[Haskell-cafe] Graham-Scan Algorithm exercise from Chapter 3 Real World Haskell
I have started the Graham Scan Algorithm exercise. I'm getting tripped up by the sortByCotangent* function. Here's what I have so far data Direction = DStraight | DLeft | DRight deriving (Eq,Show) type PointXY = (Double,Double) calcTurn :: PointXY - PointXY - PointXY - Direction calcTurn a b c | crossProduct == 0 = DStraight | crossProduct 0 = DLeft | otherwise = DRight where crossProduct = ((fst b - fst a) * (snd c - snd a)) - ((snd b - snd a) * (fst c - fst a)) calcDirectionList :: [PointXY] - [Direction] calcDirectionList (x:y:z:zs) = (calcTurn x y z) : (calcDirectionList (y:z:zs)) calcDirectionList _ = [] sortListByY :: [PointXY] - [PointXY] sortListByY [] = [] sortListByY [a] = [a] sortListByY (a:as) = insert (sortListByY as) where insert [] = [a] insert (b:bs) | snd a = snd b = a : b : bs | otherwise = b : insert bs sortListByCoTangent :: [PointXY] - [PointXY] sortListByCoTangent [] = [] sortListByCoTangent [a] = [a] sortListByCoTangent (a:as) = a : insert (sortListByCoTangent as) where insert :: [PointXY] - [PointXY] insert [] = [a] insert [b] = [b] insert (b:c:cs) | (myCoTan a b) = (myCoTan a c) = b : c : cs | otherwise = c : b : insert cs where myCoTan :: PointXY - PointXY - Double myCoTan p1 p2 = (fst p2 - fst p1) / (snd p2 - snd p1) test data *Main sortListByCoTangent (sortListByY [(1,2),(2,6),(3,10),(4,9),(5,10),(2,20),(6,15)]) [(1.0,2.0),(5.0,10.0),(2.0,6.0),(4.0,9.0),(6.0,15.0),(3.0,10.0),(2.0,20.0)] (1,0,2.0) is correct. That's the pivot point. It screws up from there. I suspect my insert is hosed, but I'm having difficulty analyzing the logic of the code. I'd like hints/help but with the following boundaries. (1) I want to stick with the parts of the language that's been introduced in the text so far. I know there are solutions that make this problem trivial, however using those misses the point. (2) I'd prefer going over the logic of my code, versus what is supposed to happen. I'm trying to learn how to troubleshoot haskell code, more than implement the graham scan algorithm. I appreciate any help/hints Michael Litchard *It seems the wikipedia page on the graham scan algorithm is wrong concerning the following part of the algorithm. ...instead, it suffices to calculate the tangent of this angle, which can be done with simple arithmetic. Someone from #haskell said that it's the cotangent I want, and my math tutor confirmed. If this is the case, I suppose we should submit a correction. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Expect module?
Hi all, Is there a Haskell-Expect module? Something that would allow me to control an external Unix program via its stdin/stdout/stderr? Cheers, Erik -- - Erik de Castro Lopo - Seen on comp.lang.c : klaushuot : C rocks the world. Plain and simple. It's the ultimate solution, period. user923005 : Every tool should be a screwdriver. You can hammer with it, if you pound hard enough. You can pry with it -- just like a crowbar. Or, perhaps, there are room for other tools in the bag. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Expect module?
Hi Erik, Is there a Haskell-Expect module? Something that would allow me to control an external Unix program via its stdin/stdout/stderr? System.Process does what you want, I think: http://hackage.haskell.org/packages/archive/process/1.0.1.1/doc/html/System-Process.html Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: walking a directory tree efficiently
Massimiliano Gubinelli wrote: Hi, what about avoid the use of the unfold over the tree and construct it directly (e.g. see http://hpaste.org/13919#a3)? Nice solution! I wonder if there is (an easy) possibility to construct the tree lazily so that output start immediately for large trees. I think the modular approach would be that of using a fold-left enumerator that produces the list of paths and navigation operations by traversing the dir hierarchy Depth First: data DirTraversalInfo = Path String | DirUp | DirDown I guess you know: http://okmij.org/ftp/papers/LL3-collections-talk.pdf best, Massimiliano Gubinelli Ciao Paolo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Graham-Scan Algorithm exercise from Chapter 3 Real World Haskell
Am Montag, 19. Januar 2009 09:32 schrieb Michael Litchard: I have started the Graham Scan Algorithm exercise. I'm getting tripped up by the sortByCotangent* function. Here's what I have so far data Direction = DStraight | DLeft | DRight deriving (Eq,Show) type PointXY = (Double,Double) calcTurn :: PointXY - PointXY - PointXY - Direction calcTurn a b c | crossProduct == 0 = DStraight | crossProduct 0 = DLeft | otherwise = DRight where crossProduct = ((fst b - fst a) * (snd c - snd a)) - ((snd b - snd a) * (fst c - fst a)) calcDirectionList :: [PointXY] - [Direction] calcDirectionList (x:y:z:zs) = (calcTurn x y z) : (calcDirectionList (y:z:zs)) calcDirectionList _ = [] sortListByY :: [PointXY] - [PointXY] sortListByY [] = [] sortListByY [a] = [a] sortListByY (a:as) = insert (sortListByY as) where insert [] = [a] insert (b:bs) | snd a = snd b = a : b : bs | otherwise = b : insert bs I think it would be easier to see what the code does if you had it sortListByY [] = [] sortListByY (a:as) = insertByY a (sortListByY as) where insertByY a (b:bs) | snd a = snd b = a:b:bs | otherwise = b:insertByY a bs insertByY a [] = [a] analogously for sortListByCoTangent. sortListByCoTangent :: [PointXY] - [PointXY] sortListByCoTangent [] = [] sortListByCoTangent [a] = [a] sortListByCoTangent (a:as) = a : insert (sortListByCoTangent as) where insert :: [PointXY] - [PointXY] insert [] = [a] ^^ shouldn't that be insert [] = [], if at all? However, this will never be encountered, so you can delete it. insert [b] = [b] insert (b:c:cs) | (myCoTan a b) = (myCoTan a c) = b : c : cs | otherwise = c : b : insert cs There's the oops. You can only pass one point at a time, so it should be ... b:insert (c:cs) resp. ... c:insert (b:cs) where myCoTan :: PointXY - PointXY - Double myCoTan p1 p2 = (fst p2 - fst p1) / (snd p2 - snd p1) test data *Main sortListByCoTangent (sortListByY [(1,2),(2,6),(3,10),(4,9),(5,10),(2,20),(6,15)]) [(1.0,2.0),(5.0,10.0),(2.0,6.0),(4.0,9.0),(6.0,15.0),(3.0,10.0),(2.0,20.0)] (1,0,2.0) is correct. That's the pivot point. It screws up from there. I suspect my insert is hosed, but I'm having difficulty analyzing the logic of the code. I'd like hints/help but with the following boundaries. (1) I want to stick with the parts of the language that's been introduced in the text so far. I know there are solutions that make this problem trivial, however using those misses the point. (2) I'd prefer going over the logic of my code, versus what is supposed to happen. I'm trying to learn how to troubleshoot haskell code, more than implement the graham scan algorithm. Walk through your code by hand for very small inputs (say four or five vertices in several orders). Then you see how exactly it works, and find more easily what's wrong (and what to rewrite in a clearer fashion). I appreciate any help/hints Michael Litchard *It seems the wikipedia page on the graham scan algorithm is wrong concerning the following part of the algorithm. ...instead, it suffices to calculate the tangent of this angle, which can be done with simple arithmetic. Someone from #haskell said that it's the cotangent I want, and my math tutor confirmed. If this is the case, I suppose we should submit a correction. Actually, both will do. Using the tangent requires a little sophistication in sorting, though (first positive tangent in increasing order, then infinity if it appears, finally negative tangent in decreasing order), so it's not technically wrong, but the cotangent is better. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Comments from OCaml Hacker Brian Hurt
david48 wrote: Apfelmus, Heinrich wrote: Hm, what about the option of opening Bird's Introduction on Functional Programming using Haskell in the section about fold? Monoid is on page 62 in the translated copy I've got here. I don't think that I would try to learn a programming language, for example Python, without obtaining a paper book on it. I would, if the online documentation makes it possible, and then I would buy a paper book later, to go further or for reference. That's how I learned Haskell, and much later I've bought my first book. Interesting, I wouldn't want to miss actual paper when learning difficult topics. Also, some great resources like the contents of Bird's book just aren't available online ;). I'd recommend to borrow it from a library, though, the current amazon price is quite outrageous. Regards, apfelmus -- http://apfelmus.nfshost.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Graham-Scan Algorithm exercise from Chapter 3 Real World Haskell
Am Montag, 19. Januar 2009 10:17 schrieb Daniel Fischer: Am Montag, 19. Januar 2009 09:32 schrieb Michael Litchard: I have started the Graham Scan Algorithm exercise. I'm getting tripped up by the sortByCotangent* function. Here's what I have so far data Direction = DStraight | DLeft | DRight deriving (Eq,Show) type PointXY = (Double,Double) calcTurn :: PointXY - PointXY - PointXY - Direction calcTurn a b c | crossProduct == 0 = DStraight | crossProduct 0 = DLeft | otherwise = DRight where crossProduct = ((fst b - fst a) * (snd c - snd a)) - ((snd b - snd a) * (fst c - fst a)) calcDirectionList :: [PointXY] - [Direction] calcDirectionList (x:y:z:zs) = (calcTurn x y z) : (calcDirectionList (y:z:zs)) calcDirectionList _ = [] sortListByY :: [PointXY] - [PointXY] sortListByY [] = [] sortListByY [a] = [a] sortListByY (a:as) = insert (sortListByY as) where insert [] = [a] insert (b:bs) | snd a = snd b = a : b : bs | otherwise = b : insert bs I think it would be easier to see what the code does if you had it sortListByY [] = [] sortListByY (a:as) = insertByY a (sortListByY as) where insertByY a (b:bs) | snd a = snd b = a:b:bs | otherwise = b:insertByY a bs insertByY a [] = [a] analogously for sortListByCoTangent. sortListByCoTangent :: [PointXY] - [PointXY] sortListByCoTangent [] = [] sortListByCoTangent [a] = [a] sortListByCoTangent (a:as) = a : insert (sortListByCoTangent as) where insert :: [PointXY] - [PointXY] insert [] = [a] ^^ shouldn't that be insert [] = [], if at all? However, this will never be encountered, so you can delete it. insert [b] = [b] insert (b:c:cs) | (myCoTan a b) = (myCoTan a c) = b : c : cs | otherwise = c : b : insert cs There's the oops. You can only pass one point at a time, so it should be ... b:insert (c:cs) resp. ... c:insert (b:cs) where myCoTan :: PointXY - PointXY - Double myCoTan p1 p2 = (fst p2 - fst p1) / (snd p2 - snd p1) test data *Main sortListByCoTangent (sortListByY [(1,2),(2,6),(3,10),(4,9),(5,10),(2,20),(6,15)]) [(1.0,2.0),(5.0,10.0),(2.0,6.0),(4.0,9.0),(6.0,15.0),(3.0,10.0),(2.0,20.0 )] (1,0,2.0) is correct. That's the pivot point. It screws up from there. I suspect my insert is hosed, but I'm having difficulty analyzing the logic of the code. I'd like hints/help but with the following boundaries. (1) I want to stick with the parts of the language that's been introduced in the text so far. I know there are solutions that make this problem trivial, however using those misses the point. (2) I'd prefer going over the logic of my code, versus what is supposed to happen. I'm trying to learn how to troubleshoot haskell code, more than implement the graham scan algorithm. Walk through your code by hand for very small inputs (say four or five vertices in several orders). Then you see how exactly it works, and find more easily what's wrong (and what to rewrite in a clearer fashion). I appreciate any help/hints Another thing, your sortListByCoTangent is inefficient because you unnecessarily sort all tails of the list according to their first element, while you only want to sort according to the very first element of the entire list. Also, you recompute the cotangent of all segments, it would probably be better to calculate it only once. sortListByCoTangent [] = [] sortListByCoTangent [a] = [a] sortListByCoTangent (a:bs) = a:map point (sortBC (map addCT bs)) where addCT b = (fst b - fst a, snd b - snd a, b) point (dx,dy,p) = p sortBC [] = [] sortBC (t:ts) = insert t (sortBC ts) insert t [] = [t] insert (dx1,dy1,p1) ((dx2,dy2,p2):ts) | dx1*dy2 dx2*dy1 = (dx2,dy2,p2):insert (dx1,dy1,p1) ts | otherwise = (dx1,dy1,p1):(dx2,dy2,p2):ts sorts only once. However, it is still an insertion sort, which is not the most efficient sorting method. Michael Litchard *It seems the wikipedia page on the graham scan algorithm is wrong concerning the following part of the algorithm. ...instead, it suffices to calculate the tangent of this angle, which can be done with simple arithmetic. Someone from #haskell said that it's the cotangent I want, and my math tutor confirmed. If this is the case, I suppose we should submit a correction. Actually, both will do. Using the tangent requires a little sophistication in sorting, though (first positive tangent in increasing order, then infinity if it appears, finally negative tangent in decreasing order),
[Haskell-cafe] Factoring into type classes
Hi everyone, I have a general program design question, but I can't really think of good examples so it will be a bit vague. There was a discussion on Show not long ago which brought up the problem that there are several ways to show a data structure, and it depends on the context (or should I call it a use case?) which of these we actually want, e.g. human readable form, debug information, serialisation for later reading and so on, and one of the solutions was to propose a family of show functions that carry the intended use in their name. However, there are other type classes that are too general to assign such concrete uses to. For instance, if a data structure can have more than one meaningful (and useful) Functor or Monoid instance, what should one do? Should one refrain from instantiating these classes altogether and just use the names of operations directly? If one still decides to pick a certain set of operations as an instance, what are the factors that should guide this decision? What about designing libraries, how much should one prefer standard classes for their interfaces? It seems to me that there is practically no literature on design issues like these, and it would be nice to hear some opinions from experienced Haskellers. Gergely -- http://www.fastmail.fm - The way an email service should be ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
As for multiple Monoid or Functor instances, simply define a newtype, as it is done in Data.Monoid. What part of your question does that answer and what part doesn't it? 2009/1/19 Patai Gergely patai_gerg...@fastmail.fm: Hi everyone, I have a general program design question, but I can't really think of good examples so it will be a bit vague. There was a discussion on Show not long ago which brought up the problem that there are several ways to show a data structure, and it depends on the context (or should I call it a use case?) which of these we actually want, e.g. human readable form, debug information, serialisation for later reading and so on, and one of the solutions was to propose a family of show functions that carry the intended use in their name. However, there are other type classes that are too general to assign such concrete uses to. For instance, if a data structure can have more than one meaningful (and useful) Functor or Monoid instance, what should one do? Should one refrain from instantiating these classes altogether and just use the names of operations directly? If one still decides to pick a certain set of operations as an instance, what are the factors that should guide this decision? What about designing libraries, how much should one prefer standard classes for their interfaces? It seems to me that there is practically no literature on design issues like these, and it would be nice to hear some opinions from experienced Haskellers. Gergely -- http://www.fastmail.fm - The way an email service should be ___ 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] Different return type?
