- Haskell-Cafe - Haskell.org

Installing FreeGLUT
by Ronald Guida 02 Aug '07

02 Aug '07

Hi, I am trying to use freeglut with GHCi 6.6.1, and I'm stuck. I downloaded freeglut 2.4.0 and compiled it. I am on a Windows XP machine, and I found that freeglut compiled "out of the box" in MS Visual Studio.Net 2003. My difficulty is that GHCi is finding GLUT 2.2.1 and not freeglut. How do I get GHC and GHCi to recognize and use freeglut instead? Note: I'm using Windows XP; an upgrade to Linux is not an option. Thank you -- Ron

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Some Haskell platformish questions
by David Pollak 02 Aug '07

02 Aug '07

Howdy, I'm considering building a desktop app using Haskell. The primary target for the app is Windows, but if it runs on Linux and Mac (Intel and PPC), that'd be a bonus. I've got a bunch of questions that hopefully folks can answer. Well, before I start, you might well be asking "Gee David, you're a Scala kind of guy... I mean, you're the primary contributor to lift... why not use Scala?" Well, I'm looking to build something that compiles down to native code and has a smaller download (and runtime) footprint than the JVM offers. I've come to love functional programming and am taking the current side project as an opportunity to learn Haskell. So... on to the questions: - Can GHC generate stand-alone executables with all the dependencies linked in such that I can distribute the single file without worrying about including a bunch of DLLs/SOs? The answer seems to be yes, but I wanted to confirm. - How much of a distribution footprint is the Haskell runtime? If I have a "Hello World" app, roughly how big will the EXE be (if one includes the JRE in the runtime, a Java/Scala program has a minimum footprint of 20M... that's big.) - Same goes for the runtime... I've looked at the stats on the Language Shootout home page and these look encouraging, but I wanted to see if the reasonable footprint is a reality. - How real/solid/stable is the wxHaskell widgets package? Is it being well maintained? Is there (okay... this is pie in the sky) an GUI Builder for it? - How are the Windows/COM bindings in Haskell... would it be possible to, for example, embed an IE Browser COM control in a a wxHaskell window? - I found a package to do HTTP requests in Haskell but it does not seem to support HTTPS. Is there an HTTPS client package for Haskell? - How are Strings internally represented? Are they single byte or multi-byte characters? How easy it is to translate to/from internal representation to UTF-8? - How's the XML support? Will the XML parser handle non-Latin characters and properly encode stuff? Does XML get parsed down into easily mappable/filterable collections? - Is there support for SHA256 (I saw an SSLeay package which had support for a lot of stuff, but not SHA256)? - I understand that Haskell has "a better approach" to parallelizing tasks, but I have not seen much about the actual manifestation of this... would someone be so kind as to give me a pointer? - On a related note, I have become a fan (via Scala) of Erlang-style Actors and asynchronous message passing. Are there any similar packages for Haskell? - I tend to do most of my coding in either Emacs or Eclipse... how's the Haskell support in either? Is there a preferred editor (I don't mean to start any wars here... :-) - Are there any production Haskell-based desktop apps of note? Anyway... sorry for the long list of questions. I look forward to hearing from you all and learning more about Haskell. Thanks, David -- lift, the fast, powerful, easy web framework http://liftweb.net

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Cartesian product of a Set
by Andy Gimblett 02 Aug '07

02 Aug '07

Hi all, Is this a reasonable way to compute the cartesian product of a Set? > cartesian :: Ord a => S.Set a -> S.Set (a,a) > cartesian x = S.fromList [(i,j) | i <- xs, j <- xs] > where xs = S.toList x It's a fairly "obvious" way to do it, but I wondered if there were any hidden gotchas. I'm particularly concerned by toList (O(n)) fromList (O(n log n)) - but for other reasons I'd really like to be using Set rather than List for this (I think). Many thanks for any thoughts, -Andy -- Andy Gimblett Computer Science Department University of Wales Swansea http://www.cs.swan.ac.uk/~csandy/

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monads and groups -- instead of loops
by Greg Meredith 02 Aug '07

02 Aug '07

Haskellians, Though the actual metaphor in the monads-via-loops doesn't seem to fly with this audience, i like the spirit of the communication and the implicit challenge: find a pithy slogan that -- for a particular audience, like imperative programmers -- serves to uncover the essence of the notion. i can't really address that audience as my first real exposure to programming was scheme and i moved into concurrency and reflection after that and only ever used imperative languages as means to an end. That said, i think i found another metaphor that summarizes the notion for me. In the same way that the group axioms organize notions of symmetry, including addition, multiplication, reflections, translations, rotations, ... the monad(ic axioms) organize(s) notions of snapshot (return) and update (bind), including state, i/o, control, .... In short group : symmetry :: monad : update Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Re: monads and groups -- instead of loops
by Greg Meredith 02 Aug '07

02 Aug '07

Arie, Thanks for your thoughtful reply. Comments in-lined. Best wishes, --greg Date: Thu, 2 Aug 2007 03:06:51 +0200 (CEST) > From: "Arie Peterson" <ariep(a)xs4all.nl> > Subject: Re: [Haskell-cafe] Re: monads and groups -- instead of loops > To: haskell-cafe(a)haskell.org > Message-ID: < 5043.213.84.177.94.1186016811.squirrel(a)webmail.xs4all.nl> > Content-Type: text/plain;charset=iso-8859-1 > > Math alert: mild category theory. > > Greg Meredith wrote: > > > But, along these lines i have been wondering for a while... the monad > laws > > present an alternative categorification of monoid. At least it's > > alternative to monoidoid. > > I wouldn't call monads categorifications of monoids, strictly speaking. > A monad is a monoid object in a category of endofunctors (which is a > monoidal category under composition). Sorry, i was being as fast and loose with the term as the rest of the communities concerned with 'categorification' seem to be. What do you mean by a 'monoidoid'? I only know it as a perverse synonym of > 'category' :-). Indeed. > In the spirit of this thought, does anyone know of an > > expansion of the monad axioms to include an inverse action? Here, i am > > following an analogy > > > > monoidoid : monad :: groupoid : ??? > > First of all, I don't actually know the answer. > > The canonical option would be a group object in the endofunctor category > (let's call the latter C). This does not make sense, however: in order to > formulate the axiom for the inverse, we would need the monoidal structure > of C (composition of functors) to behave more like a categorical product > (to wit, it should have diagonal morphisms diag :: m a -> m (m a) ). It seems to me that there are two basic possibilities, here. One is that the ambient categories over which one formulates computational monads are almost always some type of Linear-Cartesian situation. So, you can possibly exploit the additional structure there. That's certainly been the general flavor of the situation that motivates me. Otherwise, you can go the route of trying to excavate structure that might give meaningful interpretations. This has appeal in that it is more general and might actually uncover something, but as you observe it's not immediate. i haven't wrestled with the idea in anger, yet, because i thought it such an obvious thing to try that someone would have already done the work and was hoping just to get a reference. Your note suggests that it might be worth digging a little. i wonder... does a Hopf algebra-like structure do the job? Maybe there is a way to get it to work, though. After all, what we (in FP) > call a commutative monad, is not commutative in the usual mathematical > sense (again, C does not have enough structure to even talk about > commutativity). > > > > My intuition tells me this could be quite generally useful to computing > in > > situation where boxing and updating have natural (or yet to be > discovered) > > candidates for undo operations. i'm given to understand reversible > > computing > > might be a good thing to be thinking about if QC ever gets real... ;-) > > If this structure is to be grouplike, the inverse of an action should be > not only a post-inverse, but also a pre-inverse. Is that would you have in > mind? > > > (If I'm not making sense, please shout (or ignore ;-) ).) > > > Greetings, > > Arie > -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com

