo :: a - b
instance X a a where foo = id
instance X Int Bool where foo = (== 0)
But if instead you specify
instance (a ~ b) = X a b where foo = id
you can't specify the Int Bool instance without overlap.
-- ryan
On Mon, Jul 30, 2012 at 12:32 PM, Artyom Kazak artyom.ka...@gmail.comwrote:
Hello,
I have accidentally written my version of polyvariadic composition
combinator, `mcomp`. It differs from Oleg’s version (
http://okmij.org/ftp/Haskell/**polyvariadic.html#polyvar-comphttp://okmij.org/ftp/Haskell/polyvariadic.html#polyvar-comp)
in three aspects: a) it is simpler, b) it works without enumerating basic
cases (all existing types, in other words), and c) it needs more type
extensions.
{-# LANGUAGE
MultiParamTypeClasses
, FunctionalDependencies
, FlexibleInstances
, UndecidableInstances
, TypeFamilies , OverlappingInstances
#-}
class Mcomp a ar b br | a br - b where
mcomp :: a - (ar - br) - b
instance (a ~ ar, b ~ br) = Mcomp a ar b br where
mcomp a f = f a
instance (Mcomp a ar b br) = Mcomp (x - a) ar (x - b) br where
mcomp a f = \x - mcomp (a x) f
My question is: why doesn’t it work when I replace
instance (a ~ ar, b ~ br) = Mcomp a ar b br
with
instance Mcomp a a b b
? I thought that equal letters mean equal types…
__**_
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[Haskell-cafe] CRIP: the Curiously Reoccuring Instance Pattern
With apologies to Jim Coplien :) I've been seeing this pattern in a surprising number of instance definitions lately: instance (a ~ ar, b ~ br) = Mcomp a ar b br [1] instance (b ~ c, CanFilterFunc b a) = CanFilter (b - c) a [2] The trick is that since instance selection is done entirely on the instance head, these instances are strictly more general than the ones they replace: instance Mcomp a a b b instance CanFilterFunc b = CanFilter (b - b) a The compiler has to do a lot more work to select these instances; it has to prove that the matching types actually match before it can select the instance; if it can't, it won't select an instance, and instead will complain about no instance CLASS Int a. But with the CRIP, you help the compiler--it chooses the general instance, and then gets a constraint to solve. The constraint forces the two types to unify, or else there is a type error. What I'm wondering is--are there many cases where you really want the non-constraint-generating behavior? It seems like (aside from contrived, ahem, instances) whenever you have instance CLASS A B where A and B share some type variables, that there aren't any valid instances of the same class where they don't share the types in that way. For example, I've never really seen a class in practice with instances like class Foo a b instance Foo a a instance Foo ConcreteTypeA ConcreteTypeB Note that it's very difficult to use polymorphic types in the second instance without risking overlap. TL;DR: I, for one, welcome our new type equality constraint overlords. -- ryan [1] http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/99611 [2] http://www.yesodweb.com/blog/2012/07/classy-prelude ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is (==) commutative?
Thanks Wren, for the explanations (also in your previous mail)! On 07/30/2012 01:29 PM, wren ng thornton wrote: On 7/24/12 9:19 PM, Christian Sternagel wrote: (x == y) = True == x = y (x == y) = False == not (x = y) (x == _|_) = _|_ (_|_ == y) = _|_ Those axioms state that (==) is sound w.r.t. to meta-equality and strict in both it's arguments. An immediate problem that arises here is: what exactly does meta-equality denote in Isabelle/HOLCF? If it is meant to denote syntactic identity or Leibniz equality a la Coq, then the first axiom is certainly too strong for any interesting data types (e.g., Rational, Data.Set, Data.Map, Data.IntSet,...) Yes, we also stumbled about this (but I was only discussing it on the isabelle mailing list and not here, sorry). The conclusion was that for many type classes there are several interesting (whatever that means) instances. Thus, in the formalization we will use different formal type classes for different intended use-cases of Haskell type classes (e.g., one eq class for syntactic equality and another one that merely requires an equivalence relation, ...). That just means that there will not be a bijection between the formal class hierarchy and the actual class hierarchy of Haskell, but should not pose any further problems (I hope). Btw: I don't agree that the axioms are too strong for *any* interesting data types ;). What about Bool, Int, [a], ...? (Again, this depends on how we interpret interesting; in formalizations the threshold for being interesting is typically lower.) cheers chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] specifying using type class
Generally the way this is done in Haskell is that the interface to the type
is specified in a typeclass (or, alternatively, in a module export list,
for concrete types), and the axioms are specified in a method to be tested
in some framework (i.e. QuickCheck, SmallCheck, SmartCheck) which can
automatically generate instances of your type and test that the axioms hold.