Hello! I wouldn't use either. It seems like it complicates things quite a lot and it looks like this could be solved more simply by setting up the data types or organizing functions differently. Is there a specific problem that you're solving or are you just curious about different return types based on input types? Cheerio! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Different return type?
John Ky newho...@gmail.com writes:
Hi,
Possibly a silly question but is it possible to have a function that has a
different return type based on it's first argument?
Are you sure that's what you really want?
For instance
data Person = Person { name :: String, ... }
data Business = Business { business_number :: Int, ...}
data Entity = Person {...} | Business {...}
key person = name person
key business = business_number business
data Key = PersonKey String | BusinessKey Int
It seems likely that you are at least sometimes going to
want to pass the result of key to some other function.
...?
--
Jón Fairbairn jon.fairba...@cl.cam.ac.uk
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2008-04-26)
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[Haskell-cafe] Re: ANNOUNCE: Coadjute 0.0.1, generic build tool
Brandon S. Allbery KF8NH wrote: On 2009 Jan 18, at 13:47, Matti Niemenmaa wrote: 3. Coadjute keeps track of command line arguments (see docs for details): for me this is really a killer feature, I don't know of anything else which does this. It's been done many times before; it never seems to catch on. My personal favorite was Shape (http://user.cs.tu-berlin.de/~shape/) which I used for a few local projects in the late 80s. SCons is perhaps the most popular tool in this class (and itself a Pythonization of the original Perl Cons; maybe it's time for HCons?), followed by Apache's Ant (I don't think that actually caches command lines or binaries though), then Jam and successors. Actually, you're right, and I misspoke: I know about SCons and didn't like it (see below for brief rant)—Coadjute is the only truly /generic/ build tool I knew of which does it. The main thing I'll mention that turned me off SCons was that it tries to do too much stuff automatically: it's very much built around the idea that you just give it a filename and it figures out what to do. E.g. foo.hs and it detects what files it depends on, what Haskell compilers you've got, picks one (arbitrarily?), and compiles all the sources into a binary. (On my Windows machine this meant fun delays of multiple seconds prior to every build, as it scanned all the directories in PATH for several binaries which it never found: looking for various C compilers while I haven't even specified C dependencies.) I found it quite difficult (verbose and non-obvious) and probably against its philosophies to just say here is foo.hs, it depends on foo.c, don't do anything else with them and just run the following command if foo.hs needs to be built. To be completely honest I don't like the idea of SCons in itself: it seems to be something in between platform-specific tools like Cabal and generic ones like make or Coadjute. Maybe I'm just too dense, or maybe it's a documentation problem, but it's just not something I'd use for pretty much anything. (C/C++ don't really have tools geared specifically for them (CMake?) so that might be one case where I would.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Coadjute 0.0.1, generic build tool
2009/1/18 Matti Niemenmaa matti.niemenmaa+n...@iki.fi: Announcing the release of Coadjute, version 0.0.1! [...] Portability is striven towards in two ways: Is it intended to work on Windows? (I don't want to spend time downloading and trying to set it up if it was never intended to be Windows-compatible.) Paul. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
On Mon, Jan 19, 2009 at 3:58 AM, Patai Gergely patai_gerg...@fastmail.fmwrote: However, there are other type classes that are too general to assign such concrete uses to. For instance, if a data structure can have more than one meaningful (and useful) Functor or Monoid instance, As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Different return type?
John Ky newho...@gmail.com wrote in article
bd4fcb030901181744i2b26172bv2328974ff911f...@mail.gmail.com in
gmane.comp.lang.haskell.cafe:
data Person = Person { name :: String, ... }
data Business = Business { business_number :: Int, ...}
key person = name person
key business = business_number business
Let's make this concrete:
data Person = Person { name :: String, age :: Integer }
data Business = Business { business_number :: Int, revenue :: Double }
key person = name person
key business = business_number business
Even without dependent types, you can do the following (but of course,
you lose some syntactic sugar for records):
data Individual k v = Individual { key :: k, value :: v }
type Person = Individual String Integer
type Business = Individual Int Double
name :: Person - String
name = key
age :: Person - Integer
age = value
business_number :: Business - Int
business_number = key
revenue :: Business - Double
revenue = value
--
Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig
May all beings be well and happy!~
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[Haskell-cafe] Re: ANNOUNCE: Coadjute 0.0.1, generic build tool
Paul Moore wrote: 2009/1/18 Matti Niemenmaa matti.niemenmaa+n...@iki.fi: Announcing the release of Coadjute, version 0.0.1! [...] Portability is striven towards in two ways: Is it intended to work on Windows? (I don't want to spend time downloading and trying to set it up if it was never intended to be Windows-compatible.) Yes, all my software is unless I explicitly say it isn't. :-) In fact, I've both developed and used it mostly on Windows. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Bug in Text.Regex.PCRE - do not accept national symbol in pattern
Alexandr,
Thanks for sending me this question about unicode and regex-pcre. I will
share with the mailing list. This is an encoding issue.
From the haddock documentation for regex-pcre:
http://hackage.haskell.org/packages/archive/regex-pcre/0.94.1/doc/html/Text-Regex-PCRE.html
Using the provided CompOption and ExecOption values and if configUTF8 is
True, then you might be able to send UTF8 encoded ByteStrings to PCRE and get
sensible results. This is currently untested.
This is a literate Haskell post so you can save with file extension
.lhs and pass this to ghci.
The answer is a combination of man 3 pcre and the haddock
documentation for haskell-pcre and using makeRegexOpts. I show one
possible way to use utf8 below, via the 'utf8-string' package from
hackage. There are other ways to use the same package and other
packages available.
{-# LANGUAGE FlexibleContexts #-}
import Text.Regex.PCRE hiding ((=~))
--import Text.Regex.PCRE.Wrap(configUtf8)
import qualified Data.ByteString.UTF8 as U
import qualified System.IO.UTF8 as U
import Data.Bits((.|.))
Here I copied the original source for (=~) from
http://hackage.haskell.org/packages/archive/regex-pcre/0.94.1/doc/html/Text-Regex-PCRE-Wrap.html#v%3A%3D~
I then editied it to create a custom (=~) that defines its own
options. You can add compNoUTF8Check for performance/safety tradeoff
(see man 3 pcre).
makeRegexUtf8 :: (RegexMaker Regex CompOption ExecOption source) = source -
Regex
makeRegexUtf8 r = let co = defaultCompOpt .|. compUTF8 -- need compUTF8 flag
when using makeRegexOpts
-- co = defaultCompOpt .|. compUTF8 .|. compNoUTF8Check
--
in makeRegexOpts co defaultExecOpt r
(=~) :: (RegexMaker Regex CompOption ExecOption source,RegexContext Regex
source1 target)
= source1 - source - target
(=~) x r = let q = makeRegexUtf8 r
in match q x
If you are going to use the same pattern against many different texts
then you should NOT use (=~). Instead you should call makeRegexUtf8
and reuse the resulting Regex value. Otherwise you have to recompile
the pattern for each match performed.
Below, 're_test' was changed internally to convert the [Char] into a
ByteString holding a utf8 encoded representation. The 'makeRegexOpts'
and 'match' calls will then run the libpcre routines directly on the
the memory that backs the ByteString. This is an optimal was to use
the library.
re_test :: String - String - Bool
re_test re str = (U.fromString str) =~ (U.fromString re)
-- test for national symbols
main = do
putStrLn $ If this line ends with True then your libpcre has UTF8 support:
++ show configUTF8
let pattern1,pattern2,pattern3,text :: String
pattern1 = ^п.*
pattern2 = ^..ив.*
pattern3 = ^..$
text = привет
U.putStrLn $ The 3 patterns are: ++ pattern1 ++ , ++ pattern2 ++ ,
and ++pattern3
U.putStrLn $ The text to be matched is ++ text
putStrLn $ The length of the text to be matched is ++show (length text)
putStrLn All three lines below should print True
print $ re_test pattern1 text
print $ re_test pattern2 text
print $ re_test pattern3 text
The output when I run this on my machine is
If this line ends with True then your libpcre has UTF8 support: True
The 3 patterns are: ^п.*, ^..ив.*, and ^..$
The text to be matched is привет
The length of the text to be matched is 6
All three lines below should print True
True
True
True
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Re: [Haskell-cafe] Factoring into type classes
2009/1/19 Luke Palmer lrpal...@gmail.com: As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. (,) ? -Antoine ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
2009/1/19 Luke Palmer lrpal...@gmail.com: On Mon, Jan 19, 2009 at 3:58 AM, Patai Gergely patai_gerg...@fastmail.fm wrote: However, there are other type classes that are too general to assign such concrete uses to. For instance, if a data structure can have more than one meaningful (and useful) Functor or Monoid instance, As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. Luke The recent, and great, blog post about moniods [1] discusses the fact that (Num a) could be one of several different monoids and how that was handled. [1] http://sigfpe.blogspot.com/2009/01/haskell-monoids-and-their-uses.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
Am Montag, 19. Januar 2009 14:31 schrieb Antoine Latter: 2009/1/19 Luke Palmer lrpal...@gmail.com: As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. (,) ? -Antoine Wrong kind. And (,) a has only one useful instance. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
Am Montag, den 19.01.2009, 14:47 +0100 schrieb Daniel Fischer: Am Montag, 19. Januar 2009 14:31 schrieb Antoine Latter: 2009/1/19 Luke Palmer lrpal...@gmail.com: As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. (,) ? -Antoine Wrong kind. And (,) a has only one useful instance. What about instance Functor ((,) a) where fmap f (x,y) = (x, f y) and instance Functor (, a) where fmap f (x, y) = (f x, y) ? Of course, the latter is not legal Haskell. But if it was, then it might be useful. Is there any way to declare this Functor instance, possibly with some GHC extensions? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Coadjute 0.0.1, generic build tool
Excerpts from Matti Niemenmaa's message of Sun Jan 18 19:47:46 +0100 2009: Henning Thielemann wrote: Matti Niemenmaa schrieb: Announcing the release of Coadjute, version 0.0.1! Web site: http://iki.fi/matti.niemenmaa/coadjute/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Coadjute snip How does it compare to http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hake Short answer: the question that comes to mind is how does hake compare to make? Coadjute seems to be more capable, in general, but then I don't know pretty much anything about hake. Somewhat longer answer: Coadjute is better in that: 1. hake's documentation is rather sparse. I have no idea what most functions do, or even what exactly the main program does. 2. hake doesn't seem to do parallel builds, but I'm not sure because of point 1. 3. Coadjute keeps track of command line arguments (see docs for details): for me this is really a killer feature, I don't know of anything else which does this. ocamlbuild does this. 4. hake always uses timestamps, Coadjute can use MD5 hashes as well. 5. Coadjute can have arbitrary path specifications, hake's rules seem to be based on file extensions only, thus applying only to the current directory. Peter Miller's Recursive Make Considered Harmful comes to mind: http://miller.emu.id.au/pmiller/books/rmch/ [...] Moreover, it seems that Coadjute and ocamlbuild share a fair number of design choices, maybe having a look at it could be fruitful. Best regards, -- Nicolas Pouillard ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Fwd: [Haskell-cafe] runghc Setup.hs doitall
I finally installed cabal-install in windows.. I had a copy of sh.exe for windows time ago so I could use bootstrap. The only additional thing needed is wget(http://users.ugent.be/~bpuype/wget/). finally I moved the resulting cabal.exe to ghc/bin 2009/1/19 Duncan Coutts duncan.cou...@worc.ox.ac.uk On Mon, 2009-01-19 at 01:36 +0100, Alberto G. Corona wrote: The problem with Windows can be solved once and for all when CygWIN will be considered for Windows instead of MinGW. Is that possible? The standard Windows build of GHC does not use Cygwin. So all the programs built with GHC (like cabal) are native Windows programs and do not understand Cygwin paths etc. If you want to build GHC for Cygwin then there is quite a bit of work for you to do to update GHC to build that way. I get the general impression that most users would prefer not to have to install Cygwin or MinGW at all. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: Coadjute 0.0.1, generic build tool
Nicolas Pouillard wrote: Excerpts from Matti Niemenmaa's message of Sun Jan 18 19:47:46 +0100 2009: 3. Coadjute keeps track of command line arguments (see docs for details): for me this is really a killer feature, I don't know of anything else which does this. ocamlbuild does this. 4. hake always uses timestamps, Coadjute can use MD5 hashes as well. 5. Coadjute can have arbitrary path specifications, hake's rules seem to be based on file extensions only, thus applying only to the current directory. Peter Miller's Recursive Make Considered Harmful comes to mind: http://miller.emu.id.au/pmiller/books/rmch/ [...] Moreover, it seems that Coadjute and ocamlbuild share a fair number of design choices, maybe having a look at it could be fruitful. Thanks, I hadn't heard of that one. It seems to be somewhat oriented towards OCaml but I'll take a look. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
The desugaring of (, a) would involve some type level lambda, and that's not something that is available (yet). -- Lennart On Mon, Jan 19, 2009 at 1:49 PM, Holger Siegel holgersiege...@yahoo.de wrote: Am Montag, den 19.01.2009, 14:47 +0100 schrieb Daniel Fischer: Am Montag, 19. Januar 2009 14:31 schrieb Antoine Latter: 2009/1/19 Luke Palmer lrpal...@gmail.com: As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. (,) ? -Antoine Wrong kind. And (,) a has only one useful instance. What about instance Functor ((,) a) where fmap f (x,y) = (x, f y) and instance Functor (, a) where fmap f (x, y) = (f x, y) ? Of course, the latter is not legal Haskell. But if it was, then it might be useful. Is there any way to declare this Functor instance, possibly with some GHC extensions? ___ 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] How to make code least strict?
