Re: [Haskell-cafe] Accumulating related XML nodes using HXT
Daniel McAllansmith [EMAIL PROTECTED] writes:
Hello.
I have some html from which I want to extract records.
Each record is represented within a number of tr nodes, and all records
tr
nodes are contained by the same parent node.
This is very poorly written HTML. The original structure of the data
is destroyed - the parse tree no longer reflects the data structure.
(If a record is to be displayed in several rows, there are proper
ways.) It is syntactically incorrect: nested tr, and color in hr.
(Just ask http://validator.w3.org/ .) I trust that you are parsing
this because you realize it is all wrong and you want to
programmatically convert it to proper markup.
Since the file is unstructured, I choose not to sweat over restoring
the structure in an HXT arrow. The HXT arrow will return a flat list,
just as the file is a flat ensemble. The list looks like:
[/prod17, Television, (code: 17), A very nice telly.,
/prod24, Cyclotron, (code: 24), Mind your fillings.]
I then use a pure function to decompose this list four items at a time
to emit the desired records. This is trivial outside HXT arrows. I
use tuples, and every field is a string; you can easily change the
code to produce Prod's, turn (code: 17) into the number 17, etc.
Here is a complete, validated HTML 4 file containing the table, just
so that my program below actually has valid input.
!DOCTYPE HTML PUBLIC -//W3C//DTD HTML 4.01//EN
http://www.w3.org/TR/html4/strict.dtd;
html
head
meta http-equiv=Content-Type content=text/html;charset=utf-8
titleProducts/title
/head
body
table
tr
tdstrongProduct:/strong/td
tdstronga href=/prod17Television/a/strong (code: 17)/td
/tr
tr
tdstrongDescription:/strong/td
tdA very nice telly./td
/tr
tr
tdhr/td
/tr
tr
tdstrongProduct:/strong/td
tdstronga href=/prod24Cyclotron/a/strong (code: 24)/td
/tr
tr
tdstrongDescription:/strong/td
tdMind your fillings./td
/tr
tr
tdhr/td
/tr
/table
/body
/html
Here is my program:
import Text.XML.HXT.Arrow
main =
do { unstructured - runX (p table.html)
; let structured = s unstructured
; print structured
}
p filename =
readDocument [(a_parse_html,1)] filename
deep (isElem hasName table)
getChildren isElem hasName tr
getChildren isElem hasName td
getChildren
p1 + p2
p1 =
isElem hasName strong
getChildren isElem hasName a
getAttrValue href + (getChildren getText)
p2 =
getText
s (a:b:c:d: rest) = (a,b,c,d) : s rest
s _ = []
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Re: [Haskell-cafe] A type in search of a name...
Brian Hulley [EMAIL PROTECTED] writes: You'll never believe it but I've been struggling last night and all of today to try and think up a name for the following type and I'm still nowhere near a solution: data ??? = VarId | VarSym | ConId | ConSym Perhaps Atom. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is Haskell a Keynesian language?
Henning Thielemann [EMAIL PROTECTED] writes: Here is another approach of questionable classification of languages. :-) A lazy functional program is demand driven, an imperative program is supply driven. So is Haskell a Keynesian language and C++ a Say language? Great, now we can talk about the Invisible Hand performing evaluations... Alice: The Invisible Hand is holding up more memory than I thought. My program is using O(n) space just to compute length! Bob: You've violated Nash equilibrium! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell] Evil code
Thomas Davie [EMAIL PROTECTED] writes: The evil code and CPSness were actually intended to be completely separate entities. I needed (and to a lesser extent now still need), examples that are (a) very higher order, and (b) evil and hard to understand the runtime behaviour of. Generally code that produces and consumes lots of tuples, e.g., common implementations of splitAt, span, mapAccumL (these are list functions), the State monad (Control.Monad.State; you can strip out the instance Monad thing and package the code as ordinary functions). Also Phil Wadler's Monads for Functional Programming http://homepages.inf.ed.ac.uk/wadler/topics/monads.html#marktoberdorf contains a monadic parser. In section 5.11 improving laziness two different ways of coding and their respective laziness effects are shown. This can be further exemplified by debugger traces, methinks. ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
[Haskell-cafe] Re: Haskell web forum
As requested, I continue here the thread on the proposal for a web forum. You will soon enough find out what I think of web forums. Let me first show you what this mailing list looks like to me using software of my choice: ! [ 192: Mark Carroll] [Haskell-cafe] Card trick ! [ 37: Thomas Conway ] Re: [Haskell-cafe] How can we detect and fix E [ 19: Albert Lai ] Re: [Haskell-cafe] Either e Monad E [ 19: Deokhwan Kim] [ 62: Bas van Dijk] E [ 47: [EMAIL PROTECTED] [Haskell-cafe] Re: Optimization problem [ 51: Ross Paterson ] E [ 42: Conor McBride ] [ 56: Robert Dockins ] [ 42: Conor McBride ] [ 61: [EMAIL PROTECTED] [Haskell-cafe] Re: Optimization prob [ 14: Ross Paterson ] [ 154: Jan-Willem Maessen ] Re: [Haskell-cafe] Traversing a graph in STM [ 52: Josef Svenningsson ] [ 18: Bulat Ziganshin ] Re[2]: [Haskell-cafe] Traversing a graph [ 26: Sebastian