For example:
class QueueLike q where
empty :: q a
insert :: a - q a - q a
viewFirst :: q a - Maybe (a, q a)
size :: q a - Integer
-- can use a single proxy type if have kind polymorphism, but that's an
experimental feature right now
data Proxy2 (q :: * - *) = Proxy2
instance Arbitrary (Proxy2 q) where arbitrary = return Proxy2
prop_insertIncrementsSize :: forall q. QueueLike q = q () - Bool
prop_insertIncrementsSize q = size (insert () q) == size q + 1
prop_emptyQueueIsEmpty :: forall q. QueueLike q = Proxy2 q = Bool
prop_emptyQueueIsEmpty Proxy2 = maybe True (const False) $ view (empty :: q
())
Then you specialize these properties to your type and test them:
instance QueueLike [] where ...
ghci quickCheck (prop_insertIncrementsSize :: [()] - Bool)
Valid, passed 100 tests
or
Failed with: [(), (), ()]
QuickCheck randomly generates objects of your data structure and tests your
property against them. While not as strong as a proof, programs with 100%
quickcheck coverage are *extremely* reliable. SmartCheck is an extension
of QuickCheck that tries to reduce test cases to the minimum failing size.
SmallCheck does exhaustive testing on the properties for finite data
structures up to a particular size. It's quite useful when you can prove
your algorithms 'generalize' after a particular point.
There aren't any libraries that I know of for dependent-type style program
proof for haskell; I'm not sure it's possible. The systems I know of have
you program in a more strongly typed language (Coq/agda) and export Haskell
programs once they are proven safe. Many of these rely on unsafeCoerce in
the Haskell code because they have proven stronger properties about the
types than Haskell can; I look at that code with some trepidation--I am not
sure what guarantees the compiler makes about unsafeCoerce.
-- ryan
On Sun, Jul 22, 2012 at 7:19 AM, Patrick Browne patrick.bro...@dit.iewrote:
{-
Below is a *specification* of a queue.
If possible I would like to write the equations in type class.
Does the type class need two type variables?
How do I represent the constructors?
Can the equations be written in the type class rather than the instance?
-}
module QUEUE_SPEC where
data Queue e = New | Insert (Queue e) e deriving Show
isEmpty :: Queue e - Bool
isEmpty New = True
isEmpty (Insert q e) = False
first :: Queue e - e
first (Insert q e) = if (isEmpty q) then e else (first q)
rest :: Queue e - Queue e
rest (Insert q e ) = if (isEmpty q) then New else (Insert (rest q) e)
size :: Queue e - Int
size New = 0
size (Insert q e) = succ (size q)
{-
some tests of above code
size (Insert (Insert (Insert New 5) 6) 3)
rest (Insert (Insert (Insert New 5) 6) 3)
My first stab at a class
class QUEUE_SPEC q e where
new :: q e
insert :: q e - q e
isEmpty :: q e - Bool
first :: q e - e
rest :: q e - q e
size :: q e - Int
-}
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[Haskell-cafe] What does unpacking an MVar really mean?
I admit I don't know exactly how MVars are implemented, but given that they can be aliased and have indefinite extent, I would think that they look something vaguely like a cdatatype ** var, basically a pointer to an MVar (which is itself a pointer, modulo some other things such as a thread queue.) And, I would think that unpacking such an structure would basically be eliminating one layer of indirection, so it would then look vague like a cdatatype * var.But again, given aliasing and indefinite extent, this would seem to be a difficult thing to do. Actually this isn't too difficult if an MVar only exists in a single unpacked structure: other references to the MVar can simply be pointers into the structure. But the case where an MVar is unpacked into two different structures suggests that, at least in some cases, an unpacked MVar is still a cdatatype ** var; So, is my understanding more or less correct? Does anybody have a good, succinct explanation of how MVars are implemented, and how they are unpacked? One final question, assuming that unpacking an MVar really does eliminate a layer of indirection, and that other references to that MVar are simply pointers into a larger structure, what happens to that larger structure when there are no more references to it (but still some references to the MVar?)Given the complications that must arise out of a doubly unpacked MVar, I'm going to guess that the larger structure does get garbage collected in this case, and that the MVar becomes dislodged from this structure. Would that MVar then be placed directly inside another unpacked reference, if one is available? Best, Leon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What does unpacking an MVar really mean?