What guidelines should one follow to make Haskell code least-strict? -- Robin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: How to make code least strict?
Robin Green wrote: What guidelines should one follow to make Haskell code least-strict? Obviously the use of seq and bang-patterns make code more strict. Code is strict when it evaluates values to determine a pattern match. So avoiding that makes code lazier. Values are evaluated when decisions have to be make in order to choose what an expression will evaluate to. Avoiding case statements and things that de-sugar to case statements such as if then else and pattern matching. Put off examining the input values. Occasionally the use of lazy patterns, preceded by ~, can help make code both more compact and less strict. Consider that the order of pattern matching can matter as well, the simplest common case being zip: zip xs [] = [] zip [] ys = [] zip (x:xs) (y:ys) = (x,y) : zip xs ys The order of the first two lines of zip's definition affects whether zip [] (error boom) or zip (error bam) [] will be an error. This shows that least-strict is not a unique goal. For the choice I just made the zip [] (error boom) will cause an error because the first definition line of zip checks the second argument, while zip (error bam) [] will evaluate to []. The other way to reduce strictness is to be more polymorphic because this reduces what can be sensibly done with the arguments. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] runghc Setup.hs doitall
C:\Documents and Settings\Administratorcabal install plugins Resolving dependencies... cabal: dependencies conflict: ghc-6.10.1 requires Cabal ==1.6.0.1 however Cabal-1.6.0.1 was excluded because plugins-1.3.1 requires Cabal ==1.4.* 2009/1/19 Alberto G. Corona agocor...@gmail.com I finally installed cabal-install in windows.. I had a copy of sh.exe for windows time ago so I could use bootstrap. The only additional thing needed is wget(http://users.ugent.be/~bpuype/wget/). finally I moved the resulting cabal.exe to ghc/bin 2009/1/19 Duncan Coutts duncan.cou...@worc.ox.ac.uk On Mon, 2009-01-19 at 01:36 +0100, Alberto G. Corona wrote: The problem with Windows can be solved once and for all when CygWIN will be considered for Windows instead of MinGW. Is that possible? The standard Windows build of GHC does not use Cygwin. So all the programs built with GHC (like cabal) are native Windows programs and do not understand Cygwin paths etc. If you want to build GHC for Cygwin then there is quite a bit of work for you to do to update GHC to build that way. I get the general impression that most users would prefer not to have to install Cygwin or MinGW at all. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] plugins can not be installed in ghc 6.10.1
Do really pluigins needs Cabal (=1.4 1.5) ??? C:\Documents and Settings\Administratorcabal install plugins Resolving dependencies... cabal: dependencies conflict: ghc-6.10.1 requires Cabal ==1.6.0.1 however Cabal-1.6.0.1 was excluded because plugins-1.3.1 requires Cabal ==1.4.* ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] plugins can not be installed in ghc 6.10.1
-BEGIN PGP SIGNED MESSAGE- Hash: SHA512 2009/1/19 Alberto G. Corona : -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkl0uKYACgkQvpDo5Pfl1oJ30wCfQzX80TulZxyyLLyaAcU/LPVc PPMAoJl8tjhfrlWwoQ9yVGXlXStMDs+O =lf6T -END PGP SIGNATURE- Do really pluigins needs Cabal (=1.4 1.5) ??? C:\Documents and Settings\Administratorcabal install plugins Resolving dependencies... cabal: dependencies conflict: ghc-6.10.1 requires Cabal ==1.6.0.1 however Cabal-1.6.0.1 was excluded because plugins-1.3.1 requires Cabal ==1.4.* ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Yes. Suppose you change the deps: hunk ./plugins.cabal 42 - Cabal = 1.4 1.5, + Cabal = 1.4, And then try to compile, you'll see (at least for me): [ 3 of 16] Compiling System.Plugins.ParsePkgConfCabal ( src/System/Plugins/ParsePkgConfCabal.hs, dist/build/System/Plugins/ParsePkgConfCabal.o ) src/System/Plugins/ParsePkgConfCabal.hs:287:34: Ambiguous occurrence `depends' It could refer to either `Distribution.InstalledPackageInfo.depends', imported from Distribution.InstalledPackageInfo at src/System/Plugins/ParsePkgConfCabal.hs:8:0-39 or `Distribution.Package.depends', imported from Distribution.Package at src/System/Plugins/ParsePkgConfCabal.hs:9:0-26 src/System/Plugins/ParsePkgConfCabal.hs:300:36: Ambiguous occurrence `depends' It could refer to either `Distribution.InstalledPackageInfo.depends', imported from Distribution.InstalledPackageInfo at src/System/Plugins/ParsePkgConfCabal.hs:8:0-39 or `Distribution.Package.depends', imported from Distribution.Package at src/System/Plugins/ParsePkgConfCabal.hs:9:0-26 -- gwern ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to make code least strict?
On Mon, Jan 19, 2009 at 4:48 PM, Robin Green gree...@greenrd.org wrote: What guidelines should one follow to make Haskell code least-strict? There was a great Cafe discussion started by Henning on just this. He provided this link: http://www.haskell.org/haskellwiki/Maintaining_laziness ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
As a side curiosity, I would love to see an example of any data structure
which has more than one Functor instance. Especially those which have
more than one useful functor instance.
data Record a b = R { field1 :: a, field2 :: b }
If I want to use fmap to transform either field, I have to declare the
type to have the corresponding type variable at the end, i.e. choosing
Record a b or Record b a is already a design decision, and it is
driven by the standard Functor class in this case. I can define custom
functions fmap1 and fmap2 manually, but then I don't get the advantages
of overloading, like fmapping over a data structure containing my
records.
Now I understand that I can't get everything, and my question is mainly
what to do when such a dilemma comes up. Those who have already
encountered such a dilemma: how did it come up and what did you do to
solve it?
Gergely
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Re: [Haskell-cafe] Expect module?
Quoth Neil Mitchell ndmitch...@gmail.com: Is there a Haskell-Expect module? Something that would allow me to control an external Unix program via its stdin/stdout/stderr? System.Process does what you want, I think: It might not. Expect uses pseudottys (cf. openpty()), and select(). System.Process supports what you might think of as the naive model, using pipes for the I/O device and making some simplifying assumptions about the behavior of the external program. It's more portable and works for many common cases, at least the popen() and system() usages that probably account for 98 percent of process invocations. But once you need output from a process _before_ it exits, you will encounter the problem with pipes: output to a pipe is normally block buffered, so it doesn't reliably get to the pipe on time. The C I/O library treats a pty device like a tty and line buffers output. (But unfortunately, a pty device is not just a pipe with that single property - I wouldn't replace pipes with ptys just as a matter of course, because depending on the OS they may do things like discard date on overflow, or they may be a severely limited system resource.) select() also helps with another potential problem, when the same process is writing to two or more pipes, which are fixed size devices. In GHC, I suppose the potential deadlock might similarly be avoided using threads. I don't know if GHC supports openpty() - having a little trouble getting data from its web site this morning. But of course there's more to Expect than just the raw system calls. Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANN: hledger 0.3 released
The cabal file still includes the vty dependency, but simply removing it made it compile. -- From: Simon Michael si...@joyful.com Sent: Sunday, January 18, 2009 7:04 PM To: Sebastian Sylvan sebastian.syl...@gmail.com Cc: hled...@googlegroups.com; haskell-cafe@haskell.org Subject: Re: ANN: hledger 0.3 released I've pushed a patch which should omit the vty dependency and ui command on windows. Sebastian, could you darcs get the latest code from http://joyful.com/repos/hledger and see if cabal configure and build works for you on windows ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] plugins can not be installed in ghc 6.10.1
agocorona: Do really pluigins needs Cabal (=1.4 1.5) ??? C:\Documents and Settings\Administratorcabal install plugins Resolving dependencies... cabal: dependencies conflict: ghc-6.10.1 requires Cabal ==1.6.0.1 however Cabal-1.6.0.1 was excluded because plugins-1.3.1 requires Cabal ==1.4.* Patches welcome. Probably switching to the 1.6 API won't be too hard. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Expect module?
On 2009 Jan 19, at 3:47, Neil Mitchell wrote: Is there a Haskell-Expect module? Something that would allow me to control an external Unix program via its stdin/stdout/stderr? System.Process does what you want, I think: http://hackage.haskell.org/packages/archive/process/1.0.1.1/doc/html/System-Process.html I don't see any pty stuff in there offhand. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell WikiProject
Is anyone else interested in forming a Haskell WikiProject on Wikipedia, to collaborate on improving and maintaining the coverage and quality of articles on Haskell-related software and topics (broadly defined)? Not just programming topics specific to Haskell, but also ones of interest to the Haskell community. Some of you might already be doing this from time to time, but forming an explicit WikiProject might help to: * Highlight things that could use some attention * Divide up tasks (based on expertise or interest) * Recruit more editors (sticking a banner on article Talk pages can let editors know the WikiProject exists) * Eventually (something for the future, maybe!) work together on a Wikipedia Haskell Portal * And of course, improve the visibility of Haskell on Wikipedia, which should help our community Here's a good example to start with. The article on Eager evaluation could do with some improvement - and possibly should be merged into the Lazy evaluation article, I'm not sure: http://en.wikipedia.org/wiki/Eager_evaluation We could also probably create some more articles on projects written in Haskell, and add more references to Haskell research papers. Software projects don't have to be polished to be covered in Wikipedia - or even working! - they essentially just have to be notable, as the Wikipedia guidelines define it. By the way (getting a bit offtopic here) an annoying limitation of the Wikipedia category system, that you couldn't run queries like Give me all the articles in the Haskell category that are also in the Unreferenced category has now been partially addressed by the experimental prototype of Category Intersection: http://toolserver.org/~dschwen/intersection/ This is slightly better than Googling, because crucially, it searches *recursively* through categories. That means it will turn up articles that are in a subcategory of Category:Haskell programming language but don't explicitly mention Haskell. Don't know if there any such articles yet, but it's worth bearing in mind that you can do this. I think it will, in principle, make topic-specific maintenance a bit more convenient - and it's what I've been waiting for before getting involved in topic-specific maintenance. If you want to just express interest in signing up for such a WikiProject (no commitment required whatsoever!), please reply privately via email or publicly on my User Talk page (User talk:Greenrd) - to avoid clogging up this mailing list. -- Robin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to make code least strict?