Sylvan] [ 22: Ashley Yakeley ] [Haskell-cafe] Re: Wiki contact An elaboration of the cool features I have always enjoyed is in order: On browsing threads, both inter-thread jumps and intra-thread navigation, the screenshot speaks for itself. (If for any perverted reason I want the messages listed chronologically rather than well-organized, the software can do it too.) The messages marked with E are messages I have just read. With this mark, they will not show up next time I browse this mailing list again. Take note that I can mark and unmark any subset of messages, unlike certain people's linear-time idea that one single timestamp distinguishes already read from new across the board. (I have always heard that non-linear thinking is superior to linear thinking. I think I believe it for at least this application.) (Is this an example of what is meant when someone said that a web forum requires less technical knowledge, i.e., programmers on a web forum will not need to know about subsets?) (Speaking of which, is the thread-browsing part also an example, i.e., programmers on a web forum will not need to know about trees and forests?) Though the marked messages will not show up next time, they are not thrown away yet. They are kept on my disk for quite a while. (I get to set how many days they stay.) The software offers several ways to show them, but I think the most useful one is this. Suppose I look at apfelmus's message on Optimization problem (two are shown, I'm referring to the first one), and wonder, gee, what is it replying to? To find out, I position myself to that message, then press a button, then the software will show the desired parent message. In fact, the updated screen looks like this (I just include the thread in question): E [ 18: Ross Paterson ] Re: [Haskell-cafe] Re: Optimization problem E [ 47: [EMAIL PROTECTED] [ 51: Ross Paterson ] E [ 42: Conor McBride ] [ 56: Robert Dockins ] [ 42: Conor McBride ] [ 61: [EMAIL PROTECTED] [Haskell-cafe] Re: Optimizatio [ 14: Ross Paterson ] So even if apfelmus's message quotes nothing from Ross's, I can still find out, provided it is recently enough to be still on my disk. Take note that, on a linear-thinking web forum, there could be any number of intervening messages in the same thread between Ross's and apfelmus's, and even though it is true that Ross's is immortalized in the database of the forum, the real question is how to fish for it. (ObRant again about how web forums are friendly to programmers who have no clue about trees.) Here is a feature unrelated to threading, and in fact it is much more fundamental and pervasive (since almost all email software, not just mine, provides this), and in fact it has much to do with pervasive computing too. Here in Canada, in cafes one can get wireless Internet access, but it has to be paid by the minutes. So suppose I want to visit haskell-cafe when I visit a Starbucks cafe, and there are 30 messages I want to read, and I can read them at the rate of half a minute per message. If haskell-cafe were on a web forum, I would end up keeping my wireless connection for 15 minutes. (Could I disconnect and reconnect between messages? First of all that's really a pain. Secondly, let's say I do that, the billing is still such that I connect for 2 seconds and I'm still charged for 1 minute.) But since haskell-cafe is on a mailing list, I can just connect for a few seconds to download the messages, then disconnect and read offline; I'm charged for at most 1 minute. (Alternatively but equivalently, suppose I want to read haskell-cafe while riding subway - underground trains. Here in Toronto the subway doesn't provide wireless Internet access yet, and probably won't for another decade. If haskell-cafe
Re: [Haskell-cafe] Either e Monad
Deokhwan Kim [EMAIL PROTECTED] writes: Where is the Monad instance declaration of Either e? It is in Control.Monad.Error as well. Strange: the doc doesn't state it. I found out in ghci using: :module +Control.Monad.Error :info Either The relevant result is: instance Error e = Monad (Either e) -- Imported from Control.Monad.Error ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell] Haskell XML
Till Mossakowski [EMAIL PROTECTED] writes: Could someone summarize the pros and cons of HXT versus HaXml versus HSX? From my perspective (therefore perhaps important but hardly comprehensive), HXT vs HaXml: HXT provides arrows. The con is you have to learn arrows. The pro is the arrows provided have the same capability as monads of state and IO. Dually, HaXml does not provide arrows or equivalent. The provided functions of type Content - [Content] (Content is the node type) are not very flexible or helpful if you want to thread a state through a parse tree, e.g., a recent question in haskell-cafe on how to insert random numbers as attribute values into a document using HaXml. (I lied about the con of HXT. You don't have to learn arrows if you really resist --- monads of the same capability are provided too.) I have no comment on HSX, and you already know the language vs library contrast. ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell-cafe] HaXml question
Tim Newsham [EMAIL PROTECTED] writes:
I thought this one would be easy but I'm starting to think its not.