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Parser inversions [Was: CC-delcont-0.1...]
by oleg＠pobox.com 02 Aug '07

02 Aug '07

Dan Doel wrote about `inverting' a parser -- first, a pure parser consuming a string and later a parser written in a monadic style and consuming a monadic list: data MList' m a = MNil | MCons a (MList m a) type MList m a = m (MList' m a) The second attempt proved fully successful: > So, success! Tokens are printed out as soon as the lexer is able to recognize > them, properly interleaved with other IO side effects, and resuming from an > intermediate parse does not cause duplication of output. Indeed, one may say `inverting' a parser is differentiating it. That is, we wish parse incrementally, to observe semantic actions as they are being performed. If the parser is a pure function then the only possible observation is that of its result given a specific input. To differentiate such a parser we feed it progressively longer inputs and observe differences in the output, the stream of parsed tokens. The first attempt followed this `classical calculus' recipe and wasn't fully successful, even though the parser used in that attempt was a good, _continuous_ parser. The parser for conventional arithmetic expressions such as "1+2" can generate parsed token 1 after it saw the first two characters, no matter what follows further in the input stream. Giving longer input to the parser will _progressively_ cause longer output. Alas, there are _discontinuous_ parsers, such XML DOM parsers. Let's consider the input <root><a>text</a><b/></root> Until a pure parser sees </root> (that is, the entire input document) it produces nothing at all (except errors that the XML document is ill-formed). Indeed, ill-formed documents have no Infoset (no `meaning') and hence DOM parsers must not return any parsed data for incomplete documents. As in calculus, differentiating such discontinuous functions is not helpful. Now, if the parser is monadic then effects and their sequence become observable too. We can truly observe the sequence of executed semantic actions. These actions are executed incrementally, as the parser recognized a part of input to which a semantic action applies. Delimited continuations, as a `scriptable debugger', let us observe the execution of the parser step by step. One way to differentiate is to embed `breakpoints' in the input stream, in the get-char-like routine the parser uses to obtain next piece of input. Such routines are typically under user control, who can embed breakpoints, aka shift. The MList data structure is a clever way to `hide' get-char routines out of sight. Another way to observe the sequence of actions is to embed breakpoints into semantic actions themselves. In many parsers, the grammar with semantic actions is provided by users and thus again under their control. This approach can `invert' a DOM XML parser into a streaming, pull-like SAX XML parser, provided the DOM parser exports suitable callbacks: http://okmij.org/ftp/Scheme/xml.html#SSAX-pull > So, that wasn't really that hard to hack up. However, I should mention > that it wasn't trivial, either. When converting list functions to > MList functions, you have to be very careful not to perform side > effects twice. Indeed: the major complexity of LogicT was making sure effects are not done twice. > ... many recursive data structures can be expressed as the fixed-point > of shape functors. The kicker is, you can get the monadic version for > free. Indeed, open recursion at the term level lets us transparently interpose, for example, memoization. The benefits of coding recursive data types with explicit open recursion have been described in Two-Level Types and Parameterized Modules Time Sheard and Emir Pasalic JFP v14 (5):547-587, Sept. 2004 http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps In particular, please see the Section 8, History, near the end of the paper. Thank you indeed for packaging CC monad and LogicT!

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Backpatching
by Thomas Conway 02 Aug '07

02 Aug '07

Hi All, One of the things I've been working on lately is some ASN.1 stuff.One of the first things I wrote in Haskell was an ASN.1 parser. It only worked for a subset, and I'm revisiting it to make it handle a larger subset. One of the things that gets messy is that in lots of places you can put either a thing or a reference to a thing (i.e. the name of a thing defined elsewhere). For example, consider the production: NamedNumber ::= identifier "(" SignedNumber ")" | identifier "(" DefinedValue ")" If we ignore the second alternative, the natural translation into a Parsec parser would look like: namedNumber = do name <- identifier val <- parens signedNumber return (name, val) Now to handle the second alternative is easy enough: namedNumber = do name <- identifier val <- parens (fmap Left signedNumber <|> fmap Right definedValue) return (name, val) however because names can be used before they are defined the result typegoes from being type NamedNumber = (Name,Integer) to type NamedNumber = (Name,Either Integer Name) Nothing too terrible so far. The messiness comes in when you considerthe number of places that you have to replace a type 't' with (Either t Name). I'd really like to avoid having to do this. If I were using Prolog, I could finesse the problem by introducing afree variable and filling it in when I come across the definition[*]. Logic variable backpatching. :-) So one possibility would be to return a closure: ... return $ \bindings -> (name,resolve val bindings) resolve :: (Either t Name) -> Map Name t -> t or something like that. Then when you get to the end, you apply the bindings and voila, out pops the simple type. I'm not sure this will work quite as well as it sounds. This sounds like a common problem type. Is there a well known solution to this sort of problem? cheers, Tom [*] And then at the end use var/1 to look for undefined names. Urk. Actually, if I were using Prolog in the way most Prolog programmers use it, I wouldn't be thinking about the types. -- Dr Thomas Conway drtomc(a)gmail.com Silence is the perfectest herald of joy: I were but little happy, if I could say how much.

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How can I optimize pattern matching on views?
by Ryan Ingram 02 Aug '07

02 Aug '07

Background: I participated in this year's ICFP programming contest<http://www.icfpcontest.org>and our team did quite well, coming in 37th. Our simulator (in somewhat naive C++ with a good algorithm) took about 45 seconds to run the original problem, and afterwards one of my coworkers took the same algorithm and optimized it to run in about 6-10 seconds. The rest of the email will have minor spoilers, so skip it if you want to work on the problem yourself. I'm using that problem as a good testcase for learning to write high-performance Haskell. I'm using the same data structure and basic algorithm as the C++ version, but I'm having problems getting the performance where I think it should be. The relevant parts of the code: > import qualified Data.ByteString.Base as B > import qualified Data.ByteString.Char8 as BC > > type DNABlock = B.ByteString > type DNA = [DNABlock] I represent the DNA string as a simplified rope<http://en.wikipedia.org/wiki/Rope_(computer_science)>that supports fast reading from the front & prepending. To access this in a reasonable fashion, I used a view to let me treat the data as a string. Here's a sample bit of code using that view: > matchConsts :: Int -> DNA -> (Int, DNA) > matchConsts len dna | len `seq` dna `seq` False = undefined -- force strictness > matchConsts len dna = case dnaView dna of > ('F':_) -> matchConsts (len+1) (dropDna 1 dna) > ('C':_) -> matchConsts (len+1) (dropDna 1 dna) > ('P':_) -> matchConsts (len+1) (dropDna 1 dna) > ('I':'C':_) -> matchConsts (len+2) (dropDna 2 dna) > _ -> (len, dna) > dropDna :: Int -> DNA -> DNA > dropDna n dna | n `seq` dna `seq` False = undefined -- force strictness > dropDna _ [] = [] > dropDna 0 dna = d > dropDna n (d:ds) > | n >= BC.length d = dropDna (n - BC.length d) ds > | otherwise = B.unsafeDrop n d : ds > {-# INLINE dropDna #-} Profiling showed that almost all of my time & allocations were spent in my view function: > dnaView :: DNA -> String > -- dnaView d = foldr (++) [] (map BC.unpack d) > -- This was ridiculously slow for some reason that I don't entirely understand > dnaView [] = [] > dnaView (d:ds) > | BC.null d = dnaView ds > | otherwise = (w2c $ B.unsafeHead d) : dnaView (B.unsafeTail d : ds) > {-# INLINE dnaView #-} The question I have for you, the haskell-cafe-reading-audience, is how can I get GHC to do smart code-gen for this? I want "case dnaView dna of ..." to not allocate and instead fuse the list generation with the pattern-match. -- ryan

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Indentation woes
by anon 01 Aug '07

01 Aug '07

Greetings, I wish to be able to indent my code like so: > longFunctionName various and sundry arguments > | guard1 = body1 > | guard2 = body2 > | ... > where declarations That is, with guards and where clauses indented to the same level as the function name. This seems like a perfectly reasonable indentation style to me. It also happens to be the preferred style in Clean, another layout-sensitive functional language. I believe it is not uncommon in ML dialects as well. So why is it that I'm not allowed to use it in Haskell?