On Mon, 19 Jan 2009 17:36:30 + Thomas DuBuisson thomas.dubuis...@gmail.com wrote: On Mon, Jan 19, 2009 at 4:48 PM, Robin Green gree...@greenrd.org wrote: What guidelines should one follow to make Haskell code least-strict? There was a great Cafe discussion started by Henning on just this. He provided this link: http://www.haskell.org/haskellwiki/Maintaining_laziness Thanks - wow, my memory is terrible! I submitted this page to the Haskell reddit myself 20 days ago! I had a sneaking feeling of deja vu after I asked the question. :-D -- Robin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell WikiProject
greenrd: Is anyone else interested in forming a Haskell WikiProject on Wikipedia, to collaborate on improving and maintaining the coverage and quality of articles on Haskell-related software and topics (broadly defined)? Not just programming topics specific to Haskell, but also ones of interest to the Haskell community. Some of you might already be doing this from time to time, but forming an explicit WikiProject might help to: * Highlight things that could use some attention * Divide up tasks (based on expertise or interest) * Recruit more editors (sticking a banner on article Talk pages can let editors know the WikiProject exists) * Eventually (something for the future, maybe!) work together on a Wikipedia Haskell Portal * And of course, improve the visibility of Haskell on Wikipedia, which should help our community Here's a good example to start with. The article on Eager evaluation could do with some improvement - and possibly should be merged into the Lazy evaluation article, I'm not sure: http://en.wikipedia.org/wiki/Eager_evaluation We could also probably create some more articles on projects written in Haskell, and add more references to Haskell research papers. Software projects don't have to be polished to be covered in Wikipedia - or even working! - they essentially just have to be notable, as the Wikipedia guidelines define it. By the way (getting a bit offtopic here) an annoying limitation of the Wikipedia category system, that you couldn't run queries like Give me all the articles in the Haskell category that are also in the Unreferenced category has now been partially addressed by the experimental prototype of Category Intersection: http://toolserver.org/~dschwen/intersection/ This is slightly better than Googling, because crucially, it searches *recursively* through categories. That means it will turn up articles that are in a subcategory of Category:Haskell programming language but don't explicitly mention Haskell. Don't know if there any such articles yet, but it's worth bearing in mind that you can do this. I think it will, in principle, make topic-specific maintenance a bit more convenient - and it's what I've been waiting for before getting involved in topic-specific maintenance. If you want to just express interest in signing up for such a WikiProject (no commitment required whatsoever!), please reply privately via email or publicly on my User Talk page (User talk:Greenrd) - to avoid clogging up this mailing list. Yes! Also, we have many good writers who've written extensively on topics on blogs who I'm sure would be happy to donate content. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
Hello,
The multitude of newtypes in the Monoid module are a good indication
that the Monoid class is not a good fit for the class system (it is
ironic that discussing it resulted in such a huge thread recently :-).
How I'd approach the situation that you describe would depend on
the context (did I design the class, or am I just using it? am I
writing a library that is to be used by other people, or is the class
just used in an internal part of my program?, etc.) but, in general,
here are some ideas:
1. If one type can be made into an instance of a class in multipe ways
and I have no control over the class:
- I would provide non-overloaded versions for each implementation
- if there is a natural one (something that is quite commonly
used) I would use it for an instance
- if most uses are equally likely to be useful, then I would not
provide an instance but just use the non-overloaded functions. If I
did provide an instance, then I would be careful to document the
choice I made.
2. If I have control over the class I may consider changing it:
- Consider using a different class, that has operations that are
more specific to what I am doing (e.g., use a PrettyPrint class
instead of Show class)
- If many types are members of the same classes, then it may be
useful to combine them (i.e., add multiple methods that perform the
different operations).
I think that I have done all of the above in different situations, and
so I don't think that there is a single correct answer. I usually
avoid using the newtype trick as I find it inconvenient: usually
the newtype does not have the same operations as the underlying type
and so it cannot be used directly, and if you are going to wrap thing
just when you use the class methods, then you may as well use the
non-overloaded operations.
Hope that this helps,
Iavor
On Mon, Jan 19, 2009 at 9:40 AM, Patai Gergely
patai_gerg...@fastmail.fm wrote:
As a side curiosity, I would love to see an example of any data structure
which has more than one Functor instance. Especially those which have
more than one useful functor instance.
data Record a b = R { field1 :: a, field2 :: b }
If I want to use fmap to transform either field, I have to declare the
type to have the corresponding type variable at the end, i.e. choosing
Record a b or Record b a is already a design decision, and it is
driven by the standard Functor class in this case. I can define custom
functions fmap1 and fmap2 manually, but then I don't get the advantages
of overloading, like fmapping over a data structure containing my
records.
Now I understand that I can't get everything, and my question is mainly
what to do when such a dilemma comes up. Those who have already
encountered such a dilemma: how did it come up and what did you do to
solve it?
Gergely
--
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[Haskell-cafe] Re: some ideas for Haskell', from Python
Manlio Perillo wrote: Brandon S. Allbery KF8NH ha scritto: ...in theory. In practice GHC needs help with circular imports, and some cycles might be impossible to resolve. This is interesting. Where can I find some examples? Is this explained in the Real World Haskell book? I have no idea about RWH, but there are certainly mutual import cycles that cannot be resolved by using hs-boot files with GHC. Consider three modules A and B and C, which are A-B-C permutations of module A(A,AKBC, AKCB) import B(B,BKAC) import C(C,CKAB) data A AKBC :: Either B C AKCB :: Either C B instance Show (A,BKAC,CKAB) where ... There is no way to break the ?K?? import cycle with just hs-boot files. I had to solve this by generating helper modules. Call the data A the rank-1 declarations. Then the ?K?? are built on rank-1 types such as B and C and are rank-2 declarations. The rank-1 declarations can all be put in hs-boot files but the rank-2 declaration import cycle cannot be broken with the same hs-boot files. Some of these need to be put in separate modules. It may be possible to make a useful definition of rank-3 and higher declarations. -- Chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell WikiProject
* Robin Green gree...@greenrd.org [2009-01-19 18:46:43+] Here's a good example to start with. The article on Eager evaluation could do with some improvement - and possibly should be merged into the Lazy evaluation article, I'm not sure: http://en.wikipedia.org/wiki/Eager_evaluation I was also disappointed with the article on Typed lambda calculus (compare it with the article on (untyped) Lambda calculus). http://en.wikipedia.org/wiki/Typed_lambda_calculus http://en.wikipedia.org/wiki/Lambda_calculus -- Roman I. Cheplyaka :: http://ro-che.info/ Don't let school get in the way of your education. - Mark Twain ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
On Mon, 2009-01-19 at 10:59 -0800, Iavor Diatchki wrote: Hello, The multitude of newtypes in the Monoid module are a good indication that the Monoid class is not a good fit for the class system I would say rather that the class system is not a good fit for Monoid. Proposals for local instances, multiple instances, instance import/export control, etc. come up quite frequently on this list; the phenomena in question are not restricted to Monoid. I usually avoid using the newtype trick as I find it inconvenient: usually the newtype does not have the same operations as the underlying type and so it cannot be used directly, and if you are going to wrap thing just when you use the class methods, OTOH, I think you mean here `when you use class methods and when you use overloaded functions'. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Open unqualified imports
On Fri, Jan 16, 2009 at 06:42:46AM -0800, eyal.lo...@gmail.com wrote: Closed-unqualified import: import Data.Map(Map, lookup) One problem with this style is that you can get lots of conflicts from your VCS if you have multiple people working on the same module. Thanks Ian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Expect module?
Donn Cave wrote: Quoth Neil Mitchell ndmitch...@gmail.com: Is there a Haskell-Expect module? Something that would allow me to control an external Unix program via its stdin/stdout/stderr? System.Process does what you want, I think: It might not. Expect uses pseudottys (cf. openpty()), and select(). Yep, I definitely need the pty as the program I need to run asks for a password. Pipes won't work. Erik -- - Erik de Castro Lopo - The earth is degenerating these days. Bribery and corruption abound. Children no longer mind parents ...and it is evident that the end of the world is approaching fast. -- Assyrian Tablet Engraved in 2800 B.C. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Improved documentation for Bool
rocon...@theorem.ca wrote: I noticed the Bool datatype isn't well documented. Since Bool is not a common English word, I figured it could use some haddock to help clarify it for newcomers. My only problem with it is that it's called Bool, while every other programming language on Earth calls it Boolean. (Or at least, the languages that *have* a name for it...) But I'm far more perturbed by names like Eq, Ord, Num, Ix (??), and so on. The worst thing about C is the unecessary abbriviations; let's not copy them, eh? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Employment
Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
http://www.haskell.org/haskellwiki/Haskell_in_industry could be of interest to you On Mon, Jan 19, 2009 at 2:34 PM, Andrew Coppin andrewcop...@btinternet.comwrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
On Mon, 2009-01-19 at 19:33 +, Andrew Coppin wrote: rocon...@theorem.ca wrote: I noticed the Bool datatype isn't well documented. Since Bool is not a common English word, I figured it could use some haddock to help clarify it for newcomers. My only problem with it is that it's called Bool, while every other programming language on Earth calls it Boolean. (Or at least, the languages that *have* a name for it...) Except C++? But then again: But I'm far more perturbed by names like Eq, Ord, Num, Ix (??), and so on. The worst thing about C is the unecessary abbriviations; [sic] let's not copy them, eh? I agree. I've always felt that class EqualsClass randomTypeSelectedByTheUser = TotalOrderClass randomTypeSelectedByTheUser where compareXToY :: randomTypeSelectedByTheUser - randomTypeSelectedByTheUser - OrderingValue lessThanOrEqualTo :: randomTypeSelectedByTheUser - randomTypeSelectedByTheUser - Boolean lessThan :: randomTypeSelectedByTheUser - randomTypeSelectedByTheUser - Boolean was both more understandable to the reader, and easier to remember and reproduce for the writer. Or, in other words, leave well enough alone; we should always err in the direction of being like C, to avoid erring in the direction of being like Java. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
On Mon, Jan 19, 2009 at 11:34 AM, Andrew Coppin andrewcop...@btinternet.com wrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) I'm using it at work in simulations... not shipping anything with it yet, but we do ship Erlang :-) Haskell may work it's way in eventually too. ___ 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] Haskell WikiProject
-BEGIN PGP SIGNED MESSAGE- Hash: SHA512 On Mon, Jan 19, 2009 at 1:46 PM, Robin Green wrote: Is anyone else interested in forming a Haskell WikiProject on Wikipedia, to collaborate on improving and maintaining the coverage and quality of articles on Haskell-related software and topics (broadly defined)? Not just programming topics specific to Haskell, but also ones of interest to the Haskell community. Some of you might already be doing this from time to time, but forming an explicit WikiProject might help to: * Highlight things that could use some attention * Divide up tasks (based on expertise or interest) * Recruit more editors (sticking a banner on article Talk pages can let editors know the WikiProject exists) * Eventually (something for the future, maybe!) work together on a Wikipedia Haskell Portal * And of course, improve the visibility of Haskell on Wikipedia, which should help our community Here's a good example to start with. The article on Eager evaluation could do with some improvement - and possibly should be merged into the Lazy evaluation article, I'm not sure: http://en.wikipedia.org/wiki/Eager_evaluation We could also probably create some more articles on projects written in Haskell, and add more references to Haskell research papers. Software projects don't have to be polished to be covered in Wikipedia - or even working! - they essentially just have to be notable, as the Wikipedia guidelines define it. By the way (getting a bit offtopic here) an annoying limitation of the Wikipedia category system, that you couldn't run queries like Give me all the articles in the Haskell category that are also in the Unreferenced category has now been partially addressed by the experimental prototype of Category Intersection: http://toolserver.org/~dschwen/intersection/ This is slightly better than Googling, because crucially, it searches *recursively* through categories. That means it will turn up articles that are in a subcategory of Category:Haskell programming language but don't explicitly mention Haskell. Don't know if there any such articles yet, but it's worth bearing in mind that you can do this. I think it will, in principle, make topic-specific maintenance a bit more convenient - and it's what I've been waiting for before getting involved in topic-specific maintenance. If you want to just express interest in signing up for such a WikiProject (no commitment required whatsoever!), please reply privately via email or publicly on my User Talk page (User talk:Greenrd) - to avoid clogging up this mailing list. As a longtime Wikipedian (almost as long as you), I'm not too enthusiastic about this. More than once I've seen some editor enthusiastically going around, saying Hey, you know what this neglected area of Wikipedia needs? A Wikiproject! That'll solve all our problems! And then they go form the Star Wars wikiproject or the Evangelion workgroup, and things go along as before. (Meet the new project banner, same as the old banner...) What would solve all that area's problem is a lot of hard work by a lot of people over months and years. A wikiproject does little to help out with this, and in fact, is liable to suck up the effort of the few people who would otherwise be out actually improving articles. - --- To avoid sounding *too* bitter and curmudgeonly and burnt-out, I'd like to make a counter-suggestion. Instead of a Wikiproject, why don't you draw up a list of volunteers and set up a weekly cleanup drive? It would work like this: 1) Every Sunday, you pick some neglected FP topic - preferably manageable in scope like datastructures are eg. [[Rope (computer science)]] or [[Finger tree]]. 2) You track down all the academic and realworld references, download all the PDFs, and plop them down on some website where everyone can access them. You'll remove them after a few days of course. 3) Then you use one of the many notification-bot programs to contact everyone on the list, saying 'Here is this week's article, here are the references. Go to!' 4) For good measure, you'll email Haskell-cafe and post a link to the - -cafe email on Reddit. This is all perfectly doable on a weekly basis, and I can basically guarantee you tgat this will do more to clean up FP articles than any Project-space page of templates and banners would. - -- gwern -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkl02nUACgkQvpDo5Pfl1oLT9QCeLCqvec+3xyWSkguWXnAHLZJB 7/sAnihxxosIYf7++geo/bCTfYPPe+t8 =nXc3 -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