I am playing with HaXml and I want to transform an XML tree into
another tree. The transforms are simple enough, but the kicker
is that I want them to be stateful. In this example, the state
is a random number generator. So the transformation depends on
the current node and the random number generator state. I want
to transform every node in the tree.
Indeed, the HaXml functions are pure, and in particular foldXml does
not thread a state through.
To introduce state, you provide your own state monad (fortunately
there is always Control.Monad.State). To traverse the whole tree in
this monad, you write your own recursion and deconstruct the nodes
yourself (because foldXml is too specific to be re-usable).
Here is my example. It replaces attribute values by random integers
between 0 and 99, so it is a similar task but slightly different from
yours. Some names are inspired by yours, but I have simplified their
nature: The state I thread through is not a stream of generators, but
rather a stream of numbers; as long as this stream comes from
Random.randomRs, I'm good.
import Text.XML.HaXml
import Control.Monad.State
import Random
newtweak :: [Int] - CFilter
newtweak xs c = [evalState (reco c) xs]
reco :: Content - State [Int] Content
reco (CElem (Elem nm ats cs)) =
do { ats' - mapM newtweakAttr ats
; cs' - mapM reco cs
; return (CElem (Elem nm ats' cs'))
}
reco c = return c
newtweakAttr (k,_) =
do { n - gets head
; modify tail
; return (k, AttValue[Left (show n)])
}
main = do
gen - getStdGen
processXmlWith (newtweak (randomRs (0,99) gen))
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Re: [Haskell-cafe] Why Not Haskell?
Brian Hulley [EMAIL PROTECTED] writes:
Also, the bottom line imho is that Haskell is a difficult language to
understand, and this is compounded by the apparent cleverness of
unreadable code like:
c = (.) . (.)
when a normal person would just write:
c f g a b = f (g a b)
All mainstream languages are also difficult to understand, with
similarly clever, unreadable code. Let's have a fun quiz! Guess the
mainstream languages in question:
0. What language would allow
4[hello world]
when a normal person would just write
hello world[4]
I first saw this in Dr. Dobb's Journal a decade ago; the author
said that someone actually used it in interviews!
1. What language, supporting a kind of both parametric polymorphism
and subclass polymorphism, allows and actually features such a class
declaration as
class EnumT extends EnumT { ... }
2. What language allows you to test primality in constant runtime?
That is, move all the work to compile time, using its polymorphism.
* * *
I have programmed and watched programming for more than two decades.
I have observed that the rise and fall of popularity is, of course,
driven by many factors: cultural, social, economical, religious,
political, propagandic, ... but superiority is never one of them.
(Library abundance is, I say, less of a cause and more of an effect.
You have some popularity and then you have more contributors;
conversely you lose popularity and then you lose authors. Yes there
is some feeding back, but the bootstrapping is more significant. Perl
has a large library, but that's because it has got an impressive
following. And where did that following come from? Mostly economic
(there was a demand, a niche) and religious (it's like natural
languages).)
Recall that some decent technology that once attained as much as 49%
market share could still decline and vanish in less than a decade. I
am referring to Betamax video tapes. It lost to VHS video tapes, of
lesser picture quality and 51% market share. What did Betamax miss?