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Re: [Haskell] View patterns in GHC: Request for feedback
by Simon Marlow 01 Aug '07

01 Aug '07

Dan Licata wrote: > Simon PJ and I are implementing view patterns, a way of pattern matching > against abstract datatypes, in GHC. Our design is described here: > > http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns > > If you have any comments or suggestions about this design, we'd love to > hear them. You can respond to this list (and we can take it to > haskell-cafe if the thread gets long) or, if you prefer, directly to me. At the risk of being a spoil-sport, I have a somewhat negative take on view patterns. Not because I think they're particularly bad, but because I don't think they're significantly useful enough to warrant adding to the language, at least if we also have pattern guards. All of the examples on the wiki page can be written in the same number of lines, sometimes with fewer characters, using pattern guards or some other existing Haskell idiom (e.g. the bit parsing example is much more nicely expressed using a bit-parsing monad). I believe adding yet another idiom will be detrimental: too much choice is bad. To my eyes, mixing bound and binding occurrences of variables in patterns in an arbitrarily nested way is sure to lead to confusion. The existing idioms all have a one-level deep notion of bound/binding scope, and in order to get more nesting you have to start naming things: this is good, IMO. Not that I think we should omit a language feature because it *could* be used to write obfuscated code; no, in this case I think nesting more than one level deep will *always* lead to obfuscated code. The use of the right-arrow is confusing, especially on the left of a case alternative or in a lambda expression. The main argument in favour of view patterns is: > it's possible that people would start routinely hiding the data > representation and exporting view functions instead, which would be an > excellent thing. My impression is that most of the time a data structure has a clever internal representation that you don't want to expose anyway. This is supported by the data structures we have in the base package, all these are abstract: Data.Set, Data.IntSet, Data.Map, Data.IntMap Data.Graph, Data.Sequence, Data.Bytestring, Data.Array, Data.HashTable, Data.Ratio(Rational) and most export view-like things (e.g. Data.Set.toList). The modules I found that export non-abstract types that should really be abstract: Data.Complex, Data.Tree So I don't think there's an overwhelming amount of stuff that would change if we had view patterns. In GHC itself most of our data structures are already abstract too. The View class is nice, even with just pattern guards. I'd be in favour of making it standard and actively encouraging its use. Cheers, Simon

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Re[6]: [Haskell-cafe] i need wxHaskell compiled for ghc 6.6.1 on Windows
by Bulat Ziganshin 01 Aug '07

01 Aug '07

Hello shelarcy, Wednesday, August 1, 2007, 4:03:52 AM, you wrote: >> problems. the only question that remains - does this version supports >> unicode? > Yes. Current darcs repository version support only unicode > enabled version. great, it's all what i need. but i'm still curious about other features enabled when building this package. can i see config.gcc or build.cfg or setup.h or any other file that shows feature list? also it will be useful to include such file with the next builds of wxHaskell -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Newbie question about Haskell skills progress
by peterv 01 Aug '07

01 Aug '07

Having only a couple of days of practice programming Haskell (but having read lots of books and docs), I find myself writing very explicit low level code using inner "aux" functions (accumulators and loops). Then I force myself to revise the code, replacing these aux functions with suitable higher-order functions from the library. However, I would like to use these higher order functions right away, without using low-level aux constructs, which is most likely caused by my very long history of imperative programming. Is this the "normal" way of progressing in Haskell, or should I consider a different approach? Thanks, Peter

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infinite list of random elements
by Chad Scherrer 01 Aug '07

01 Aug '07

I'm trying to do something I thought would be pretty simple, but it's giving me trouble. Given a list, say [1,2,3], I'd like to be able to generate an infinite list of random elements from that list, in this case maybe [1,2,1,3,2,1,3,2,3,1,2,...]. I'm using IO for random purely due to laziness (my own, not Haskell's). I was thinking the best way to do this might be to first write this function: randomElts :: [a] -> [IO a] randomElts [] = [] randomElts [x] = repeat (return x) randomElts xs = repeat r where bds = (1, length xs) xArr = listArray bds xs r = do i <- randomRIO bds return (xArr ! i) Then I should be able to do this in ghci: > sequence . take 5 $ randomElts [1,2,3] [*** Exception: stack overflow Any idea what's going on? I thought laziness (Haskell's, not my own) would save me on this one. Thanks! Chad

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Operational Semantics of Haskell
by Lewis-Sandy, Darrell 01 Aug '07

01 Aug '07

Is there a good source for the operational semantics of Haskell? I am trying to squeeze the most efficiency out of a bit of code and am looking to remove unnecessary reductions.

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Fwd: Re: Re[4]: [Haskell-cafe] i need wxHaskell compiled for ghc 6.6.1 on Windows
by shelarcy 01 Aug '07

01 Aug '07

Oops, I made mistake to send this mail only for Bulat. ------- Forwarded message ------- From: shelarcy <shelarcy(a)gmail.com> To: "Bulat Ziganshin" <Bulat.Ziganshin(a)gmail.com> Subject: Re: Re[4]: [Haskell-cafe] i need wxHaskell compiled for ghc 6.6.1 on Windows Date: Wed, 01 Aug 2007 09:03:52 +0900 Hello Bulat, On Wed, 25 Jul 2007 22:18:58 +0900, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > Tuesday, July 24, 2007, 2:32:01 AM, you wrote: > >> So I put newer Windows binary on my project's file space. > >> http://sourceforge.net/project/showfiles.php?group_id=168626 > > thank you very much!!! now i'm really happy - it works without any > problems. the only question that remains - does this version supports > unicode? Yes. Current darcs repository version support only unicode enabled version. http://article.gmane.org/gmane.comp.lang.haskell.wxhaskell.general/198 So you can use UTF-8 string in your program. Best Regards, -- shelarcy <shelarcy hotmail.co.jp> http://page.freett.com/shelarcy/

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Newbie question about automatic memoization
by peterv 01 Aug '07

01 Aug '07

Does Haskell support any form of automatic memorization? For example, does the function iterate f x which expands to [x, f(x), f(f(x)), f(f(f(x))), … gets slower and slower each iteration, or can it take advantage of the fact that f is referentially transparent and hence can be "memoized / cached"? Thanks, Peter No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.10.25/926 - Release Date: 29/07/2007 23:14

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problem building lambdabot
by Michael Vanier 01 Aug '07