I'm not sure I totally understand your question about 'unpacking' an MVar,
but I'm going to assume you mean data structures that use the {-# UNPACK
#-} pragma, like in Control.Concurrent.Future [1] and
Control.Concurrent.NamedLock [2].
Here is how MVar is defined in GHC [3]:
data MVar a = MVar (MVar# RealWorld a)
A MVar# s a is an unboxed pointer to a structure understood by the GHC
runtime.
So yes, you can imagine a MVar as a pointer-to-pointer. The structure it
points at likely has another pointer to the embedded boxed a, so it may
even be pointer-to-pointer-to-pointer.
The MVar data structure exists to allow laziness; for example
let x = unsafePerformIO (newMVar ()) in ()
is likely to not allocate an MVar#, just a thunk that would create an MVar
if it was evaluated. Unboxed objects (represented by convetion with # in
GHC), on the other hand, are strict, so if you have an MVar# RealWorld (),
you know it points to a valid MVar#.
From [2] we have
data NLItem = NLItem {-# UNPACK #-} !Int
{-# UNPACK #-} !(MVar ())
All the {-# UNPACK #-} pragma does is embed the contents of a strict
single-constructor data declaration *directly* into the structure
containing it; it's like you declared NLItem as such:
data NLItem = NLItem Word# (MVar# RealWorld ())
except that if you call functions that want an Int or MVar thunk, the
compiler will automatically re-box them in a new I#/MVar constructor.
Many copies of pointers to the same MVar# may exist; they are all
'identical' MVars; equality is defined as such:
instance Eq (MVar a) where
(MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
where sameMVar# is a primitive that is probably just raw pointer equality.
Because of this, boxed MVars can be garbage collected without necessarily
garbage-collecting the MVar# it holds, if a live reference to that MVar#
still exists elsewhere.
-- ryan
[1]
http://hackage.haskell.org/packages/archive/future/2.0.0/doc/html/src/Control-Concurrent-Future.html
[2]
http://hackage.haskell.org/packages/archive/named-lock/0.1/doc/html/src/Control-Concurrent-NamedLock.html
[3]
http://www.haskell.org/ghc/docs/7.4.1/html/libraries/base/src/GHC-MVar.html#MVar
On Mon, Jul 30, 2012 at 9:25 PM, Leon Smith leon.p.sm...@gmail.com wrote:
I admit I don't know exactly how MVars are implemented, but given that
they can be aliased and have indefinite extent, I would think that they
look something vaguely like a cdatatype ** var, basically a pointer to an
MVar (which is itself a pointer, modulo some other things such as a thread
queue.)
And, I would think that unpacking such an structure would basically be
eliminating one layer of indirection, so it would then look vague like a
cdatatype * var.But again, given aliasing and indefinite extent, this
would seem to be a difficult thing to do.
Actually this isn't too difficult if an MVar only exists in a single
unpacked structure: other references to the MVar can simply be pointers
into the structure. But the case where an MVar is unpacked into two
different structures suggests that, at least in some cases, an unpacked
MVar is still a cdatatype ** var;
So, is my understanding more or less correct? Does anybody have a good,
succinct explanation of how MVars are implemented, and how they are
unpacked?
One final question, assuming that unpacking an MVar really does
eliminate a layer of indirection, and that other references to that MVar
are simply pointers into a larger structure, what happens to that larger
structure when there are no more references to it (but still some
references to the MVar?)Given the complications that must arise out of
a doubly unpacked MVar, I'm going to guess that the larger structure
does get garbage collected in this case, and that the MVar becomes
dislodged from this structure. Would that MVar then be placed directly
inside another unpacked reference, if one is available?
Best,
Leon
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Re: [Haskell-cafe] What does unpacking an MVar really mean?
Let me clarify a bit.
I am familiar with the source of Control.Concurrent.MVar, and I do see {-#
UNPACK #-}'ed MVars around, for example in GHC's IO manager. What I
should have asked is, what does an MVar# look like? This cannot be
inferred from Haskell source; though I suppose I could have tried to read
the Runtime source.