Barclays Capital use it for Equity Derivative modeling and pricing - it's a small team at the moment, but the whole project is in Haskell. I don't work on it myself so I couldn't give you any details (plus I would get fired for blabbing!), I work in an adjacent group. Haskell certainly lends itself to complex financial maths simulation tho, so I think they've made a good choice. On 19/01/2009 19:34, Andrew Coppin andrewcop...@btinternet.com wrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) ___ 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] Employment
And of course, there's at least half a dozen people on this list at working at Galois. And all documented on the wiki, http://haskell.org/haskellwiki/Haskell_in_industry See you guys at CUFP 09! http://cufp.galois.com/ -- Don pbeadling: Barclays Capital use it for Equity Derivative modeling and pricing - it's a small team at the moment, but the whole project is in Haskell. I don't work on it myself so I couldn't give you any details (plus I would get fired for blabbing!), I work in an adjacent group. Haskell certainly lends itself to complex financial maths simulation tho, so I think they've made a good choice. On 19/01/2009 19:34, Andrew Coppin andrewcop...@btinternet.com wrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) ___ 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 mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
Hi, On Mon, Jan 19, 2009 at 11:06 AM, Jonathan Cast jonathancc...@fastmail.fm wrote: On Mon, 2009-01-19 at 10:59 -0800, Iavor Diatchki wrote: Hello, The multitude of newtypes in the Monoid module are a good indication that the Monoid class is not a good fit for the class system I would say rather that the class system is not a good fit for Monoid. Proposals for local instances, multiple instances, instance import/export control, etc. come up quite frequently on this list; the phenomena in question are not restricted to Monoid. I disagree with you but that is a moot point because we are discussing Haskell, which does not have any of these features. Also, I find that in many situations where people want to use them, simpler solutions (like some of the ideas I mentioned in my previous post) suffice. That is not to say that we should stop trying to figure out how to improve the class system, but language changes require a lot more work than improving the design of the libraries. I usually avoid using the newtype trick as I find it inconvenient: usually the newtype does not have the same operations as the underlying type and so it cannot be used directly, and if you are going to wrap thing just when you use the class methods, OTOH, I think you mean here `when you use class methods and when you use overloaded functions'. Sure, the point is that you are essentially adding a type annotation, which is like using a non-overloaded function. Compare, for example: mappend add x y and getSum (mappend (Sum x) (Sum y)). I think that the first one is quite a bit more readable but, of course, this is somewhat subjective. -Iavor ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
This is one of the shortcomings of haskell not to mention other programming languages. Mathemathicist would find it very annoying. Instead of instance Monoid Integer where mappend = (+) mempty = 0 instance Monoid Integer where mappend = (*) mempty = 1 which is not legal and the workaround Num a = Monoid (Sum a) Num a = Monoid (Product a) wich is cumbersome A mathematician would say something like: instance Monoid Integer with operation + where mappend = (+) mempty = 0 and instance Monoid Integer with operation * where mappend = (*) mempty = 1 But talking about shortcomings, personally I prefer to implement first a form of assertion that permits the checking of the class properties automatically for each new instance. This is far more important in práctical terms. 2009/1/19 Thomas DuBuisson thomas.dubuis...@gmail.com 2009/1/19 Luke Palmer lrpal...@gmail.com: On Mon, Jan 19, 2009 at 3:58 AM, Patai Gergely patai_gerg...@fastmail.fm wrote: However, there are other type classes that are too general to assign such concrete uses to. For instance, if a data structure can have more than one meaningful (and useful) Functor or Monoid instance, As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. Luke The recent, and great, blog post about moniods [1] discusses the fact that (Num a) could be one of several different monoids and how that was handled. [1] http://sigfpe.blogspot.com/2009/01/haskell-monoids-and-their-uses.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to make code least strict?
On Mon, Jan 19, 2009 at 9:10 AM, ChrisK hask...@list.mightyreason.com wrote: Consider that the order of pattern matching can matter as well, the simplest common case being zip: zip xs [] = [] zip [] ys = [] zip (x:xs) (y:ys) = (x,y) : zip xs ys If you are obsessive about least-strictness and performance isn't a giant concern, this seems like a perfect use for Conal's unamb[1] operator. zipR xs [] = [] zipR [] ys = [] zipR (x:xs) (y:ys) = (x,y) : zip xs ys zipL [] ys = [] zipL xs [] = [] zipL (x:xs) (y:ys) = (x,y) : zip xs ys zip xs ys = unamb (zipL xs ys) (zipR xs ys) This runs both zipL and zipR in parallel until one of them gives a result; if neither of them is _|_ they are guaranteed to be identical, so we can unambiguously choose whichever one gives a result first. -- ryan [1] http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
On Mon, Jan 19, 2009 at 11:33 AM, Andrew Coppin andrewcop...@btinternet.com wrote: My only problem with it is that it's called Bool, while every other programming language on Earth calls it Boolean. (Or at least, the languages that *have* a name for it...) Python: bool ocaml: bool C++: bool C99: bool C#: bool But I'm far more perturbed by names like Eq, Ord, Num, Ix (??), and so on. The worst thing about C is the unecessary abbriviations; let's not copy them, eh? They're short so they're quick to parse (for a human) and read. They're easy to type. If you have a constraint like (Eq a,Num a,Ord a,Show a,Ix a) you can see all five type classes at a single glance without having to scan your eye across the line. They're highly mnemonic in the sense that once I'd learnt what they meant it became hard to forget them again. What exactly is wrong with them? -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
On Mon, 2009-01-19 at 12:10 -0800, Iavor Diatchki wrote: I usually avoid using the newtype trick as I find it inconvenient: usually the newtype does not have the same operations as the underlying type and so it cannot be used directly, and if you are going to wrap thing just when you use the class methods, OTOH, I think you mean here `when you use class methods and when you use overloaded functions'. Sure, the point is that you are essentially adding a type annotation, which is like using a non-overloaded function. Compare, for example: mappend add x y and getSum (mappend (Sum x) (Sum y)). I think that the first one is quite a bit more readable but, of course, this is somewhat subjective. Right. Of course, this issue comes up quite frequently; even sort :: Ord alpha = [alpha] - [alpha] Needs to be specialized to non-standard Ord instances. I think that, if we're going to restrict type classes to only those cases where we never want to specialize an overloaded function to a non-standard instance, that we're going to end up with Eq, Num, and maybe Functor as classes. I'm not sure a general language mechanism is really needed just for those three. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
On Mon, 2009-01-19 at 21:18 +0100, Alberto G. Corona wrote: This is one of the shortcomings of haskell not to mention other programming languages. Mathemathicist would find it very annoying. Instead of instance Monoid Integer where mappend = (+) mempty = 0 instance Monoid Integer where mappend = (*) mempty = 1 which is not legal and the workaround Num a = Monoid (Sum a) Num a = Monoid (Product a) wich is cumbersome A mathematician would say something like: instance Monoid Integer with operation + where mappend = (+) mempty = 0 and instance Monoid Integer with operation * where mappend = (*) mempty = 1 Check out the OBJ family of languages, particularly OBJ3 and (I think) Maude. But talking about shortcomings, personally I prefer to implement first a form of assertion that permits the checking of the class properties automatically for each new instance. This is far more important in práctical terms. 2009/1/19 Thomas DuBuisson thomas.dubuis...@gmail.com 2009/1/19 Luke Palmer lrpal...@gmail.com: On Mon, Jan 19, 2009 at 3:58 AM, Patai Gergely patai_gerg...@fastmail.fm wrote: However, there are other type classes that are too general to assign such concrete uses to. For instance, if a data structure can have more than one meaningful (and useful) Functor or Monoid instance, As a side curiosity, I would love to see an example of any data structure which has more than one Functor instance. Especially those which have more than one useful functor instance. Luke The recent, and great, blog post about moniods [1] discusses the fact that (Num a) could be one of several different monoids and how that was handled. [1] http://sigfpe.blogspot.com/2009/01/haskell-monoids-and-their-uses.html ___ 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 mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell Weekly News: Issue 101 - January 19, 2009
--- Haskell Weekly News http://sequence.complete.org/hwn/20090119 Issue 101 - January 19, 2009 --- Welcome to issue 101 of HWN, a newsletter covering developments in the [1]Haskell community. Gee whiz, people, stop being so darn productive or you're going to burn me out. Seriously. Announcements curl-1.3.4. Sigbjorn Finne [2]announced that a new version of [3]curl, a complete Haskell binding to the libcurl API, is now available and have been uploaded to Hackage. The most notable change is the overloading of representation of response buffers (and headers), allowing for the use of ByteStrings. Turbinado V0.4. Alson Kemp [4]announced the release of version 0.4 of [5]Turbinado, an easy to use Model-View-Controller-ish web framework for Haskell. Highlights for the 0.4 release include a dramatically improved ORM which handles foreign keys, and improved documentation. Hackage about to reach 1000 releases. Don Stewart [6]announced that Hackage is about to reach the 1000 release mark, 2 years after it went live. Some pretty charts [7]can be seen here. leapseconds-announced-2009. Bjorn Buckwalter [8]announced the release of the [9]leapseconds-announced package, which contains a single module and a single function implementing the Data.Time.Clock.TAI.LeapSecondTable interface. zipper-0.1. Andres Loeh [10]announced [11]zipper-0.1, a library offering a generic zipper for systems of recursive datatypes. multirec-0.2. Andres Loeh [12]announced [13]multirec-0.2, a library which provides a mechanism to talk about fixed points of systems of datatypes that may be mutually recursive. On top of these representations, generic functions such as the fold or the Zipper can then be defined. ghci-haskeline 0.1. Judah Jacobson [14]announced the first release of [15]ghci-haskeline. This package uses the GHC API to reimplement ghci with the [16]Haskeline library as a backend. Haskeline is a library for line input in command-line programs, similar to readline or editline, which is written in Haskell and thus (hopefully) more easily integrated into other Haskell programs. The Monad.Reader (13) - Call for copy. Wouter Swierstra [17]announced a Call for Copy for Issue 13 of [18]The Monad.Reader. The submission deadline is February 13, 2009. Please get in touch with Wouter if you intend to submit something. Cabal 2.0. Duncan Coutts [19]announced that he has started a [20]wiki page to collect ideas for Cabal 2. The basic idea for Cabal 2 is to learn lessons from our how the existing design has fared and how we can make a better design to tackle an expanded set of goals. Announcing Haskell protocol-buffers 1.4.0 (the smashing recursive edition). Chris Kuklewicz [21]announced version 1.4.0 (the smashing recursive edition) of [22]protocol-buffers, a Haskell interface to Google's ..language-neutral, platform-neutral, extensible way of serializing structured data for use in communications protocols, data storage, and more. Haskell WikiProject. Robin Green [23]asked: is anyone else interested in forming a Haskell WikiProject on Wikipedia, to collaborate on improving and maintaining the coverage and quality of articles on Haskell-related software and topics (broadly defined)? darcs 2.2.0. Petr Rockai [24]announced the release of darcs 2.2.0, with both a [25]source tarball and a [26]cabalized tarball available. This version features many improvements and bug fixes; see Petr's original announcement for a list. hledger 0.3. Simon Michael [27]announced the release of [28]hledger 0.3, a partial haskell clone of John Wiegley's ledger text-based accounting tool. It generates transaction and balance reports from a plain text ledger file, and demonstrates a functional implementation of ledger. language-sh-0.0.3.1. Stephen Hicks [29]announced the [30]language-sh package, a set of modules for parsing, manipulating, and printing sh-style shell scripts. It's being developed alongside shsh, the [31]Simple Haskell Shell. Coadjute 0.0.1, generic build tool. Matti Niemenmaa [32]announced version 0.0.1 of [33]Coadjute, a generic build tool intended as an easier to use and more portable replacement for make. dataenc 0.12. Magnus Therning [34]announced version 0.12 of [35]dataenc, a data encoding library currently providing Uuencode, Base64, Base64Url, Base32, Base32Hex, Base16, Base85, and (new in 0.12) yEncoding. 3 applications of indexed composition as a language design principle. Greg Meredith [36]announced that he has found a way to generalize the LogicT transformer, and calculated it's [37]application to three fairly interesting examples. HTTPbis / HTTP-4000.x package
Re: [Haskell-cafe] Employment
Haskell's all I use at work, although no-one requires it. I know that Ravi Nanavati's company uses Haskell pretty exclusively, and there's also Galois and a couple of financial houses. I was pretty impressed with the professional turnout for ICFP 2008. -- Jeff On Mon, Jan 19, 2009 at 2:34 PM, Andrew Coppin andrewcop...@btinternet.com wrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) ___ 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] Factoring into type classes
I'd prefer something like
Sum :: Monoid Integer
Sum = Monoid {mappend = (+), mempty = 0}
Prod :: Monoid Integer
Prod = Monoid {mappend = (*), mempty = 1}
instance Sum in [some code using mempty and mappend]
On 19 Jan 2009, at 23:18, Alberto G. Corona wrote:
This is one of the shortcomings of haskell not to mention other
programming languages. Mathemathicist would find it very annoying.