01 Aug '07

When I try to build lambdabot, I get this: Configuring lambdabot-4.0... configure: Dependency base-any: using base-2.1.1 configure: Dependency unix-any: using unix-2.1 configure: Dependency network-any: using network-2.0.1 configure: Dependency parsec-any: using parsec-2.0 configure: Dependency mtl-any: using mtl-1.0.1 configure: Dependency haskell-src-any: using haskell-src-1.0.1 configure: Dependency readline-any: using readline-1.0 configure: Dependency QuickCheck-any: using QuickCheck-1.0.1 Setup.hs: cannot satisfy dependency arrows-any I'm using ghc 6.6.1 compiled from source (by me) on Debian Linux. I thought that arrows were included with the ghc distribution. Does anyone know what's happening and how to fix it? TIA, Mike

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curious hxt error
by brad clawsie 01 Aug '07

01 Aug '07

i am having a problem with hxt, i was wondering if anyone here has experience with it. in particular, i find that the xread function chokes on xml files with xml declarations, and i am not sure why. consider this sample script: module Main where import Text.XML.HXT.Parser main = do xml <- getContents print $ head $ xread xml and this file content (test.xml): <?xml version="1.0"?> <foo><bar>BAR1</bar><bar>BAR2</bar></foo> running: $ runhaskell hxt.hs < test.xml NTree (XError 2 "\"string: \"<?xml version=\\\"1.0\\\"?>\\n<foo><bar>BAR1</ba...\"\" (line 1, column 6):\nunexpected xml\nexpecting legal XML name character\n") [] which clearly indicates a choke on the xml declaration but if i remove the xml declaration, i see no errors and i am able to get a valid data structure printing out any ideas? thanks brad

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problem implementing an EDSL in Haskell
by Daniil Elovkov 01 Aug '07

01 Aug '07

Hello folks Haskell is considered good for embedded DSLs. I'm trying to implement some simple EDSL in a typeful manner and having a problem with looking up variable values. I've got an Expression GADT, which admits variables. The problem is with writing compute function which will lookup variable values in a type-safe manner. The exp. data type is like this data Exp a where Const a :: Val a -> Exp a Var :: a -> String -> Exp a -- where the first comp. isn't used,only for type info. .... So, obviously, I have to perform lookups in some 'scope' to compute the expression. By scope I mean the list of (name,value) pairs. How do I represent the scope? Well, I haven't gone that far as to encode statically the information about the type of every variable in scope. Instead, I used existentials to hide their types and put'em all in a list. For that purpose I introduced pack/unpack. -- value with dynamic type annotation -- m here and below can be Val, Exp, etc. -- to represent Val Int, Exp Int, etc. data Type m = TInt (m Int) | TString (m String) | TDouble (m Double) class Typed a where typ :: m a -> Type m instance Typed Int where typ x = TInt x instance Typed String where typ x = TString x instance Typed Double where typ x = TDouble x data Opaque m = forall a. (Typed a) => Opaque (m a) -- extract to an annotated representation extract :: Opaque m -> Type m extract (Opaque a) = typ a How would you suggest, I write compute function? My try was compute :: Scope -> Exp t -> Val t compute scope (Const x) = x -- trivial compute scope (Var t name) = -- intereseting part let opq = lookup name scope val = case opq of Nothing -> error "not in scope" Just opq -> extract opq expType = aux t in case val `matches` expType of -- I'd like to make some 'matches' func. Nothing -> error "type error" -- which would either produce an error Just v -> v -- or return the value, based on run-time tags matches :: Typed m -> Typed m -> Maybe (m a) BUT of course this type is bad, there's no 'a' in the left side matches (TInt x) (TInt _) = Just x matches (TString x) (TString _) = Just x matches (TDouble x) (TDouble _) = Just x matches _ _ = Nothing So, clearly the problem is in that Type m has no evidence of a, which was its very purpose. Ok, so I made data FType m a where FInt :: m Int -> FType m Int FString :: m String -> FType m String FDouble :: m Double -> Aux m a class Typed a where typ :: m a -> Type m -- as before ftyp :: m a -> FType m a -- new one and again obvious instance instance Typed Int where ftyp x = FInt x ... And of course, I'd like to get that information somehow extract2 (Opaque a) = ftyp a I rewrote 'matches' accordingly but the problem is already in the type of extract2 its type Opaque m -> (forall a. (Typed a) => m a) is not good to ghc, less polymorphic than expected So, in principle it must be doable, since opaque data retains its dictionary, and by that I can get a dynamic tag, say FInt x, where x is proved to be Int. What would you suggest? Thank you

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Re: Haskell-Cafe Digest, Vol 47, Issue 237
by Chad Scherrer 01 Aug '07

01 Aug '07

Maybe I'm going about this the wrong way, but using the State monad for random numbers was always leaky for me. Any time I use it, I have to go through and strategically place "seq"s to counter the excessive laziness. IO and ST have worked well, and seem to be faster as well (though that's purely self-anecdotal evidence). I'm stuck in IO anyway for a lot of this (reading data from files to build my lookup tables), so ST didn't seem worth the extra trouble either. Admittedly it would have been good practice, though. What monad do most people use for random numbers? If performance is an issue, do you still use State? Generating an infinite list of randoms seems like a good approach if you just use them in one place, but beyond that you end up passing around the tail of the list at each step, so then you may as well wrap the generator in State, I think. Thanks, Chad > Chad Scherrer wrote: > > I prefer the purely functional approach as well, but I've > > been bitten several times by laziness causing space leaks in this > > context. I'm on a bit of a time crunch for this, so I avoided the > > risk. > > Well, space leaks won't magically disappear if you use IO a . > > Regards, > apfelmus

1 0

RLE in Haskell: why does the type variable get instantiated?
by Chris Eidhof 31 Jul '07

31 Jul '07

Hey Haskell-Cafe, I was trying out the code in Dons's article [1], and I noticed a weird thing when doing it in GHCi. When binding the function composition to a variable, the type suddenly changes. I'm not completely sure why this happens. Is this because GHCi is in a monad and wants to find an instance for the type variable? Here's my GHCi session: Prelude> :m +Control.Arrow Prelude Control.Arrow> :m + List Prelude Control.Arrow List> :t map (length &&& head) . group map (length &&& head) . group :: (Eq a) => [a] -> [(Int, a)] Prelude Control.Arrow List> let encode = map (length &&& head) . group Prelude Control.Arrow List> :t encode encode :: [Integer] -> [(Int, Integer)] Thanks, -chris [1]: http://cgi.cse.unsw.edu.au/~dons/blog/2007/07/31

5 4

Conditional compilation of Setup.hs
by Bayley, Alistair 31 Jul '07

31 Jul '07

I'd like to add a #ifdef to Takusen's Setup.hs, so that we can have a single source file that will compile with ghc-6.6 and ghc-6.6.1. With ghc-6.6 and Cabal-1.1.6.1 we use splitFileName and joinPaths from Distribution.Compat.FilePath. With ghc-6.6.1 (which includes Cabal-1.1.6.2) these have been moved to System.FilePath. I'd like to do something like the following: #ifdef __CABAL_VERSION__ > 117 import System.FilePath (splitFileName, combine) joinPaths = combine #else import Distribution.Compat.FilePath (splitFileName, joinPaths) #endif Is something like this possible with Cabal? It's either that, or make Takusen's install depend on filepath. I'm not sure which is the least desirable, but I'm open to suggestions. To be fair, we already require our ghc-6.6 users to upgrade Cabal from 1.1.6 to 1.1.6.1, so making them install filepath instead perhaps isn't so bad, and is no more effort. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

3 3

Constraing satisfaction
by drop669＠gmail.com 31 Jul '07

31 Jul '07

Hi. I have a set of problems in the field of constaint satisfaction and I'm looking for a tool for this. In simplest form, these task are like Sudoku puzzle solver, school schedule creator, etc. Prolog language comes to mind at first. But if to choose Haskell? Can be these tasks defined in Haskell with the same elegance as in Prolog, or I'll need some additional framework or library?