Now, one would hope that and (MVar# RealWorld footype) would
approximately correspond to a footype * mvar; variable in C. The
problem is this cannot _always_ be the case, because you can alias the
(MVar# RealWorld footype) by placing a single MVar into two unpacked
columns in two different data structures.So you would need to be able
to still sometimes represent an MVar# by a footype ** mvar at runtime,
even though one would hope that it would be represented by a footype *
mvar in one particular data structure.
On Tue, Jul 31, 2012 at 1:04 AM, Ryan Ingram ryani.s...@gmail.com wrote:
Because of this, boxed MVars can be garbage collected without necessarily
garbage-collecting the MVar# it holds, if a live reference to that MVar#
still exists elsewhere.
I was asking the dual question: if the MVar# exists in some data
structure, can that data structure still be garbage collected when there
is a reference to the MVar#, but not the data structure it is contained
within.
Best,
Leon
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Re: [Haskell-cafe] How to define a Monad instance
To take this a step further, if what you really want is the syntax sugar
for do-notation (and I understand that, I love sweet, sweet syntactical
sugar), you are probably implementing a Writer monad over some monoid.
Here's two data structures that can encode this type;
data Replacer1 k a = Replacer1 (k - Maybe k) a
data Replacer2 k a = Replacer2 [(k,k)] a
instance Monad Replacer1 where
return x = Replacer1 (\_ - Nothing) x
Replacer1 ka a = f = result where
Replacer1 kb b = f a
result = Replacer1 (\x - ka x `mplus` kb x) b
(!) :: Eq k = k - k - Replacer1 k ()
x ! y = Replacer1 (\k - if k == x then Just y else Nothing) ()
replace1 :: Replacer1 k () - [k] - [k]-- look ma, no Eq requirement!
replace1 (Replacer k ()) = map (\x - fromMaybe x $ k x) -- from Data.Maybe
table1 :: Replacer1 Char ()
table1 = do
'a' ! 'b'
'A' ! 'B'
test = replace1 table1 All I want
-- Exercise: what changes if we switch ka and kb in the result of (=)?
When does it matter?
-- Exercises for you to implement:
instance Monad Replacer2 k where
replacer :: Eq k = Replacer2 k - [k] - [k]
($) :: k - k - Replacer2 k
-- Exercise: Lets make use of the fact that we're a monad!
--
-- What if the operator ! had a different type?
-- (!) :: Eq k = k - k - Replacer k Integer
-- which returns the count of replacements done.
--
-- table3 = do
-- count - 'a' ! 'b'
-- when (count 3) ('A' ! 'B')
-- return ()
--
-- Do any of the data structures I've given work? Why or why not?
-- Can you come up with a way to implement this?
-- ryan
On Sat, Jul 28, 2012 at 10:07 AM, Steffen Schuldenzucker
sschuldenzuc...@uni-bonn.de wrote:
On 07/28/2012 03:35 PM, Thiago Negri wrote:
[...]
As Monads are used for sequencing, first thing I did was to define the
following data type:
data TableDefinition a = Match a a (TableDefinition a) | Restart
So TableDefinition a is like [(a, a)].
[...]
So, to create a replacement table:
table' :: TableDefinition Char
table' =
Match 'a' 'b'
(Match 'A' 'B'
Restart)
It look like a Monad (for me), as I can sequence any number of
replacement values:
table'' :: TableDefinition Char
table'' = Match 'a' 'c'
(Match 'c' 'a'
(Match 'b' 'e'
(Match 'e' 'b'
Restart)))
Yes, but monads aren't just about sequencing. I like to see a monad as a
generalized computation (e.g. nondeterministic, involving IO, involving
state etc). Therefore, you should ask yourself if TableDefinition can be
seen as some kind of abstract computation. In particular, can you
execute a computation and extract its result? as in
do
r - Match 'a' 'c' Restart
if r == 'y' then Restart else Match 2 3 (Match 3 4 Restart)
Doesn't immediately make sense to me. In particular think about the
different possible result types of a TableDefinition computation.
If all you want is sequencing, you might be looking for a Monoid instance
instead, corresponding to the Monoid instance of [b], where b=(a,a) here.
[...]