Apparently, nothing. It seems that the only difference you can put
your finger at - and even this is elusive - is price. VHS was
slightly cheaper, and apparently that did it.
If you were born after Betamax had vanished, I congratulate you, on
two counts. First, you skipped the dark age during which there was no
Haskell, no Gofer, not even Scheme; there was Lisp but even then not
all functions were first-class. Second, don't feel bad about missing
the video war, as you will soon see an even better one, this time
between Blu-Ray and HD-DVD. Any bet?
I am trying to say this, with much foregoing digression: we could
equip Haskell with the perfect library, the perfect IDEs and tools,
the perfect tutorials and examples, the license that pleases
everyone... every nice thing mentioned in this thread, and it may
still not become popular. Betamax had everything and 49% market share
(if Haskell had 49% mind share, we would be really thrilled, right?),
and it could still vanish.
How to make Haskell more popular? How to make anything at all more
popular? I am inclined to think it's a purely social question.
Nothing short of a rigorous social science can answer it. All the nice
things mentioned in this thread, we should strive to build for our own
sake of course, but they don't answer the question.
I have long stopped asking that question. Once again, I say we should
strive to build all the missing things mentioned. What impact will
they make to the grand scheme of things, we don't know. If there will
be none, don't be surprised; it's life. One day we may have a
rigorous social science that can explain it. Until then, I share with
you a line a Greek friend puts in his .plan file:
Man plans and God laughs.
* * *
Answers to quiz:
0. http://c-faq.com/aryptr/joke.html
1. http://weblogs.java.net/blog/arnold/archive/2005/06/generics_consid_1.html
2. http://homepage.mac.com/sigfpe/Computing/peano.html
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Re: [Haskell-cafe] Re: Haskell-Cafe Digest, Vol 33, Issue 9
Alberto G. Corona [EMAIL PROTECTED] writes: stmcache= newTVar 0 I will explain what this doesn't with an analogy. import Data.IORef notglobal = newIORef True main = do a - notglobal b - notglobal writeIORef a False x - readIORef b print x To better show what's going on, I also provide this for contrast: import Data.IORef import System.IO.Unsafe global = unsafePerformIO (newIORef True) main = do x - readIORef global print x writeIORef global False x - readIORef global print x ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell vs OCaml
I particularly like OCaml's provision of subtyping. As a member of the ML family, it's module system is also quite formidable. Of course the imperative constructs are also pretty convenient when you just want to be quirky. But I miss the monad do-notation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Function application like a Unix pipe
I offer a simpler, more direct, and pre-existing correspondence between a functional programming construct and unix pipes: http://www.vex.net/~trebla/weblog/pointfree.html Scherrer, Chad [EMAIL PROTECTED] writes: I'm still trying to settle on a feel for good programming style in Haskell. One thing I've been making some use of lately is (\|) = flip ($) infixl 0 \| Then expressions like f4 $ f3 $ f2 $ f1 $ x become x \| f1 \| f2 \| f3 \| f4 I've seen something like this on haWiki using (#), but I prefer this notation because it looks like a Unix pipe, which is exactly how it's used. [...] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question on Haskell type
Huong Nguyen [EMAIL PROTECTED] writes:
newtype Parser a = Parser(String - [(a, String)])
[...]
parse :: Parser a - String - [(a, String)]
parse p cs = p cs
\end{code}
Try this instead:
parse (Parser p) cs = p cs
(You forgot to deconstruct! :) )
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Re: [Haskell-cafe] a new Monad
Malcolm Wallace [EMAIL PROTECTED] writes: Microsoft has announced the following: Developers can also expect a new scripting language for management applications, called Monad. If we embedded the Monad language, as a DSL, into Haskell using a Haskell monad, would we get to call it the Monad monad? :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Best way to build strings?