3 3

Latest AngloHaskell news and a request
by Philippa Cowderoy 31 Jul '07

31 Jul '07

Two pieces of news regarding AngloHaskell: 1) We've been offered WiFi access at Microsoft Research for any attendees who want it. We'll need a name, email address and company/institution affiliation where appropriate - see wiki for details. 2) We're being given lunch on Friday! Finally, a request: We're a little short on crashspace, and I suspect this is affecting who's able to show up. Could people who can only make it with crashspace (or who can only make one day without) add their names on the wiki? Is there anyone in the area able to offer people at least some space on a floor? Last year we had offers early, and I think this helped get plans moving. For those who could use the reminder, the wiki page is at http://www.haskell.org/haskellwiki/AngloHaskell Many thanks, and I hope to see you there! -- flippa(a)flippac.org Society does not owe people jobs. Society owes it to itself to find people jobs.

1 0

GSL special functions (Re: [Haskell] math library for Haskell)
by Alberto Ruiz 31 Jul '07

31 Jul '07

(moved to Haskell-cafe) On Wednesday 25 July 2007 12:52, Gregory Wright wrote: > On Jul 25, 2007, at 6:39 AM, Chris Kuklewicz wrote: > > Alberto Ruiz wrote: > >> I have included a binding to gsl_sf_gamma in the darcs repo of > >> GSLHaskell: > >> http://dis.um.es/~alberto/GSLHaskell/doc/GSL-Special.html#v%3Agamma > >> I will try to upload to hackage a recent version of the library in > >> a few days. > >> Alberto > > > > It occurs to me that generating the binding for all the GSL > > "Special" functions by hand may be labor intensive. Would it be > > reasonable to have a (perhaps customized) tool generate the > > wrapping code? Absolutely! I am busy working on my applications and rewriting the internal support for the linear algebra wrappers, so I usually add new functions only when I need them for my own work. But a GSL library without the simplest functions is not very useful... > I have some functions that make wrapping the GSL special functions > much simpler. I've used them > to wrap all of the Airy and Bessel functions. The process could be > automated, but there are a few > more cases than convenient. (For example, the computationally > expensive functions take an additional > argument specifying how much precision is desired.) It wouldn't be > too much work to automate the > most common cases (double arg, double return and double arg, error > struct return); perhaps doing the > remainder by hand is reasonable. That's also right, so I have written a (very crude and ad-hoc) tool to generate wrappers for the most common cases. The results look promising: http://perception.inf.um.es/~aruiz/darcs/HSSL/doc/html/GSL-Special.html They are available in the new version of the library. In principle it is already working but needs more testing. I plan to upload a version to hackage (the repo can be found in the GSLHaskell web page). A problem with automatic wrappers is that we should still add a small piece of useful haddock documentation and appropriate tests for each function. > I also have bindings for all of the ODE integration routines and all > of the numerical differentiation > routines. All are low level, just exposing the underlying GSL > routines without any "convenience wrapper". > I can make them available if anyone is interested. > > Best Wishes, > Greg Good! I'd like to include a high level interface to the ODE routines. Perhaps it could be based on your bindings... Best regards, Alberto

1 0

Questions about threads
by John Goerzen 31 Jul '07

31 Jul '07

Hi everyone, I have been confused by some things about threads for a long time. I'm hoping someone out there can help clear this up. I'll clean up and document on the wiki if we get conclusive answers. So it seems there are four scenarios for firing off threads: A) Threaded RTS, forkIO B) Threaded RTS, forkOS C) Non-threaded RTS, forkIO D) Non-threaded RTS, forkOS So the questions, for each of the four models, are: 1) What is the impact of firing off a thread to execute a pure (non-IO) computation under each model? Will multiple pure computations be allowed to run in parallel, or will only one run at a time? (While the computation may be outside the IO monad, of course at the end it will have to use IO to communicate the result back.) 2) What is the impact of IO under each model? Will GHC internally use select/poll/whatever? Or will each thread get a dedicated OS thread that uses synchronous I/O? 3) When signals are received, which thread receives them? 4) When forkProcess is executed, which thread(s) are duplicated to the forked process? 5) What does an FFI import "safe" mean under each model? 6) What does an FFI import "unsafe" mean under each model? 7) What is the expected future level of support for each model? This is of significant concern to me, as it appears that the threaded RTS is only supported on an extremely limited set of architectures (most programs won't even link on Debian's autobuilders if I use -threaded). Also I have heard comments that the non-threaded RTS may be dropped in the future. 8) What is the expected level of support for STM in combination with each threaded model? 9) How does par mix with each threaded model? Is it equivolent to forkOS or forkIO? Thanks, -- John

4 3

MAPI provider?
by John Goerzen 31 Jul '07

31 Jul '07

Hi, I apologize if this is off-topic or stupid. I'm not very familiar (at all) with Windows programming... I have a Haskell program that I want to get called when someone sends a message on Windows. This sending a message will happen from various programs that call MAPI. It doesn't seem very easy to plug into MAPI from Haskell as a MAPI *provider*, though if anybody has examples, that'd be great. Does anybody know of an MAPI provider that receives send requests, writes the data to a file, and then just invokes a program passing the file as a parameter? That would let me do all I need to plug in. Thanks, -- John

1 0

Re: [Haskell] View patterns in GHC: Request for feedback
by Dan Licata 30 Jul '07

30 Jul '07

[I think we should move the rest of this thread to haskell-cafe, since it's getting long. Note that there have already been some responses on both lists.] Hi Claus, On Jul25, Claus Reinke wrote: > >I think that the signature > > > >> type Typ > >> > >> unit :: Typ -> Maybe () > >> arrow :: Type -> Maybe (Typ,Typ) > > > >is *wrong* if what you really mean is > > > >> type Typ > >> > >> data TypView = Unit | Arrow Typ Typ > >> view :: Typ -> TypView > > different =/= wrong !-) No, of course not. All I meant to say is that sometimes you want a total view, and that a total view should be given a type that says as much. The latter says this better than the former. On the other hand, there are lots of circumstances in which you want a partial view, and I think the (... -> Maybe _) types for those are perfectly appropriate. > >That is, if what you mean is that every Typ is either Unit or an Arrow > >*and nothing else* then the latter signature should be preferred, as it > >makes this fact explicit in the type system. > > but that is not what you're saying there at all! you're saying that -within > view 'view' of Typ- Typ is mapped to either Unit or Arrow, if the mapping > is successfull. there can be other views of Typ, Right, but the only issue here is whether this particular view function is total or not. > and the types do not guarantee that 'view' itself is exhaustive over > Typ (there can be variants of Typ that 'view' fails to map to > TypView). > in the abstract deconstructor variant, this partiality is explicit in the > types, > in the concrete view type variant, it is hidden from the types, implicit in > the implementation of 'view'. But by this reasoning, every expression you write should have a Maybe type, since there's always the possibility of match failure or non-termination. In Haskell, these effects are always implicit in a type, so there's no need to add extra summands to account for them. If you think about the *values* of the returned type, then (1 + Typ * Typ) is what you want, not ((1 + 1) + (1 + Typ * Typ)). > btw, it might be useful to permit association of abstract types > with abstract deconstructors, so that an extended abstract type > (export) declaration somewhat like > > type Typ as unit -> () | arrow -> (Typ,Typ) > > tells us (and the compiler) that the abstract patterns in the size > function are exhaustive (or at least as exhaustive as clients of > the abstract type Typ are supposed to be). the proof obligation > would be on the exporter of the abstract type, and any pattern > match failures relating to this should be reported as view failures. > > doing so would declare a virtual view type, similar to the concrete > view types used in other examples, so there might be several 'as' > clauses for a single abstract type, declaring separate sets of > exhaustive abstract deconstructors. Setting aside the "transparent ordinary patterns" feature, I think this is exactly what you get above: - the data declaration for TypView corresponds to this "as" declaration - the view function of type Typ -> TypView can be analyzed for exhaustiveness using GHC's exhaustiveness checker - a run-time match failure of view will be reported as such So I don't think I understand what you're going for here. Could you explain? -Dan