I'd like to define the same data structure as:
newTable :: TableDefinition Char
newTable = do
'a' : 'b'
'A' : 'B'
But I can't figure a way to define a Monad instance for that. :(
The desugaring of the example looks like this:
('a' : 'b') ('A' : 'B')
Only () is used, but not (=) (i.e. results are always discarded). If
this is the only case that makes sense, you're probably looking for a
Monoid instead (see above)
-- Steffen
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Re: [Haskell-cafe] How to define a Monad instance
A couple typos:
instance Monad Replacer1 where
-
instance Monad (Replacer1 k) where
instance Monad Replacer2 k where
-
instance Monad (Replacer2 k) where
I haven't tested any of this code, so you may have to fix some minor type
errors.
On Mon, Jul 30, 2012 at 10:38 PM, Ryan Ingram ryani.s...@gmail.com wrote:
To take this a step further, if what you really want is the syntax sugar
for do-notation (and I understand that, I love sweet, sweet syntactical
sugar), you are probably implementing a Writer monad over some monoid.
Here's two data structures that can encode this type;
data Replacer1 k a = Replacer1 (k - Maybe k) a
data Replacer2 k a = Replacer2 [(k,k)] a
instance Monad Replacer1 where
return x = Replacer1 (\_ - Nothing) x
Replacer1 ka a = f = result where
Replacer1 kb b = f a
result = Replacer1 (\x - ka x `mplus` kb x) b
(!) :: Eq k = k - k - Replacer1 k ()
x ! y = Replacer1 (\k - if k == x then Just y else Nothing) ()
replace1 :: Replacer1 k () - [k] - [k]-- look ma, no Eq requirement!
replace1 (Replacer k ()) = map (\x - fromMaybe x $ k x) -- from Data.Maybe
table1 :: Replacer1 Char ()
table1 = do
'a' ! 'b'
'A' ! 'B'
test = replace1 table1 All I want
-- Exercise: what changes if we switch ka and kb in the result of (=)?
When does it matter?
-- Exercises for you to implement:
instance Monad Replacer2 k where
replacer :: Eq k = Replacer2 k - [k] - [k]
($) :: k - k - Replacer2 k
-- Exercise: Lets make use of the fact that we're a monad!
--
-- What if the operator ! had a different type?
-- (!) :: Eq k = k - k - Replacer k Integer
-- which returns the count of replacements done.
--
-- table3 = do
-- count - 'a' ! 'b'
-- when (count 3) ('A' ! 'B')
-- return ()
--
-- Do any of the data structures I've given work? Why or why not?
-- Can you come up with a way to implement this?
-- ryan
On Sat, Jul 28, 2012 at 10:07 AM, Steffen Schuldenzucker
sschuldenzuc...@uni-bonn.de wrote:
On 07/28/2012 03:35 PM, Thiago Negri wrote:
[...]
As Monads are used for sequencing, first thing I did was to define the
following data type:
data TableDefinition a = Match a a (TableDefinition a) | Restart
So TableDefinition a is like [(a, a)].
[...]
So, to create a replacement table:
table' :: TableDefinition Char
table' =
Match 'a' 'b'
(Match 'A' 'B'
Restart)
It look like a Monad (for me), as I can sequence any number of
replacement values:
table'' :: TableDefinition Char
table'' = Match 'a' 'c'
(Match 'c' 'a'
(Match 'b' 'e'
(Match 'e' 'b'
Restart)))
Yes, but monads aren't just about sequencing. I like to see a monad as a
generalized computation (e.g. nondeterministic, involving IO, involving
state etc). Therefore, you should ask yourself if TableDefinition can be
seen as some kind of abstract computation. In particular, can you
execute a computation and extract its result? as in
do
r - Match 'a' 'c' Restart
if r == 'y' then Restart else Match 2 3 (Match 3 4 Restart)
Doesn't immediately make sense to me. In particular think about the
different possible result types of a TableDefinition computation.
If all you want is sequencing, you might be looking for a Monoid instance
instead, corresponding to the Monoid instance of [b], where b=(a,a) here.
[...]
I'd like to define the same data structure as:
newTable :: TableDefinition Char
newTable = do
'a' : 'b'
'A' : 'B'
But I can't figure a way to define a Monad instance for that. :(
The desugaring of the example looks like this:
('a' : 'b') ('A' : 'B')
Only () is used, but not (=) (i.e. results are always discarded). If
this is the only case that makes sense, you're probably looking for a
Monoid instead (see above)
-- Steffen
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