Andy Gimblett [EMAIL PROTECTED] writes: show (External p q) = ( ++ show p ++ [] ++ show q ++ ) but to me the extensive use of ++ is not particularly readable. [...] return (%s [] %s) % (self.p, self.q) which to me seems clearer, or at least easier to work out roughly what the string's going to look like. I wish to toss out a new thought. To that end let me blow up the example to underline a scalability issue: A. q ++ ++ a ++ ++ z ++ [ ++ m ++ - ++ k ++ | ++ p ++ | ++ g ++ - ++ c ++ ] ++ h ++ ++ b ++ ++ f ++ ++ i B. printf %s %s %s [%s - %s |%s| %s - %s] %s %s %s %s q a z m k p g c h b f i B looks clearer because without parsing you can see that the output will contain a |blah| between two blah-blah's inside square brackets, etc. A looks clearer because without counting you can see that p is the thing that will go into |blah|, the first blah-blah will be m-k, etc. The best of both worlds may be something like the notation in the HOL theorem prover: ``^q ^a ^z [^m - ^k |^p| ^g - ^c] ^h ^b ^f ^i`` Do you agree that this is much better? Could someone implement something like this in GHC please? :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Going nuts
Alexandre Weffort Thenorio [EMAIL PROTECTED] writes:
outputLine keyno key orgFile = do
part1 - getLeft keyno orgFile
part2 - getRight keyno orgFile
total - part1 ++ (strUpper key) ++ part2 ++ \n
newHexFile - openFileEx newfile (BinaryMode WriteMode)
hPutStrLn newHexFile (orgFile!!0 ++ \n ++ total ++ unlines (drop 2
orgFile))
outputLine keyno key orgFile = do
let part1 = getLeft keyno orgFile
let part2 = getRight keyno orgFile
let total = part1 ++ (strUpper key) ++ part2 ++ \n
newHexFile - openFileEx newfile (BinaryMode WriteMode)
hPutStrLn newHexFile (orgFile!!0 ++ \n ++ total ++ unlines (drop 2
orgFile))
let ... = instead of - because getLeft et al. aren't IO commands.
(Why type error rather than syntax error then? Because getLeft returns
a list, and list is a monad too. E.g.,
do { part1 - getLeft keyno orgFile; return part1 }
= [ part1 | part1 - getLeft keyno orgFile ]
so part1 is inferred to be a Char.)
There will be another problem. The type of orgFile is expected to be
String here, but the callsite gives it lines(hexFile) of type [String].
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[Haskell-cafe] Re: [Haskell] Y in haskell?
Bernard Pope [EMAIL PROTECTED] writes: I also meant to add that I think these solutions are not what Lloyd is after, because they rely on recursive equations, which I believe was avoided in Lloyd's SML code. Those recursive equations are avoided in SML because SML is eager - y f = f (y f) never terminates - and an SML programmer has to thunk by hand; the obfuscation shows. In Haskell there is no such need. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: re-opening a closed stdin?
Simon Marlow [EMAIL PROTECTED] writes: One reason, I think, is lazy I/O(*). It's to stop you accidentally [...] (*) kill it! die! die! So you want main = readFile /dev/zero return () to terminate violantly rather than terminate peacefully? :) Seriously, I understand that the IO monad is the most intuitive and robust when it is strict. But sometimes it is also nice if one could map a file to a lazy string. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Optimisation and unsafePerformIO
David Sabel [EMAIL PROTECTED] writes:
{-# NOINLINE b #-}
b x = if even x then unsafePerformIO getChar else bot
bot = bot
main = do
putChar (b 4)
putChar (b 6)
I am not a compiler implementer (or lawyer, for that matter :)
But I propose this guess. First, both even 4 and even 6 get
constant-folded to True; so b 4 and b 6 both become unsafePerformIO
getChar. Then there is a common subexpression elimination.
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Re: Docs missing?