3 5

Strange behavior of executeFile
by Krzysztof Kościuszkiewicz 30 Jul '07

30 Jul '07

Fellow Haskellers, I wrote a small script that intercepts arguments and exec's the pstops program. The intention was to center and scale pages in a document before processing it by psnup. So far so good, I've ended up with something like: > runPstops :: [Flag] -> IO () > runPstops flags = > do let args = mkArgs flags > when (isVerbose flags) $ do > hPutStrLn stderr $ "pstops " ++ unwords (map show args) > executeFile "pstops" True args Nothing > main = do (opts, _) <- getOptions =<< getArgs > runPstops opts This works for files, but "randomly" fails when stdin is connected to a pipe (pstops complains that it can't seek input). I've tested "raw" pstops with pipes, files and /dev/null and it never fails, so I guess there is something wrong with my code. Can anyone enlighten me in this matter? :) Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: kokr(a)jabberpl.org Phone IRL: +353851383329, Phone PL: +48783303040 "Simplicity is the ultimate sophistication" -- Leonardo da Vinci

4 4

Space usage and CSE in Haskell
by Melissa O'Neill 30 Jul '07

30 Jul '07

When advocating functional languages like Haskell, one of the claims I've tended to make is that referential transparency allows the language to be much more aggressive about things like common subexpression elimination (CSE) than traditional imperative languages (which need to worry about preserving proper side-effect sequencing). But a recent example has left me thinking that maybe I've gone too far in my claims. First, lets consider a simple consumer program, such as: > printEveryNth c l n = do print (c', x) > printEveryNth c' xs n > where c' = c+n > x:xs = drop (n-1) l Note that we can pass this function an infinite list, such as [1..], and it won't retain the whole list as it prints out every nth element of the list. Now let's consider two possible infinite lists we might pass to our consumer function. We'll use a list of primes (inspired by the recent discussion of primes, but you can ignore the exact function being computed). Here's the first version: > primes = 2 : [x | x <- [3,5..], all (\p -> x `mod` p > 0) > (factorsToTry x)] > where > factorsToTry x = takeWhile (\p -> p*p <= x) primes As you might expect, at the point where we print the nth prime from our infinite list, we will be retaining a list that requires O(n) space. But this simple modification allows us to use only O(sqrt(n)) space at the point we print the nth prime: > primes = > 2 : [x | x <- [3,5..], all (\p -> x `mod` p > 0) (factorsToTry x)] > where > slowerPrimes = > 2 : [x | x <- [3,5..], all (\p -> x `mod` p > 0) > (factorsToTry x)] > factorsToTry x = takeWhile (\p -> p*p <= x) slowerPrimes Notice the gigantic common subexpression -- both primes and slowerPrimes define exactly the same list, but at the point where we're examining the nth element of primes, we'll only have advanced to the sqrt(n)th element of slowerPrimes. Clearly, "simplifying" the second version of primes into the first by performing CSE actually makes the code much *worse*. This "CSE- makes-it-worse" property strikes me as "interesting". So, is it "interesting"...? Has anyone worked on characterizing CSE space leaks (and avoiding CSE in those cases)? FWIW, it looks like others have run into the same problem, since bug #947 in GHC (from October 2006) seems to be along similar lines. Melissa. P.S. These issues do make massive difference in practice. There is a huge difference between taking O(n) and O(sqrt(n)) space -- the difference between a couple of megabytes for the heap and tens or hundreds of megabytes.

7 12

Using GADTs
by Matthew Pocock 30 Jul '07

30 Jul '07

Hi, I'm trying to get to grips with GADTs, and my first attempt was to convert a simple logic language into negative normal form, while attempting to push the knowledge about what consitutes negative normal form into the types. My code is below. I'm not entirely happy with it, and would appreciate any feedback. The rules are that in nnf, only named concepts, the concept Top and the concept Bottom can be negated. So, I've added three NNFNegation_* constructors capturing these legal cases. Is there a way to use one constructor, that is allowed to 'range over' these three cases and none of the others? I've ended up producing two data types, one for the general form and one for the nnf. Actually, the constraint on what constitutes nnf is fairly obvious - no complex terms are negated. Is there a way to use just the one data type but to describe two levels of constraints - one for the general case, and one for the nnf case? Or is the whole point that you capture each set of constraints in a different data type? Thanks, Matthe data Named data Equal data Conjunction data Disjunction data Negation data Existential data Universal data Top data Bottom data Concept t where CNamed :: String -> Concept Named CEqual :: Concept a -> Concept b -> Concept Equal CConjunction :: Concept a -> Concept b -> Concept Conjunction CDisjunction :: Concept a -> Concept b -> Concept Disjunction CNegation :: Concept a -> Concept Negation CExistential :: Role Named -> Concept Existential CUniversal :: Role Named -> Concept Universal CTop :: Concept Top CBottom :: Concept Bottom data NNFConcept t where NNFCNamed :: String -> NNFConcept Named NNFCEqual :: NNFConcept a -> NNFConcept b -> NNFConcept Equal NNFCConjunction :: NNFConcept a -> NNFConcept b -> NNFConcept Conjunction NNFCDisjunction :: NNFConcept a -> NNFConcept b -> NNFConcept Disjunction NNFCExistential :: Role Named -> NNFConcept Existential NNFCUniversal :: Role Named -> NNFConcept Universal NNFCTop :: NNFConcept Top NNFCBottom :: NNFConcept Bottom NNFCNegation_N :: NNFConcept Named -> Concept Negation NNFCNegation_T :: NNFConcept Top -> Concept Negation NNFCNegation_B :: NNFConcept Bottom -> Concept Negation data Role t where RNamed :: String -> RNamed Named -- terms not prefixed with a negation are already in nnf nnf :: Concept t -> NNFConcept u nnf CNamed name = NNFCNamed name nnf CEqual lhs rhs = NNFConcept (nnf lhs) (nnf rhs) nnf CConjunction lhs rhs = NNFCConjunction (nnf lhs) (nnf rhs) nnf CDijunction lhs rhs = NNFCDisjunction (nnf lhs) (nnf rhs) nnf CExistential r c = NNFCExistential r (nnf c) nnf CUniversal r c = NNFCUniversal r (nnf c) -- if negated, we must look at the term and then do The Right Thing(tm) nnf CNegation (CNamed name) = NNFCNegation_N NNFCNamed name nnf CNegation (CEqual lhs rhs) = NNFCEqual (nnf $ CNegation lhs) (nnf $ CNegation rhs) nnf CNegation (CConjunction lhs rhs) = NNFCDisjunction (nnf $ CNegation lhs) (nnf $ CNegation rhs) nnf CNegation (CDisjunction lhs rhs) = NNFCConjunction (nnf $ CNegation lhs) (nnf $ CNegation rhs) nnf CNegation (CNegation c) = nnf c nnf CNegation (CExistential r c) = NNFCUniversal r (nnf $ CNegation c) nnf CNegation (CUniveral r c) = NNFCExistential r (nnf $ CNegation c) nnf CNegation CTop = NNFCNegation_T NNFCTop nnf CNegation CBottom = NNFCNegation_B NNFCBottom

