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[Haskell Cafe] Parsec: buildExpressionParser and parens typecheck problem
by Paul Sujkov 11 Jul '09

11 Jul '09

Hi haskellers, I'm trying to use buildExpressionParser parser generator from ParsecExpr module of Parsec ( http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#ParsecExpr) It works well, except for the "parens" token parser ( http://legacy.cs.uu.nl/daan/download/parsec/parsec.html#parens) This code (sample from Expressions part of the manual) typechecks fine: expr :: Parser Integer expr = buildExpressionParser table factor <?> "expression" table = [[op "*" (*) AssocLeft, op "/" div AssocLeft] ,[op "+" (+) AssocLeft, op "-" (-) AssocLeft] ] where op s f assoc = Infix (do{ string s; return f}) assoc factor = do{ char '(' ; x <- expr ; char ')' ; return x } <|> number <?> "simple expression" but if I try to use parens: factor = parens expr <|> number <?> "simple expression" it fails to typecheck: Couldn't match expected type `GenTokenParser s u m' against inferred type `Parser Integer' In the first argument of `parens', namely `expr' In the first argument of `(<|>)', namely `parens expr' In the first argument of `(<?>)', namely `parens expr <|> number' Failed, modules loaded: none. the type of expr infers to GenParser Char () Integer, and the expected type for the parens is GenTokenParser s u m (however, manual introduces it with the type CharParser st a expected) It seems pretty weird for me, as there are numerous examples of using parens with the buildExpressionParser (e.g. http://blog.moertel.com/articles/2005/08/27/power-parsing-with-haskell-and-…) and nobody comments such an error. I know I'm missing something very simple here, maybe someone could help me with it? Thanks in advace I'm using GHC 6.10.1 and Parsec 3.0.0 -- Regards, Paul Sujkov

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Laziness enhances composability: an example
by Cristiano Paris 11 Jul '09

11 Jul '09

Hi, I'm wondering what a good example of why laziness enhances composability would be. I'm specifically looking for something that can't implemented in Python with iterators (at least not elegantly), but can actually be implemented in Haskell. Thanks, Cristiano

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first project. code reviews?
by Jared Jennings 10 Jul '09

10 Jul '09

I've written a parser for OFX 1.x (these are the files you can download from most banks detailing recent account activity). I'm not sure I've written it well. This is my first serious thing in Haskell. (Kudos to the writers of the guide for how to make a new Hackage project! It's brilliant.) http://patch-tag.com/r/ofx1/pullrepo

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Leaner Haskell.org frontpage
by Jeff Wheeler 10 Jul '09

10 Jul '09

On Thu, Jul 9, 2009 at 6:12 PM, Derek Elkins<derek.a.elkins(a)gmail.com> wrote: > I'm not a newbie and I don't use the front page terribly often, but I > do like most of the links that are on it. The Ruby page is certainly > prettier, but the layout of the Haskell page is fine in my opinion; > the difference is mainly eye-candy. On another topic, I know people > have expressed that they have liked the fact that the entire Haskell > site is a wiki; this expressing openness and community involvement. > I personally don't find the Haskell front page too cluttered and I > think most of issue in that vein could be resolved by simply making > sure the most important/newbie-oriented links are "above the fold" and > appropriately emphasized/categorized as is partially done already. I strongly feel that the homepage should be made more newbie friendly, and I think the Ruby page has done this well, disregarding the news section. I suspect most people who like the Ruby page see the "Ruby is..." section as especially effective at introducing the language, and the random snippet is a simple way to show off a bit of code before they dive into a tutorial. Furthermore, the "Download" link is useful, but since GHC can be complicated (and varies by platform), we probably want to include pretty well thought-out instructions behind the link if we include a similar feature. I also quite like the "Participate" box on the Ruby page, which is very inviting. Regarding the current Haskell homepage, I feel the events are given far too high a place on the homepage. Almost no newbies will be interested in these, and most experienced users will know of the events via the mailing lists. The headlines below that deserve significantly more attention, and perhaps should be updated with greater frequency (and dated, and have RSS). The updated package list is fine, I think. The navigation is a bit tricky, in my opinion. To a beginner (that doesn't know what GHC is), the two download links may be confusing, although I suspect most would correctly assume that "Download Haskell" was correct option. The "Find A Library" is a good link, but the Search that follows it is awkward. There are three large search choices for beginners: 1) the search at the top, which confusingly has two submit buttons (with ambiguous differences to a beginner); 2) the Search link near the top of the navigation (which links to an almost empty page that might as well be included at the link's location); and 3) the Search link underneath the About header, which doesn't seem to belong at all. Jeff Wheeler (Sorry, sent this to just Derek at first.)

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Re: [Haskell-cafe] Laziness enhances composability: an example
by Marcin Kosiba 10 Jul '09

10 Jul '09

On Thursday 09 July 2009, you wrote: > 2009/7/9 Marcin Kosiba <marcin.kosiba(a)gmail.com>: > > I thought I'll go the "smart" way and rely on the Python yield construct > > to do a CPS transformation of my code. > > While this worked to a certain extent, composability was a > > problem, because any sub-procedure which used yield (and thus was an > > iterator) required being called in a rather inelegant way. > > i confess to not understanding the inelegance. > can you provide an example where an iterator inside CPS requires > special support? Hi, To illustrate what I meant I'm attaching two examples. In example_1.py I've written code the way I think would be elegant (but it doesn't work). In example_2.py I've written code so that it works, but it isn't elegant. I know I'm abusing Python iterators here. Also, I'm not sure the way to compose iterators shown in example_2.py is the only option. Actually I'd love to see a better solution, because it would remove a lot of bloat from my code ;) Thanks! Marcin Kosiba