Instead of
instance Monoid Integer where
mappend = (+)
mempty = 0
instance Monoid Integer where
mappend = (*)
mempty = 1
which is not legal and the workaround
Num a = Monoid (Sum a)
Num a = Monoid (Product a)
wich is cumbersome
A mathematician would say something like:
instance Monoid Integer with operation + where
mappend = (+)
mempty = 0
and
instance Monoid Integer with operation * where
mappend = (*)
mempty = 1
But talking about shortcomings, personally I prefer to implement
first a form of assertion that permits the checking of the class
properties automatically for each new instance.
This is far more important in práctical terms.
2009/1/19 Thomas DuBuisson thomas.dubuis...@gmail.com
2009/1/19 Luke Palmer lrpal...@gmail.com:
On Mon, Jan 19, 2009 at 3:58 AM, Patai Gergely patai_gerg...@fastmail.fm
wrote:
However, there are other type classes that are too general to
assign
such concrete uses to. For instance, if a data structure can have
more
than one meaningful (and useful) Functor or Monoid instance,
As a side curiosity, I would love to see an example of any data
structure
which has more than one Functor instance. Especially those which
have more
than one useful functor instance.
Luke
The recent, and great, blog post about moniods [1] discusses the fact
that (Num a) could be one of several different monoids and how that
was handled.
[1] http://sigfpe.blogspot.com/2009/01/haskell-monoids-and-their-uses.html
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Re: [Haskell-cafe] Employment
Andrew Coppin wrote: Is it possible to earn money using Haskell? Does anybody here actually do this? Inquiring minds want to know... ;-) I work for a company that designs, builds, and sells lawn mowers (hustlerturf.com). We use quite a bit of Haskell, especially as a glue language for tying together data from different manufacturing-related systems. We also use it for some web apps that are deployed to our dealer network. There are also some uses for it doing sysadmin automation, such as adding/removing people from LDAP servers and the like. -- John ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] MonadTrans lift implementation
Is there a reason why the lift function in ReaderT's MonadTrans instance is implemented as: instance MonadTrans (ReaderT r) where lift m = ReaderT $ \_ - m Instead of just using the monad's return function? Could lift m be implemented as return m? instance (Monad m) = Monad (ReaderT r m) where * return a = ReaderT $ \_ - return a* m = k = ReaderT $ \r - do a - runReaderT m r runReaderT (k a) r fail msg = ReaderT $ \_ - fail msg Thanks, Daryoush ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MonadTrans lift implementation
On Mon, Jan 19, 2009 at 01:03:55PM -0800, Daryoush Mehrtash wrote: lift m = ReaderT $ \_ - m return a = ReaderT $ \_ - return a If you look carefully you will see that these are not the same. -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] LinuxFest Northwest 2009
LFNW 2009 (http://linuxfestnorthwest.org/) is going to be at the end of April, and I was wondering if anyone here is going to be there, or possibly a Haskell-related presentation. Last year I met ac from #haskell there, but it would be nice if more people came, especially with the (relatively) big group in Oregon and such. Perhaps someone here has plans already? Shachaf ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MonadTrans lift implementation
On Mon, Jan 19, 2009 at 01:13:37PM -0800, Jonathan Cast wrote: (On the other hand, your hunch that lift = return is correct --- so you get a cookie for that; it's just that return here is neither the return of the monad for m nor the return of the monad for ReaderT m. It is, instead, the return of the *applicative functor* --- on the category of monads and monad homomorphisms --- associated to the monad transformer ReaderT.) It's also a monad in the category of monads, as are ErrorT, StateT and WriteT (see Moggi, An Abstract View of Programming Languages, 1989, s4). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GLUT (glutGet undefined reference)
Hi all, I was hoping to introduce my old pal OpenGL with my new chum, Haskell. I used cabal to install GLUT on my 64-bit Ubuntu machine with GHC 6.8.2 (installed via apt-get/synaptic). I followed the wiki OpenGLTutorial1 until: ghc -package GLUT HelloWorld.hs -o HelloWorld at which point my screen is filled with errors. The errors begin with: /home/paul/.cabal/lib/GLUT-2.1.1.2/ghc-6.8.2/libHSGLUT-2.1.1.2.a(Begin.o): In function `szEn_info': (.text+0x26c): undefined reference to `glutGet' This surprised me a little because I've already seen these same errors recently on two separate Windows boxes. Somehow I'd got the idea it was (on Windows) due to installing from a binary; though I guess the story is the same with apt-get. Should I look for an apt-get switch to reinstall GHC from source instead? Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GLUT (glutGet undefined reference)
Have you tried http://netsuperbrain.com/blog/posts/freeglut-windows-hopengl-hglut/ ? 2009/1/20 Paul Keir pk...@dcs.gla.ac.uk: Hi all, I was hoping to introduce my old pal OpenGL with my new chum, Haskell. I used cabal to install GLUT on my 64-bit Ubuntu machine with GHC 6.8.2 (installed via apt-get/synaptic). I followed the wiki OpenGLTutorial1 until: ghc -package GLUT HelloWorld.hs -o HelloWorld at which point my screen is filled with errors. The errors begin with: /home/paul/.cabal/lib/GLUT-2.1.1.2/ghc-6.8.2/libHSGLUT-2.1.1.2.a(Begin.o): In function `szEn_info': (.text+0x26c): undefined reference to `glutGet' This surprised me a little because I've already seen these same errors recently on two separate Windows boxes. Somehow I'd got the idea it was (on Windows) due to installing from a binary; though I guess the story is the same with apt-get. Should I look for an apt-get switch to reinstall GHC from source instead? Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Евгений Кирпичев Разработчик Яндекс.Маркета ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GLUT (glutGet undefined reference)
Ouch. Sorry, I misread your post: I thought you were having troubles on Windows. 2009/1/20 Eugene Kirpichov ekirpic...@gmail.com: Have you tried http://netsuperbrain.com/blog/posts/freeglut-windows-hopengl-hglut/ ? 2009/1/20 Paul Keir pk...@dcs.gla.ac.uk: Hi all, I was hoping to introduce my old pal OpenGL with my new chum, Haskell. I used cabal to install GLUT on my 64-bit Ubuntu machine with GHC 6.8.2 (installed via apt-get/synaptic). I followed the wiki OpenGLTutorial1 until: ghc -package GLUT HelloWorld.hs -o HelloWorld at which point my screen is filled with errors. The errors begin with: /home/paul/.cabal/lib/GLUT-2.1.1.2/ghc-6.8.2/libHSGLUT-2.1.1.2.a(Begin.o): In function `szEn_info': (.text+0x26c): undefined reference to `glutGet' This surprised me a little because I've already seen these same errors recently on two separate Windows boxes. Somehow I'd got the idea it was (on Windows) due to installing from a binary; though I guess the story is the same with apt-get. Should I look for an apt-get switch to reinstall GHC from source instead? Regards, Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Евгений Кирпичев Разработчик Яндекс.Маркета ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MonadTrans lift implementation
On Mon, 2009-01-19 at 21:31 +, Ross Paterson wrote: On Mon, Jan 19, 2009 at 01:13:37PM -0800, Jonathan Cast wrote: (On the other hand, your hunch that lift = return is correct --- so you get a cookie for that; it's just that return here is neither the return of the monad for m nor the return of the monad for ReaderT m. It is, instead, the return of the *applicative functor* --- on the category of monads and monad homomorphisms --- associated to the monad transformer ReaderT.) It's also a monad in the category of monads, as are ErrorT, StateT and WriteT (see Moggi, An Abstract View of Programming Languages, 1989, s4). Nice! I really haven't studied monad transformers nearly as much as I should have. (I also really need to read more Moggi :) jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Factoring into type classes
On Mon, 2009-01-19 at 12:10 -0800, Iavor Diatchki wrote:
Hi,
On Mon, Jan 19, 2009 at 11:06 AM, Jonathan Cast
jonathancc...@fastmail.fm wrote:
On Mon, 2009-01-19 at 10:59 -0800, Iavor Diatchki wrote:
Hello,
The multitude of newtypes in the Monoid module are a good indication
that the Monoid class is not a good fit for the class system
I would say rather that the class system is not a good fit for Monoid.
Proposals for local instances, multiple instances, instance
import/export control, etc. come up quite frequently on this list; the
phenomena in question are not restricted to Monoid.
I disagree with you but that is a moot point because we are discussing
Haskell, which does not have any of these features. Also, I find that
in many situations where people want to use them, simpler solutions
(like some of the ideas I mentioned in my previous post) suffice.
That is not to say that we should stop trying to figure out how to
improve the class system, but language changes require a lot more work
than improving the design of the libraries.
I usually
avoid using the newtype trick as I find it inconvenient: usually
the newtype does not have the same operations as the underlying type
and so it cannot be used directly, and if you are going to wrap thing
just when you use the class methods,
OTOH, I think you mean here `when you use class methods and when you use
overloaded functions'.
Sure, the point is that you are essentially adding a type annotation,
which is like using a non-overloaded function. Compare, for example:
mappend add x y and getSum (mappend (Sum x) (Sum y)). I think
that the first one is quite a bit more readable but, of course, this
is somewhat subjective.
data Iso a b = Iso { to :: a - b, from :: b - a }
under :: Iso a b - (b - b) - (a - a)
under iso = to iso ~ from iso
under2 :: Iso a b - (b - b - b) - (a - a - a)
under2 iso = to iso ~ under iso
sumIso = Iso Sum getSum
(+) = under2 sumIso mappend
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[Haskell-cafe] xhtml + bytestring
Hi Bjorn, hi list,
the darcswatch instance I’m running is getting quite big, and it
periodically slows down my server. I managed to get quite an improvement
with this simple patch to my parsing:
http://darcs.nomeata.de/cgi-bin/darcsweb.cgi?r=darcswatch;a=commitdiff;h=20090119181919-23c07-140f8deb91a52a423a2984dce2d22f4a48999aaf.gz
Since the new HTTP library, my code runs completely on ByteStrings, only
the xhtml library expects me to feed strings. I tried to fix this and
created a patch against xhtml-3000.2.0.1 to work internally with lazy
ByteStrings. It’s API-compatible to the normal xhtml library, it just
adds showHtml', renderHtml' and prettyHtml' that output lazy
ByteStrings, and that has Html instances for strict any lazy
ByteStrings.
There were some speed and space improvements, but none horrific (at
least for DarcsWatch, most of the time goes into parsing the
repositories and mails, and into sorting that data). Unfortunately, I
can’t use it in the live installation until I upgrade the machine from
Debian etch to lenny, as the bundled bytestring library in ghc-6.6’ base
is too old.
Nevertheless, I’m sharing my patch here, maybe it’s useful for some, or
maybe it can be the base for an official xhtml release with bytestrings
inside.
To speed things up even more one should probably create a type analogous
to ShowS, i.e. (L.ByteString - L.ByteString), that allows you to
concatenate ByteString chunks cheaply.
I also noted that the current version of bytestring implements
concatMap in a way that is guaranteed to rip apart the string into
very small chunks, even if the mapped function returns the same
character most times (as it is the case for the Html escaping function).
Therefore, I wrote this function:
-- | More efficient variant of 'L.concatMap'
concatMapL' :: (Char - String) - L.ByteString - L.ByteString
concatMapL' f s = go s
where go s = let (unmodified, modified) = L.span (\c - f c == [c]) s
in case L.uncons modified of
Nothing - unmodified
Just (c,rest) - L.pack (f c) `L.append` go rest
Does this make sense? Should it maybe replace the function in the
library?