2 2

ANN: Finance.Quote.Yahoo 0.1 on hackage
by brad clawsie 30 Jul '07

30 Jul '07

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… this is a simple module to get stock quote information from yahoo finance, considered alpha quality

4 6

Text.XHtml Documentation
by David F. Place 29 Jul '07

29 Jul '07

Hello: I was just looking into using Text.XHtml for a project. The Haddock documentation points to an introduction by Andy Gill, but the link is broken. Can anyone point me to the correct location of the introduction? Thanks. Cheers, David ___________________ (---o-------o-o-o---o-o-o----( David F. Place mailto:d@vidplace.com

2 1

problems installing Takusen-0.6
by Rahul Kapoor 29 Jul '07

29 Jul '07

I am having problems installing Takusen-0.6 (ghc 6.6.1 on FreeBSD) The configure and build works fine. running ./setup install fails with: Installing: /usr/local/lib/Takusen-0.6/ghc-6.6.1 & /usr/local/bin Takusen-0.6... setup: Error: Could not find module: Database.Oracle.Enumerator with any suffix: ["hi"] I only have sqlite3 installed (no Oracle or Postgres). I followed the usual steps for the install ghc --make -o setup Setup.hs ./setup configure ./setup build ./setup install Any Ideas? Rahul

2 1

Videos of Haskell talks
by dons＠cse.unsw.edu.au 29 Jul '07

29 Jul '07

A new wiki page collecting tutorials and presentations about Haskell, is on the wiki: http://haskell.org/haskellwiki/Video_presentations Highlights include: Transactional Memory for Concurrent Programming Parametric Polymorphism and the Girard-Reynolds Isomorphism Programming in the Age of Concurrency Nested Data Parallelism in Haskell If you know of other online video material, please add it! -- Don

1 0

Using Gtk2Hs version 0.9.12 on a PPC Mac
by Thorkil Naur 29 Jul '07

29 Jul '07

Hello, (From the archives:) > [Haskell] ANNOUNCE: Gtk2Hs version 0.9.12 released > Duncan Coutts duncan.coutts at worc.ox.ac.uk > Fri Jul 27 15:20:57 EDT 2007 > Gtk2Hs - A GUI Library for Haskell based on Gtk+ > Version 0.9.12 is now available from: > ... > Duncan > (on behalf of the Gtk2Hs team) Following the advice of nominolo_ from #haskell yesterday, I added -L/opt/local/lib to my ghc command and then the helloworld demo of Gtk2Hs worked. The problem seems to be the order in which the two library directories /usr/X11R6/lib and /opt/local/lib appear on the link command: For the GTK+ example that worked, /opt/local/lib comes first. For the Gtk2Hs demo that failed, /usr/X11R6/lib comes first. I compared these two directories and found these common files: X11 libXft.2.dylib libXft.a libXft.dylib libXrender.1.dylib libXrender.a libXrender.dylib libexpat.0.dylib libexpat.a libexpat.dylib libfontconfig.1.dylib libfontconfig.a libfontconfig.dylib libfreetype.6.dylib libfreetype.a libfreetype.dylib pkgconfig Since some of these (Xrender, fontconfig, and freetype) are actually used in GTK+ (and therefore also Gtk2Hs) applications, there is clearly a potential for conflict here. In any case, adding -L/opt/local/lib to the ghc command makes this library appear before /usr/X11R6/lib on the link command and this seems to solve the problem. Subsequently, I have tried a handful of the other demos, and as far as they didn't require something that wasn't available (glade, for example), they seemed to work. Thanks and best regards Thorkil

2 3

Frag
by Jon Harrop 28 Jul '07

28 Jul '07

I just stumbled upon this fast action 3D shooter written entirely in Haskell: http://haskell.org/haskellwiki/Frag After 15 minutes trying to build it I find that it segfaults. Can anyone else get this to work? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. The OCaml Journal http://www.ffconsultancy.com/products/ocaml_journal/?e

5 7

Handling custom types in Takusen
by Salvatore Insalaco 28 Jul '07

28 Jul '07

Disclaimer: I don't know Takusen very well, and I'm talking of Sqlite backend. I noticed that in Takusen there're just two instances to implement to make any Haskell type db-serializable: DBBind / SqliteBind for serialization and DBType for deserialization. I wanted to implement blob serialization for PackedStrings, but I noticed that both DBBind and DBType classes are not public. Defining custom serialization (not just Show / Read string serialization) is very useful in applicative code, and not just in Takusen backend code. There's any way to do it without modifying Takusen Sqlite backend? Salvatore Insalaco

4 8

Memory profiler
by Jon Harrop 28 Jul '07

28 Jul '07

Is there a memory profiler for Haskell? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

2 1

Avoiding boilerplate retrieving GetOpt cmd line args
by Dave Bayer 28 Jul '07

28 Jul '07

Ok, I'm writing a command line tool, using System.Console.GetOpt to handle command line arguments. My Flags structure so far is > data Flag > = Filter String > | DateFormat String > | DocStart String > | DocEnd String ... and I want to write accessor functions that return the strings if specified, otherwise returning a default. The best I've been able to do is this: > getFilter = getString f "Markdown.pl" > where f (Filter s) = Just s > f _ = Nothing > > getDateFormat = getString f "%B %e, %Y" > where f (DateFormat s) = Just s > f _ = Nothing > > getDocStart = getString f "^{-$" > where f (DocStart s) = Just s > f _ = Nothing > > getDocEnd = getString f "^-}$" > where f (DocEnd s) = Just s > f _ = Nothing using a generic accessor function `getString`. There are eight (and growing) needless lines here, where what I really want to do is to pass the constructors `Filter`, `DateFormat`, `DocStart`, or `DocEnd` to the function `getString`. ghci types each of these as `String -> Flag`, so one at least knows how to type such a `getString`, but using a constructor-passed-as-an-argument in a pattern match is of course a "Parse error in pattern". (I expected as much, but I had to try... `String -> Flag` is not enough information to make it clear we're passing a constructor, rather than some hairy arbitrary function, so such a pattern match would be undecidable in general.) So what's the right idiom for avoiding this boilerplate? Thanks, Dave

8 8

Re: Re: Avoiding boilerplate retrieving GetOpt
by Eric Kow 27 Jul '07

27 Jul '07

> For starters, you could squish these down into something like > > flagToString :: Flag -> Maybe String > > flagToString (Filter s) = Just s > > flagToString (DateFormat s) = Just s > > ... > > flagToString _ = Nothing What am I saying?! This wouldn't work! Sorry for the noise. -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

1 0

Ray tracer language comparison
by Jon Harrop 27 Jul '07

27 Jul '07

I've added four progressively optimized implementations of the Haskell ray tracer to the language comparison: http://www.ffconsultancy.com/languages/ray_tracer/ Only the first is lazy and I haven't mentioned them in the discussion yet. -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