The doc RPM package for Red Hat 7.3 suffers the same problem as the SuSE one. Could someone please give a hand-holding guide so that we can fix it ourselves? Please? Please? ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Double your monad in 30 days
Ashley Yakeley [EMAIL PROTECTED] writes: Prelude 0 * 2 0 We can generalize this. import Monad double_your_monad_in_30_days :: (MonadPlus M) = M a double_your_monad_in_30_days = mzero `mplus` mzero ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Congrats to Mandrake
[EMAIL PROTECTED] (Ketil Z. Malde) writes: I'm not sure what the point would be, if they are in the distributions anyway? Isn't it better to install them by apt-get or up2date or whatever? (In fact, I had almost thought manually downloading packages a thing of the past, but then the IT department insisted on Red Hat) If I, a developer, wrote a program, and some distributions included it (while others didn't), I would want to acknowledge and thank them in public. If I, a potential customer choosing among Linux distributions, liked to use a program, and I saw on the program home page that so-and-so distributions included the program, I would arrange my preference accordingly. If I, an absent-minded user, heard about some great program from my friends, and saw on the program home page which distributions included it, I might realize Gosh, *my* distribution CD already has it! and save some download and install effort. Remember, I am absent-minded, so I wouldn't know what's on my CD unless someone stick it on my face. :) I probably don't need hyperlinks; I just need to know if my distribution includes it or not. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: i368-pc-os2-emx port
Simon Marlow [EMAIL PROTECTED] writes: On the other hand, you'd need something like mingw for OS/2 - does such a beast exist? The substring emx refers to an OS/2 version of gcc and libraries that make OS/2 look really like Unix from the programmer's point of view. (It probably even pre-dates cygwin.) I have two encouraging stories to tell. First, for a school assignment, I wrote an OpenGL program on emx, then ported it to school's Solaris+Mesa ten minutes before the deadline --- where porting meant removing #include os2.h. Second, I wrote a program that used tcpip sockets on Unix, then merely recompiled it on emx to use it on OS/2 as well. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Instances of IArray
I had looked into the source code of IArray a bit. Like you have found, (!) is not a class method. In fact, none of the claimed class methods in the doc is a class method, and no class method of IArray is documented. Instead, IArray class methods seem to pertain to implementation details (I mean, unsafeAt!), and user operations such as (!) are constrained polymorphic functions, e.g., (!) :: (IArray a e, Ix ix) = a ix e - ix - e From the user's point of view, there is little difference between a class method and a class-constrained polymorphic function when the provided instances are abstract types. So the documentation is telling a white lie. Of course, you see the difference when you roll your own IArray instance. I think the white lie is a balance between exposition simplicity and implementation tuning. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: State Transformer
testfunc = do
r - newSTRef ('x',0)
foo r
bar r
(c,n) - readSTRef r
return n
Jorge Adriano [EMAIL PROTECTED] writes:
Yeap, I could do it like this myself :)
The whole problem is with passing the 'r' as a parameter, which is precisly
what I'm trying to avoid.
I agree with you. My work-around is then to define foo and bar locally
to testfunc, in the scope of r:
testfunc = do
r - newSTRef ('x',0)
let foo = do
(c,n) - readSTRef r
writeSTRef r ('a', n+1)
bar = do
(c,n) - readSTRef r
writeSTRef r (c,n+2)
foo
bar
(c,n) - readSTRef r
return n
But if this looks like unsatisfactory (it does to me, too), perhaps
you have to go back to DIY monads.
DIY monads are good when: you fix the state variables, you don't want
to mention them in subprogram parameters.
The ST monad is good when: you create more state variables on the fly,
you use mutable arrays, you don't want to write your own monad and
put/get commands.
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Re: State Transformer
Theodore Norvell [EMAIL PROTECTED] writes: Jorge's question raised a question in my mind. The IOExts module has many of the same features as the ST module, why are there two ways to do the same thing? Is the ST module only there for legacy purposes? My user view is that I appreciate the presence of both. When I write a pure function that can be implemented efficiently in imperative programming, I want the ST monad to support mutable variables. E.g., to implement f n returns a list of all primes between 2 and n, I want to use a mutable array and hide it. When I write an I/O-bound routine that can be implemented conveniently with state variables, I want the IO monad to support mutable variables. Now my grief is that I cannot write a subprogram with state variables and have it reused in ST and IO. Fortunately I can write a subprogram with mutable arrays and have it reused in ST and IO, so I can write sort a given array; but I cannot write increment a given integer variable. Of course, you can tell me to use mutable arrays of length 1 to simulate mutable variables. Fine! ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: ghc image size