Greetings,
Joachim
[1] http://darcswatch.nomeata.de/
--
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
Nur in xhtml-bytestring/: dist.
diff -aur xhtml-3000.2.0.1/Text/XHtml/Debug.hs xhtml-bytestring/Text/XHtml/Debug.hs
--- xhtml-3000.2.0.1/Text/XHtml/Debug.hs 2008-09-17 16:19:47.0 +0200
+++ xhtml-bytestring/Text/XHtml/Debug.hs 2009-01-19 22:04:32.0 +0100
@@ -7,6 +7,7 @@
import Text.XHtml.Table
import Text.XHtml.Strict.Elements
import Text.XHtml.Strict.Attributes
+import qualified Data.ByteString.Lazy.Char8 as L
--
-- * Tree Displaying Combinators
@@ -84,7 +85,7 @@
debug :: HtmlElement - HtmlTree
debug (HtmlString str) = HtmlLeaf (spaceHtml +++
- linesToHtml (lines str))
+ linesToHtml (lines (L.unpack str)))
debug (HtmlTag {
markupTag = markupTag,
markupContent = markupContent,
diff -aur xhtml-3000.2.0.1/Text/XHtml/Extras.hs xhtml-bytestring/Text/XHtml/Extras.hs
--- xhtml-3000.2.0.1/Text/XHtml/Extras.hs 2008-09-17 16:19:47.0 +0200
+++ xhtml-bytestring/Text/XHtml/Extras.hs 2009-01-19 22:03:57.0 +0100
@@ -13,11 +13,11 @@
-- | Convert a 'String' to 'Html', converting
-- characters that need to be escaped to HTML entities.
stringToHtml :: String - Html
-stringToHtml = primHtml . stringToHtmlString
+stringToHtml = primHtml' . stringToHtmlString
-- | This converts a string, but keeps spaces as non-line-breakable.
lineToHtml :: String - Html
-lineToHtml = primHtml . concatMap htmlizeChar2 . stringToHtmlString
+lineToHtml = primHtml' . concatMapL' htmlizeChar2 . stringToHtmlString
where
htmlizeChar2 ' ' = nbsp;
htmlizeChar2 c = [c]
diff -aur xhtml-3000.2.0.1/Text/XHtml/Frameset.hs xhtml-bytestring/Text/XHtml/Frameset.hs
--- xhtml-3000.2.0.1/Text/XHtml/Frameset.hs 2008-09-17 16:19:47.0 +0200
+++ xhtml-bytestring/Text/XHtml/Frameset.hs 2009-01-19 22:35:55.0 +0100
@@ -11,6 +11,7 @@
primHtml,
-- * Rendering
showHtml, renderHtml, prettyHtml,
+ showHtml', renderHtml', prettyHtml',
showHtmlFragment, renderHtmlFragment, prettyHtmlFragment,
module Text.XHtml.Strict.Elements,
module Text.XHtml.Frameset.Elements,
@@ -19,7 +20,7 @@
module Text.XHtml.Extras
) where
-import Text.XHtml.Internals
+import Text.XHtml.Internals hiding (showHtml', renderHtml', prettyHtml')
import Text.XHtml.Strict.Elements
import Text.XHtml.Frameset.Elements
@@ -28,7 +29,9 @@
import Text.XHtml.Extras
-docType =
+import qualified Data.ByteString.Lazy.Char8 as L
+
+docType =
Re: [Haskell-cafe] Employment
Erik de Castro Lopo wrote: Its proabably a little harder to find a company that wants a Haskell hacker than it is to find a company where Haskell and other sane languages can be worked in over time. I think you're probably right about that. ;-) I mean, heck, *I* use Haskell at work - and I'm not even supposed to be coding things! Like many people I'm sure, I'd like to get paid to code stuff in Haskell. But I can't begin to imagine how you go about doing that... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
Andrew Wagner wrote: http://www.haskell.org/haskellwiki/Haskell_in_industry could be of interest to you Hmm, interesting... So lots happening in Portland, Oregon. Lots in Cambridge, MA. A few things in Europe. And nothing at all in the UK... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GLUT (glutGet undefined reference)
2009/1/19 Paul Keir pk...@dcs.gla.ac.uk: I was hoping to introduce my old pal OpenGL with my new chum, Haskell. I used cabal to install GLUT on my 64-bit Ubuntu machine with GHC 6.8.2 (installed via apt-get/synaptic). I'm sorry, I can't help you with your problem. But I'd recommend you using GLFW, it's a lot easier to build and use on Linux and on Windows (I never managed to install GLUT on Windows). There are some functions from GLUT that it doesn't have, but most of the time you don't use them anyway. HTH, -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Employment
Andrew Coppin wrote: Andrew Wagner wrote: http://www.haskell.org/haskellwiki/Haskell_in_industry could be of interest to you Hmm, interesting... So lots happening in Portland, Oregon. Lots in Cambridge, MA. A few things in Europe. And nothing at all in the UK... We (Credit Suisse) have Haskell developers in both London and NY, although the page only listed NY (I've now corrected it). Two other companies without locations listed - Barclays Capital and Standard Chartered - also have at least some Haskell development in the UK, and I think that's where Amgen's development is based too. Ganesh == Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html == ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to make code least strict?
I second Ryan's recommendation of using unamb [1,2,3] to give you unbiased (symmetric) laziness. The zip definition could also be written as zip xs@(x:xs') ys@(y:ys') = assuming (xs == []) [] `unamb` assuming (ys == []) [] `unamb` (x,y) : zip xs' ys' The 'assuming' function yields a value if a condition is true and otherwise is bottom: assuming :: Bool - a - a assuming True a = a assuming False _ = undefined This zip definition is a special case of the annihilator pattern, so zip = parAnnihilator (\ (x:xs') (y:ys') - (x,y) : zip xs' ys') [] where 'parAnnihilator' is defined in Data.Unamb (along with other goodies) as follows: parAnnihilator :: Eq a = (a - a - a) - a - (a - a - a) parAnnihilator op ann x y = assuming (x == ann) ann `unamb` assuming (y == ann) ann `unamb` (x `op` y) [1] http://haskell.org/haskellwiki/Unamb [2] http://hackage.haskell.org/packages/archive/unamb/latest/doc/html/Data-Unamb.html [3] http://conal.net/blog/tag/unamb/ - conal On Mon, Jan 19, 2009 at 12:27 PM, Ryan Ingram ryani.s...@gmail.com wrote: On Mon, Jan 19, 2009 at 9:10 AM, ChrisK hask...@list.mightyreason.com wrote: Consider that the order of pattern matching can matter as well, the simplest common case being zip: zip xs [] = [] zip [] ys = [] zip (x:xs) (y:ys) = (x,y) : zip xs ys If you are obsessive about least-strictness and performance isn't a giant concern, this seems like a perfect use for Conal's unamb[1] operator. zipR xs [] = [] zipR [] ys = [] zipR (x:xs) (y:ys) = (x,y) : zip xs ys zipL [] ys = [] zipL xs [] = [] zipL (x:xs) (y:ys) = (x,y) : zip xs ys zip xs ys = unamb (zipL xs ys) (zipR xs ys) This runs both zipL and zipR in parallel until one of them gives a result; if neither of them is _|_ they are guaranteed to be identical, so we can unambiguously choose whichever one gives a result first. -- ryan [1] http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice/ ___ 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] Employment
On Mon, 2009-01-19 at 21:04 +, Andrew Coppin wrote: Erik de Castro Lopo wrote: Its proabably a little harder to find a company that wants a Haskell hacker than it is to find a company where Haskell and other sane languages can be worked in over time. I think you're probably right about that. ;-) I mean, heck, *I* use Haskell at work - and I'm not even supposed to be coding things! /me feels slightly relieved, if you'll forgive my saying so :) Also, if you don't mind my asking, what *is* your job title? jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
On Mon, 2009-01-19 at 20:55 +, Andrew Coppin wrote: Dan Piponi wrote: On Mon, Jan 19, 2009 at 11:33 AM, Andrew Coppin andrewcop...@btinternet.com wrote: My only problem with it is that it's called Bool, while every other programming language on Earth calls it Boolean. (Or at least, the languages that *have* a name for it...) Python: bool ocaml: bool C++: bool C99: bool C#: bool Versus Java, Pascal, Again, we don't want to imitate these two! Smalltalk and Eiffel who all call it Boolean. Oh well. At least it's pretty obvious what it means. But I'm far more perturbed by names like Eq, Ord, Num, Ix (??), and so on. The worst thing about C is the unecessary abbriviations; let's not copy them, eh? They're short so they're quick to parse (for a human) and read. They're easy to type. If you have a constraint like (Eq a,Num a,Ord a,Show a,Ix a) you can see all five type classes at a single glance without having to scan your eye across the line. They're highly mnemonic in the sense that once I'd learnt what they meant it became hard to forget them again. What exactly is wrong with them? Would it really hurt to type a few more keystrokes and say Equal? Ordered? Index? I don't think so. Constantly? Yeah. Commonly used names should be short, or abbreviated. You can't abbreviate type classes. Sure, we don't especially want to end up with classes like StrictlyOrderedAssociativeSet or something, but a few more characters wouldn't exactly kill you. But, again, this is too difficult to change now, so we're stuck with it. PS. Ord implies Eq, so you don't need both in the same constraint. Num implies Show, so you don't need that either. So actually, (Ord a, Num a, Ix a) - or rather, (Ordered a, Number a, Index a) - would do just fine. newtype MyFoo = MyWrapsWhatever deriving (Eq, Ord, Read, Show, Num, Ix, Data, Typeable) vs. newtype MyFoo = MyWrapsWhatever deriving (Equality, Order, Read, Show, Number, Index, Data, Typeable) Yeah. Count me out. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] xhtml + bytestring
I think that your instance is too specific, although useful for the particular case of escaping. I've done my own implementation for fun: concatMap' :: (Word8 - L.ByteString) - L.ByteString - L.ByteString concatMap' f s = L.unfoldr p x0 where x0 = (LI.Empty, s, 0, 0) p (LI.Empty, LI.Empty, _, _) = Nothing p (LI.Empty, c@(LI.Chunk s s'), _, ns) | ns==S.length s = p (LI.Empty, s', 0, 0) | otherwise = p (f (S.index s ns), c, 0, ns+1) p (c@(LI.Chunk s s'), bs, nf, ns) | nf==S.length s = p (s', bs, 0, ns) | otherwise = Just (S.index s nf, (c, bs, nf+1, ns)) It turns out to be both a lot slower (don't know why) and with greater memory residence (because it's not lazy) than the built-in concatMap for bytestrings in my synthetic tests. However, it produces a bytestring with few chunks (due to a good implementation of unfoldr) in an asymptotically optimal time. I also noticed that for a case like unescaping, where 'f' produces a small string (5) and the source bytestring is large (10mln), my version spends 24% in GC whereas the standard version spends 68%. A drawback is that the resulting bytestring is not lazy at all. I wonder how one might optimize it. It looks like a function that a compiler should optimize very well without my help, although I didn't read the core etc. 2009/1/20 Joachim Breitner m...@joachim-breitner.de: Hi Bjorn, hi list, the darcswatch instance I'm running is getting quite big, and it periodically slows down my server. I managed to get quite an improvement with this simple patch to my parsing: http://darcs.nomeata.de/cgi-bin/darcsweb.cgi?r=darcswatch;a=commitdiff;h=20090119181919-23c07-140f8deb91a52a423a2984dce2d22f4a48999aaf.gz Since the new HTTP library, my code runs completely on ByteStrings, only the xhtml library expects me to feed strings. I tried to fix this and created a patch against xhtml-3000.2.0.1 to work internally with lazy ByteStrings. It's API-compatible to the normal xhtml library, it just adds showHtml', renderHtml' and prettyHtml' that output lazy ByteStrings, and that has Html instances for strict any lazy ByteStrings. There were some speed and space improvements, but none horrific (at least for DarcsWatch, most of the time goes into parsing the repositories and mails, and into sorting that data). Unfortunately, I can't use it in the live installation until I upgrade the machine from Debian etch to lenny, as the bundled bytestring library in ghc-6.6' base is too old. Nevertheless, I'm sharing my patch here, maybe it's useful for some, or maybe it can be the base for an official xhtml release with bytestrings inside. To speed things up even more one should probably create a type analogous to ShowS, i.e. (L.ByteString - L.ByteString), that allows you to concatenate ByteString chunks cheaply. I also noted that the current version of bytestring implements concatMap in a way that is guaranteed to rip apart the string into very small chunks, even if the mapped function returns the same character most times (as it is the case for the Html escaping function). Therefore, I wrote this function: -- | More efficient variant of 'L.concatMap' concatMapL' :: (Char - String) - L.ByteString - L.ByteString concatMapL' f s = go s where go s = let (unmodified, modified) = L.span (\c - f c == [c]) s in case L.uncons modified of Nothing - unmodified Just (c,rest) - L.pack (f c) `L.append` go rest Does this make sense? Should it maybe replace the function in the library? Greetings, Joachim [1] http://darcswatch.nomeata.de/ -- 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: How to make code least strict?
Actually, I see a nice pattern here for unamb + pattern matching:
zip xs ys = foldr unamb undefined [p1 xs ys, p2 xs ys, p3 xs ys] where
p1 [] _ = []
p2 _ [] = []
p3 (x:xs) (y:ys) = (x,y) : zip xs ys
Basically, split each pattern out into a separate function (which by
definition is _|_ if there is no match), then use unamb to combine
them.
The invariant you need to maintain is that potentially overlapping
pattern matches (p1 and p2, here) must return the same result.
With a little typeclass hackery you could turn this into
zip = unambPatterns [p1,p2,p3] where {- p1, p2, p3 as above -}
Sadly, I believe the performance of parallel-or-style operations is
pretty hideous right now. Conal?
-- ryan
On Mon, Jan 19, 2009 at 2:42 PM, Conal Elliott co...@conal.net wrote:
I second Ryan's recommendation of using unamb [1,2,3] to give you unbiased
(symmetric) laziness.