1 0

Re: [Haskell-cafe] Re: [Haskell] View patterns in GHC: Request for feedback
by Conor McBride 27 Jul '07

27 Jul '07

Me: > > In the dependently typed setting, it's often the case that the > > "with-scrutinee" is an expression of interest precisely because it > > occurs > > in the *type* of the function being defined. Correspondingly, an > > Epigram implementation should (and the Agda 2 implementation now does) > > abstract occurrences of the expression from the type. Dan: > Oh, I see: you use 'with' as a heuristic for guessing the motive of the > inductive family elim. How do you pick which occurrences of the > with-scrutinee to refine, and which to leave as a reference to the > original variable? You don't always want to refine all of them, do you? There are two components to this process, and they're quite separable. Let's have an example (in fantasy dependent Haskell), for safe lookup. defined :: Key -> [(Key, Val)] -> Bool defined k [] = False defined k ((k', _) : kvs) = k == k' || defined k kvs data Check :: Bool -> * where OK :: Check True lookup :: (k :: Key; kvs :: [(Key, Val)]) -> Check (defined k kvs) -> Val lookup k [] !! -- !! refutes Check False; no rhs {-before-} lookup k ((k', v) : kvs) p with k == k' {-after-} lookup k ((k', v) : kvs) OK | True = v lookup k ((k', v) : kvs) p' | False = lookup k kvs p' Left-hand sides must refine a 'problem', initially lookup k kvs p where k :: Key; kvs :: [(Key, Value)]; p :: Check (defined k kvs) Now, {-before-} the with, we have patterns refining the problem lookup k ((k', v) : kvs) p where k, k' :: Key v :: Val kvs :: [(Key, Val)] p :: Check (k == k' || defined k kvs) The job of "with" is only to generate the problem which the lines in its block must refine. We introduce a new variable, abstracting all occurrences of the scrutinee. In this case, we get the new problem {-after-}. lookup k ((k', v) : kvs) p | b where k, k' :: Key v :: Val kvs :: [(Key, Val)] b :: Bool p :: Check (b || defined k kvs) All that's happened is the abstraction of (k == k'): no matching, no mucking about with eliminators and motives. Now, when it comes to checking the following lines, we're doing the same job to check dependent patterns (translating to dependent case analysis, with whatever machinery is necessary) refining the new problem. Now, once b is matched with True or False, the type of p computes to something useful. So there's no real guesswork here. Yes, it's true that the choice to abstract all occurrences of the scrutinee is arbitrary, but "all or nothing" are the only options which make sense without a more explicit mechanism to pick the occurrences you want. Such a mechanism is readily conceivable: at worst, you just introduce a helper function with an argument for the value of the scrutinee and write its type explicitly. I guess it's a bit weird having more structure to the left-hand side. The approach here is to allow the shifting of the problem, rather than to extend the language of patterns. It's a much better fit to our needs. Would it also suit Haskell? Cheers Conor

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Lazy in either argument?
by Dan Weston 27 Jul '07

27 Jul '07

I am trying to get my feet wet with Haskell threads with the following problem, inspired by a recent post (http://www.haskell.org/pipermail/haskell-cafe/2007-July/029408.html) saying that: > Since there's no way to have a function be lazy in both arguments, the > implicit convention is to make functions strict in the first arguments > and, if applicable, lazy in the last arguments. In other words, the > convention is > > True || _|_ = True but not _|_ || True = True > > 1 + _|_ = Succ _|_ but not _|_ + 1 = Succ _|_ > > Regards, > apfelmus Maybe not lazy in both arguments, but how about lazy in either argument? The idea is to fork a thread for each of the two functions, (||) and flip (||), pick the winner, then kill off both threads. I can wrap this up in a pure function using unsafePerformIO (given the proof obligation that the results of both threads will always be equal where both are defined). The code below seems to work, except for the following problems: 1) Commenting out the type annotation f :: Bool makes the program hang 2) If I replace f = f by f = undefined, I get an annoying print of "LazyOr: Prelude.undefined" before it returns the correct value. Does anyone know why the type annotation is needed in #1, and/or how to suppress the error message in #2? Dan Weston ----------------------------------------------------------- import Control.Monad(when) import Control.Concurrent(forkIO,killThread) import Control.Concurrent.Chan(newChan,readChan,writeChan,isEmptyChan) import Foreign(unsafePerformIO) f :: Bool f = f main = putStrLn . show $ lazyBinaryOp (||) f True lazyBinaryOp p x y = unsafePerformIO $ do c <- newChan p2 <- forkIO (lazyBinaryOpThread c p x y) p1 <- forkIO (lazyBinaryOpThread c p y x) z <- readChan c killThread p1 killThread p2 return z where lazyBinaryOpThread c p x y = do case (p x y) of True -> writeChan c True False -> writeChan c False

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"Identifier" generators with QuickCheck
by J. Pablo Fernández 27 Jul '07

27 Jul '07

Hello Haskellers, I want to make a QuickCheck generator that creates identifiers, basically [a-zA-Z] as the first character and then [a-zA-Z0-9-_] for a total of 63 characters. So, I've got up to: do s <- choose (1, 63 :: Int) elements validFirstChars where validFirstChars = ['a'..'z'] ++ ['A'..'Z'] validChars = validFirstChars ++ "_-" ++ ['0'..'9'] which of course only gives me one random character. I want both, the characters, and the length to be random. Thank you. -- J. Pablo Fernández <pupeno(a)pupeno.com> (http://pupeno.com)

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Order of evaluation
by Jon Harrop 27 Jul '07

27 Jul '07

If you have a boolean-or expression: a || b will "a" be evaluated before "b" in Haskell as it is in other languages? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

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advantages of using fix to define rcursive functions
by Harald ROTTER 27 Jul '07

27 Jul '07

Hi, I read about the usage of "fix" to define recursive functions. Although I think that I understood how to use "fix", I still wonder what the advantages of "fix" are (as compared to the "conventional" approach to define recursive functions). Any hints are appreciated. Thanks Harald. " Ce courriel et les documents qui y sont attaches peuvent contenir des informations confidentielles. Si vous n'etes pas le destinataire escompte, merci d'en informer l'expediteur immediatement et de detruire ce courriel ainsi que tous les documents attaches de votre systeme informatique. Toute divulgation, distribution ou copie du present courriel et des documents attaches sans autorisation prealable de son emetteur est interdite." " This e-mail and any attached documents may contain confidential or proprietary information. If you are not the intended recipient, please advise the sender immediately and delete this e-mail and all attached documents from your computer system. Any unauthorised disclosure, distribution or copying hereof is prohibited."

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ANN: Finance.Quote.Yahoo-0.2
by brad clawsie 27 Jul '07

27 Jul '07

i have released Finance.Quote.Yahoo 0.2 i have broken the 0.1 api, be careful if you use it i have added support for historical quotes which some people requested http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… i received useful input from dale jordan and aaron tomb on this list in particular

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Optimizing Haskell compilers
by Jon Harrop 27 Jul '07

27 Jul '07

I've heard that there are a plethora of Haskell compilers available. Which others give performance comparable to GHC? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

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Fixity of >>>
by dons＠cse.unsw.edu.au 26 Jul '07

26 Jul '07

There's a theory this should work: getContents >>= lines >>> map read >>> sum >>> print But unfortunately we have: `(>>=)' [infixl 1] `(>>>)' [infixr 1] Meaning we must write: getContents >>= (lines >>> map read >>> sum >>> print) Indeed, all Arrow ops are infixr. Are there any technical/compelling reasons for this? -- Don

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