Mike Gunter [EMAIL PROTECTED] writes: Why is executable size a barrier? 1.64 megabytes (that's the size of the executable I built with GHC most recently) of disk space costs less than half a cent. I don't like this argument. Can I go to a computer store, pay a cent, and get a hard disk with space 1.64 megabytes or more? Until then, I can't believe that 1.64 megabytes of disk space costs less than half a cent. When a compiler does not perform as good as other compilers (e.g., in terms of generated code size), it is important to ask: Why does it happen? Is there anything we can do to improve it? Being critical is the first step towards progress. (Of course these questions should be asked in a constructive rather than whining way.) Why would anyone optimize code for time --- a second of electricity and labour cost less than a cent... ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Working character by character in Haskell
Andre W B Furtado [EMAIL PROTECTED] writes: [...] For example, suppose function doSomeStuffWith returns its own parameter. Using a 1.5MB file in this case, the Haskell program ends in almost 5 seconds, while the C program ends in less than 0.5 seconds... Is my Haskell program too bad implemented? (I'm using GHC 4.08-1 under a Windows 98 platform.) I indeed think that your Haskell program is inefficient and is not a translation of your C program. It reverses a huge string, which takes not only execution time but also garbage collection time and memory, which entails even more time for OS overhead and swapping out other things in the physical memory. (Many programmers complain my linear-time algorithm takes just 1 second on a 100M list, so why is it taking 5 seconds on a 200M list? They forget that because of issues such as cache locality and virtual memory, their computers do not scale.) I am wondering that if doSomeStuffWith is pure-functional, why are you writing and using copyFile instead of just using map? I mean: main :: IO () main = do bmFile - openFileEx in.txt (BinaryMode ReadMode) bmString - hGetContents bmFile writeFile out.txt (map doSomestuffWith bmString) hClose bmFile Because both hGetContents and map are lazy and use O(1) memory, this program behaves exactly as your C program plus occasional garbage collections that don't hurt too much. In partcular, the OS will see no difference (although the cache does). ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Student Programming Projects
Consider goal-directed theorem prover (or proof checker). Two existing samples are in Lawrence Paulson's ML for the Working Programmer, and yours truly's http://www.cs.utoronto.ca/~trebla/fp/prover/index.html The advantage of mine is it illustrates monads. The advantage of Paulson's is it doesn't scare people with monads :) and it includes a substitution library (yet another monad, but shhh) for use in first-order logic. ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [ghc5] getDirectoryContents: trouble on Linux/NFS
Volker Stolz [EMAIL PROTECTED] writes: ghc[i] was built from source. The Solaris binary provided on the GHC- download page works fine. I'll investigate FreeBSD as soon as I get home. Feel free to ask for more details regarding machine setup/compilation. I guess a standard question would be: when you built from source, did you build twice (once from 4.08.x and then once from 5.00 itself)? Hmm wait a second, getDirectoryContents fails, so there is no way you can build for the second time. :) ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
ghc/rts/parallel/HLComms.c
I try to build ghc 4.08.2 with parallelism turned on, i.e., GhcLibsWays=mp. At ghc/rts/parallel/HLComms.c line 550: GarbageCollect(GetRoots); it barfs because the new GarbageCollect() function expects a boolean parameter after the first parameter. (See ghc/rts/Storage.h for example.) Is this why I should use gum 4.06 instead? :) I would really like to have just one installation of ghc for every way I use it, not one for sequential execution and one for parallel execution. One installation is already faring at 90MB; having two such beasts on a shared disk at school is not nice... ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
Re: ghc/rts/parallel/HLComms.c
Albert Lai [EMAIL PROTECTED] writes: I try to build ghc 4.08.2 with parallelism turned on, i.e., GhcLibsWays=mp. At ghc/rts/parallel/HLComms.c line 550: GarbageCollect(GetRoots); it barfs because the new GarbageCollect() function expects a boolean parameter after the first parameter. (See ghc/rts/Storage.h for example.) For now, I will just steal the code from gum 4.06 and replace this (and all other occurences in HLComms.c) by GarbageCollect(GetRoots, rtsFalse); and hope it works. ___ Glasgow-haskell-bugs mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
Confused about enabling parallel ghc
I installed ghc 4.08.2 binaries for sparc solaris (and those for intel linux, too), then tried compiling a sample program (the fibonacci one in the user guide) with ghc -parallel. The compiler complained: Could not find interface file for `Prelude' in the directories ./*.hi /u/trebla/lib/ghc-4.08.2/imports/lang/*.mp_hi /u/trebla/lib/ghc-4.08.2/imports/concurrent/*.mp_hi /u/trebla/lib/ghc-4.08.2/imports/lang/*.mp_hi /u/trebla/lib/ghc-4.08.2/imports/concurrent/*.mp_hi /u/trebla/lib/ghc-4.08.2/imports/std/*.mp_hi (and similarly for Parallel.) Does it mean I must build ghc myself from source? The user guide promises a lot of binary bundles such as par. Ever since 4.0x, the ghc download page ceases to mention them. Where are they? ___ Glasgow-haskell-users mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