The zip definition could also be written as
zip xs@(x:xs') ys@(y:ys') =
assuming (xs == []) [] `unamb`
assuming (ys == []) [] `unamb`
(x,y) : zip xs' ys'
The 'assuming' function yields a value if a condition is true and otherwise
is bottom:
assuming :: Bool - a - a
assuming True a = a
assuming False _ = undefined
This zip definition is a special case of the annihilator pattern, so
zip = parAnnihilator (\ (x:xs') (y:ys') - (x,y) : zip xs' ys') []
where 'parAnnihilator' is defined in Data.Unamb (along with other goodies)
as follows:
parAnnihilator :: Eq a = (a - a - a) - a - (a - a - a)
parAnnihilator op ann x y =
assuming (x == ann) ann `unamb`
assuming (y == ann) ann `unamb`
(x `op` y)
[1] http://haskell.org/haskellwiki/Unamb
[2]
http://hackage.haskell.org/packages/archive/unamb/latest/doc/html/Data-Unamb.html
[3] http://conal.net/blog/tag/unamb/
- conal
On Mon, Jan 19, 2009 at 12:27 PM, Ryan Ingram ryani.s...@gmail.com wrote:
On Mon, Jan 19, 2009 at 9:10 AM, ChrisK hask...@list.mightyreason.com
wrote:
Consider that the order of pattern matching can matter as well, the
simplest
common case being zip:
zip xs [] = []
zip [] ys = []
zip (x:xs) (y:ys) = (x,y) : zip xs ys
If you are obsessive about least-strictness and performance isn't a
giant concern, this seems like a perfect use for Conal's unamb[1]
operator.
zipR xs [] = []
zipR [] ys = []
zipR (x:xs) (y:ys) = (x,y) : zip xs ys
zipL [] ys = []
zipL xs [] = []
zipL (x:xs) (y:ys) = (x,y) : zip xs ys
zip xs ys = unamb (zipL xs ys) (zipR xs ys)
This runs both zipL and zipR in parallel until one of them gives a
result; if neither of them is _|_ they are guaranteed to be identical,
so we can unambiguously choose whichever one gives a result first.
-- ryan
[1]
http://conal.net/blog/posts/functional-concurrency-with-unambiguous-choice/
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[Haskell-cafe] Current research on overlapping/closed type families?
What's the status of overlapping/closed type families? I'm interested
in something like the following, which can currently be implemented in
GHC with Oleg-magic using functional dependencies, but cannot, to my
knowledge, be implemented with type families:
data HTrue = HTrue
data HFalse = HFalse
type family IsFunction f
{- not legal in GHC6.10 -}
type instances
IsFunction (a - b) = HTrue
IsFunction a = HFalse
-- ryan
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Re: [Haskell-cafe] MonadTrans lift implementation
G'day all. Quoting Jonathan Cast jonathancc...@fastmail.fm: (By the way, you *do* have the equations lift (return x) = return x [...] Right. And you could, at least in principle, implement return this way in all monad transformers. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
Hmm, interesting... So lots happening in Portland, Oregon. Lots in Cambridge, MA. A few things in Europe. And nothing at all in the UK... Nothing in the UK? Lets not forget MSR and Well-Typed! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
G'day all.
On Mon, 2009-01-19 at 19:33 +, Andrew Coppin wrote:
My only problem with it is that it's called Bool, while every other
programming language on Earth calls it Boolean. (Or at least, the
languages that *have* a name for it...)
Jonathan Cast commented:
Except C++?
And perhaps more to the point, Boolean is an adjective, not a noun.
Therefore, it would be better reserved for a typeclass.
class (PartialOrder a) = JoinSemilattice a where
(||) :: a - a - a
class (MeetSemilattice a) = BoundedJoinSemilattice a where
bottom :: a
class (PartialOrder a) = MeetSemilattice a where
() :: a - a - a
class (MeetSemilattice a) = BoundedMeetSemilattice a where
top :: a
class (BoundedJoinSemilattice a, BoundedMeetSemilattice a) = Heyting a where
implies :: a - a - a
not :: a - a
not x = x `implies` bottom
class (Heyting a) = Boolean a where
{- the additional axiom that x || not x == top -}
Cheers,
Andrew Bromage
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Re: [Haskell-cafe] LinuxFest Northwest 2009
I might go - I'll certainly keep an eye on the page. It would be more likely if I could find a group driving from Portland. Tom On Mon, Jan 19, 2009 at 9:25 PM, Shachaf Ben-Kiki shac...@gmail.com wrote: LFNW 2009 (http://linuxfestnorthwest.org/) is going to be at the end of April, and I was wondering if anyone here is going to be there, or possibly a Haskell-related presentation. Last year I met ac from #haskell there, but it would be nice if more people came, especially with the (relatively) big group in Oregon and such. Perhaps someone here has plans already? Shachaf ___ 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] know a workaround for greedy context reduction?
I revisited the Strongly typed Heterogeneous Lists paper and read
about the import hierarchy technique. The basic idea is to delay
importing the instances until as late as possible, which prevents the
context simplification. The instances are effectively imported in the
top, Main module.
I'm thinking of exporting a MyLibrary.Main or MyLibrary.Instances module.
Anyone have experience with this approach in a library design? Is it
worth the user's extra import? Any pitfalls?
On Sun, Dec 7, 2008 at 4:57 PM, Nicolas Frisby nicolas.fri...@gmail.com wrote:
Seems I got ahead of myself with the bug search. I was thinking bug
because when I ascribe a type, I expect the compiler to check and then
respect it. With the most general type specification of the :type
command in mind, this does make sense. Thanks for improving my
internal notion of :type.
My grumble may seem more legitimate from a library perspective. I
implement a type-level function Append with three (preferably hidden)
ancillary classes and a single instance in order to support the
multiple modalities (in the Mercury sense) of the Append logic
function. When a user defines another function that uses the append
method, it's obfuscating for the user to see the internal classes in
the inferred type. That's what I would like to workaround.
If we consider class C the internal and consider class D and the
function f the library's exposed interface, then I'd like to see C
instead of D in the context of f and any function the user defines
with f, especially when I have supplied a preferred type for f.
f :: D a = () - a
f () = d
* :t f
f :: (C a) = () - a
No dice?
Thanks again,
Nick
On Sun, Dec 7, 2008 at 2:34 PM, Simon Peyton-Jones
simo...@microsoft.com wrote:
This is perfectly reasonable behavior I'm afraid. If you do :info d
you'll get d's original type signature. But :type takes an *arbitrary
expression* (in this case a single variable 'd', and figures out its most
general type. You could have said :t (3*3) for example.
In this case, when inferring the most general type of the expression d,
GHC tries to simplify the context (D a), and uses the instance declaration
to reduce it to (C a). And then it can't simplify it further. But you
*might* have had
instance C a
somewhere, in which case it'd have been able to simplify the (C a) away. So
GHC must try that route. If it fails, you want it to back up to a
notationally more convenient type, but GHC can't do that, I'm afraid
Simon
| -Original Message-
| From: haskell-cafe-boun...@haskell.org [mailto:haskell-cafe-
| boun...@haskell.org] On Behalf Of Nicolas Frisby
| Sent: 06 December 2008 03:23
| To: haskell Cafe
| Subject: [Haskell-cafe] know a workaround for greedy context reduction?
|
| With these three declarations
|
| {-# LANGUAGE FlexibleInstances #-}
| {-# LANGUAGE UndecidableInstances #-}
|
| class C a where c :: a
| class C a = D a where d :: a
| instance C a = D a where d = c
|
| ghci exhibits this behavior:
|
| * :t d
| d :: (C a) = a
|
| Where I would prefer d :: (D a) = a. In my actual examples, the
| context is much larger and I can't involve overlapping instances. Is
| there a known workaround? I didn't find a related bug on the GHC trac,
| and I don't know if other compilers behave in the same way.
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[Haskell-cafe] GHCi Memory Leak in Windows Vista
Hi all! Environment: Windows Vista + SP1, GHC 6.10.1. Description: If use ghci.exe and run separate ghc.exe compiler process, memory leak in ghci occurred. After exhausting available memory Vista has no responding. Steps for reproduce: 1) After run GHCi, process list has 2 processes: ghci.exe (a), ghc.exe (b), 2) Then I run separated compilation process 20 times: Command Line: for /L %i in (1,1,20) do ghc -fforce-recomp -O --make problem.hs -o problem.exe Sample Haskell Program listing (problem.hs): main :: IO () main = do print $ [x | x - [1..]] putStrLn Hello, world! 3) In process list (taskmgr.exe) I can see some processor activity and increasing value of Memory (Private Working Set) for process (b). 4) After run Memory (Private Working Set) increased about 100Mb. Best regards. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Employment
On Mon, Jan 19, 2009 at 3:04 PM, Andrew Coppin andrewcop...@btinternet.com wrote: Like many people I'm sure, I'd like to get paid to code stuff in Haskell. But I can't begin to imagine how you go about doing that... At Eaton, we're using Haskell to design automotive control systems (see http://cufp.galois.com/). In a 3 month span we went from 98K lines of Simulink, Matlab, and VisualBasic to 4K lines of Haskell. The system is now in vehicle testing and is going to production mid this year. In addition to tuning control laws and fault monitors, other fun things we are using Haskell for is to integrate our environment with an SMT solver for infinite state, bounded model checking. Unfortunately, due to economic conditions, all new openings have been temporarily put on hold. However, we expect new reqs to open as soon as conditions improve. When this happens, it would be nice if we had a single source to find qualified people. Maybe the community should consider building a database to help people who want to write Haskell for a living get in touch with employers who want to hire them. Such a database would help me counter by boss's argument that it's impossible to find and hire Haskell programmers. -Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
On Mon, Jan 19, 2009 at 7:22 PM, a...@spamcop.net wrote:
And perhaps more to the point, Boolean is an adjective, not a noun.
Therefore, it would be better reserved for a typeclass.
There's also John Meacham's Boolean package.
http://repetae.net/recent/out/Boolean.html
class (Heyting a) = Boolean a where
{- the additional axiom that x || not x == top -}
Are there any instances of Boolean that aren't isomorphic to Bool?
(I'm assuming that (||) and () are intended to be idempotent,
commutative, and associative.)
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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Re: [Haskell-cafe] Employment
Such a database would help me counter by boss's argument that it's impossible to find and hire Haskell programmers. Err, people actually say such things? And they say _we're_ out of touch with the real world? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
On Mon, Jan 19, 2009 at 6:25 PM, David Menendez d...@zednenem.com wrote: Are there any instances of Boolean that aren't isomorphic to Bool? a-Bool for any a. I think. Though I think it should be called GeorgeBoolean otherwise we might confuse it for something his father might have invented. -- Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
G'day all. Quoting David Menendez d...@zednenem.com: Are there any instances of Boolean that aren't isomorphic to Bool? Sure. Two obvious examples: - The lattice of subsets of a universe set, where or is union and is intersection and not is complement with respect to the universe. - Many-valued logic systems. - Intuitionistic logic systems. - The truth values of an arbitrary topos (i.e. the points of the subobject classifier). Look up Heyting algebra for examples. Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Improved documentation for Bool
G'day all. I wrote: - Intuitionistic logic systems. - The truth values of an arbitrary topos (i.e. the points of the subobject classifier). Sorry, I misread the question. These are _not_ instances of Boolean (or at least the latter isn't an instance in general). Cheers, Andrew Bromage ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell and C++ program
On 17 jan 2009, at 22:22, Derek Elkins wrote: On Thu, 2009-01-15 at 13:40 +0100, Apfelmus, Heinrich wrote: Eugene Kirpichov wrote: Well, your program is not equivalent to the C++ version, since it doesn't bail on incorrect input. Oops. That's because my assertion show . read = id is wrong. We only have read . show = id show . read = id (in the less defined than sense) No, you only have read . show = id which often doesn't hold in practice. show . read /= id You do not even have that; the read may remove surplus parentheses which will not be reinserted by the show. Doaitse Assuming the first identity holds, you do of course have show . read . show = show and this probably holds even in most cases where read . show = id does not hold. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Haskell and C++ program
On Mon, 2009-01-19 at 22:12 -0500, S. Doaitse Swierstra wrote: On 17 jan 2009, at 22:22, Derek Elkins wrote: On Thu, 2009-01-15 at 13:40 +0100, Apfelmus, Heinrich wrote: Eugene Kirpichov wrote: Well, your program is not equivalent to the C++ version, since it doesn't bail on incorrect input. Oops. That's because my assertion show . read = id is wrong. We only have read . show = id show . read = id (in the less defined than sense) No, you only have read . show = id which often doesn't hold in practice. show . read /= id You do not even have that; the read may remove surplus parentheses which will not be reinserted by the show. Doaitse My notation is show . read is not less than or equal to id. That covers that case. The particular example I was thinking of was actually simply whitespace. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comments from OCaml Hacker Brian Hurt
On Thu, 2009-01-15 at 18:10 -0500, Cale Gibbard wrote: My personal preference would be: class Monoid m where zero :: m (++) :: m - m - m (in the Prelude of course) - Cale I've tried doing this (and making more widespread use of typeclassed operations) by writing my own AltPrelude. Unfortunately there is still a lot of 'unrebindable' syntax (list comprehensions, 'error' forced to exist in Monad, if-then-else not using nearest Bool, etc) which makes this hard to achieve. 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