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Re: [Haskell-cafe] Leaner Haskell.org frontpage
by haskell＠kudling.de 10 Jul '09

10 Jul '09

> I've seen book providing a "chapters at a glance" part, just before the real table of content. Such an inverted pyramid is exactly the consequence Nielson draw from the "F shape pattern" (http://www.useit.com/alertbox/reading_pattern.html) And that's my critque: i don't see the most important things there are to be sayed about Haskell in the top left corner. > For the hompage we're talking about, glancing is even simpler since everything is on the same page and you can scroll it quite easily. I don't agree that "everything on one page" makes comprehension easier. > I'm not sure hiding a level of the hierarchy of information behind a few clicks make things easier. That depends on which task we are talking about: - getting an overview of all available information, or - finding exactly what you are looking for I think we should optimize for the latter, where "What is Haskell?" being the most improtant question.

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iteratee enumHandle
by Paolino 10 Jul '09

10 Jul '09

I'm testing iteratee. This is the possible bug I've found import Data.Iteratee.IO import Data.Iteratee.Base import Data.Iteratee.Char import System.IO import Control.Exception main = do h <- openFile "mamma23" ReadWriteMode hPutStrLn h "ciao" hSeek h AbsoluteSeek 0 l <- (enumHandle h stream2list :: IO (Iteratee IO String)) >>= run print $ assert (l == "ciao") () assertion failed This aside, I'd like to know if there is a way to use parsec library to make an Iteratee, so if it's possible to make parsec spit out a continuation on EOF, or maybe as in delcont language to capture it. thanks paolino

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c2hs confusion with a simple function
by Jeff Heard 09 Jul '09

09 Jul '09

The c2hs documentation at http://www.cse.unsw.edu.au/~chak/haskell/c2hs/docu/implementing.html#id3149… gives me an example to follow for this case: {#fun notebook_query_tab_label_packing as ^ `(NotebookClass nb, WidgetClass cld)' => {notebook `nb' , widget `cld' , alloca- `Bool' peekBool*, alloca- `Bool' peekBool*, alloca- `PackType' peekEnum*} -> `()'#} so I took the original C binding: void SHPGetInfo(SHPHandle, int*, int*, double*, double*) And wrote a c2hs binding: {#fun SHPGetInfo as getInfo { fromSHPHandle `SHPHandle' , alloca- `Int' , alloca- `Int' , alloca- `Double' , alloca- `Double' } -> `()' #} The error I get: Internal.chs:85:2: Couldn't match expected type `Int' against inferred type `(Ptr b -> IO c) -> IO c' Expected type: IO (Int, Int, Double, Double) Inferred type: IO ((Ptr b -> IO c) -> IO c, (Ptr b1 -> IO c1) -> IO c1, (Ptr b2 -> IO c2) -> IO c2, (Ptr b3 -> IO c3) -> IO c3) In the expression: alloca $ \ a2' -> alloca $ \ a3' -> alloca $ \ a4' -> ... In the expression: let a1' = fromSHPHandle a1 in alloca $ \ a2' -> alloca $ \ a3' -> alloca $ ... I've tried ending it with withCIntConv and withCFloatConv to no avail. The original c2hs file is attached. I'm sure there are many more problems, but this is the one I'm working on at the moemnt... -- Jeff

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[Haskell-cafe] ANN: hsparql, a SPARQL query generator/DSL and client
by Jeff Wheeler 09 Jul '09

09 Jul '09

I'm excited to announce the first version of hsparql. HSparql makes it easy to query SPARQL-compliant servers using a relatively intuitive DSL and very simple client. SPARQL is a SQL-ish query language for stores of RDF data. So, SPARQL lets you search the structured data in several big databases. I wrote it so that I could search DBpedia [1], a neat site that pulls structured data from Wikipedia's infoboxes, and also ties it to remote data services like MusicBrainz and US Census information. HSparql isn't tied to DBpedia in any way, though, and can search any server. The DSL looks something like this: > simple :: Query [Variable] > simple = do > resource <- prefix (iriRef "http://dbpedia.org/resource/") > dbpprop <- prefix (iriRef "http://dbpedia.org/property/") > foaf <- prefix (iriRef "http://xmlns.com/foaf/0.1/") > > x <- var > name <- var > page <- var > > triple x (dbpprop .:. "genre") (resource .:. "Web_browser") > > triple x (foaf .:. "name") name > triple x (foaf .:. "page") page > > return [name, page] It's currently available on Hackage [2], and at the moment the source is only available from the tarball hosted there. I'd greatly appreciate any feedback on this package. Thanks! Jeff Wheeler [1] http://dbpedia.org/ [2] http://hackage.haskell.org/package/hsparql

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Re: [Haskell-cafe] Leaner Haskell.org frontpage
by haskell＠kudling.de 09 Jul '09

09 Jul '09

> I find it very to the point and not overwhelming at all : it's easy to glance over it and find quickly what I want. Thanks for your feedback. Most people feel overwhelmed when confronted with more than 7+-2 items: http://www.smashingmagazine.com/2007/10/09/30-usability-issues-to-be-aware-… On the current Haskell frontpage there are over 60 links competing for attention. I am not sure whether we should design interfaces solely with few people having exceptional abilities in mind. This could be understood as a statement about who Haskell is made for in itself. Bye, Lenny

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