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Correct implementation of ListT besides the pipes package?
by Nicola Gigante 14 Nov '14

14 Nov '14

Hi! I understand the code on the wiki about what is wrong about the standard ListT and how to implement a correct version, but I’m wondering if there is already a package that provides such an implementation ready to use. I know the pipes package provides one, but since I don’t otherwise use pipes in my program I’d rather not depend on a big library like that only for a transformer. Is there any package that provides a correct ListT? Side question: is there a particular reason why Control.Monad.ListT doesn’t get fixed? Thank you, NIcola

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Presenting at Royal Holloway Colloquium
by Dominic Steinitz 14 Nov '14

14 Nov '14

All, I have been invited to give a TED style talk (20 mins) at the Royal Holloway Hewlett Packard Information Security Colloquium: https://www.royalholloway.ac.uk/isg/externalengagement/hpday.aspx. Now I could give an uncontroversial talk about Internet banking security using triple DES, role based access control, etc. but I am thinking about being controversial (I think that is in the spirit of TED). I’d like to say that the Information Security community is solving the wrong problems by e.g. performing security audits of code, developing tools for finding buffer overflows, etc. and what they should really be doing is encouraging development in languages that prevent this sort of behaviour. E.g. if openssl were written in Haskell, heartbleed (http://en.wikipedia.org/wiki/Heartbleed) would never have happened. What do people think about this? Are there other examples I can draw on? Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a)
by Evan Laforge 13 Nov '14

13 Nov '14

I've been using these functions lately: try :: Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a) try action alternative = maybe alternative (return . Just) =<< action tries :: Monad m => [m (Maybe a)] -> m (Maybe a) tries = foldr try (return Nothing) It's sort of like (<|>) on Maybe, or MonadPlus, but within a monad. It seems like the sort of thing that should be already available, but hoogle shows nothing. I think 'm' has to be a monad, and I can't figure out how to generalize the Maybe to MonadPlus or Alternative. It's sort of a mirror image to another function I use a lot: justm :: Monad m => m (Maybe a) -> (a -> m (Maybe b)) -> m (Maybe b) justm op1 op2 = maybe (return Nothing) op2 =<< op1 ... which is just MaybeT for when I can't be bothered to put runMaybeT and lifts and hoists on everything. So you could say 'try' is like MaybeT with the exceptional case reversed. Is 'try' just the instantiation of some standard typeclass, or is it its own thing?

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Mechanize in Haskell or Revive Shpider?
by Cody Goodman 13 Nov '14

13 Nov '14

What would be a good way to model mechanize in Haskell? For instance the python mechanize example is quite simple: def google_search(keyword): br = mechanize.Browser() br.open("https://www.google.com") br.select_form(name="gbqf") br["q"] = keyword response = br.submit() print html_to_text(response.read()) I was thinking I could just use a StateT monad transformer with the current pages html as a cursor. Then have something like: get "http://www.google.com" >>= (\c -> c $// form (Name "gbqf") >=> input (Name "q") &| submit . modifyVal "query" "Haskell") That's kinda horrible but it's a place to start. Or would I be better off trying to revive Shpider?

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Fwd: Generating valid html
by Mike Meyer 13 Nov '14

13 Nov '14

On Thu, Nov 13, 2014 at 12:56 PM, Evan Laforge <qdunkan(a)gmail.com> wrote: > On Wed, Nov 12, 2014 at 2:44 PM, Wojtek Narczyński <wojtek(a)power.com.pl> > wrote: > > How to create a DSL for HTML generation, that would (statically) allow > only > > valid HTML to be generated? Parent - child relations, valid attributes > only. > > > > Or does such a DSL already exist, perhaps? > > WASH did that, a long time ago. They mentioned that they > intentionally relaxed the rules, since being totally correct was > annoying to actually use. There are docs here: > http://www2.informatik.uni-freiburg.de/~thiemann/WASH/ > Well, a lot of folks felt that any validation beyond "it works in my browser" was annoying to actually use. I, on the other hand, used NSGML in emacs and the actual DTD without much problem. But there were multiple DTDs, some of which also relaxed things because so many people felt the strict rules were annoying to actually use. It didn't seem to catch on, so maybe it was too heavy for its benefits. > Based on a decade of trying to get people to write valid HTML - so that it would work in browsers other than the most popular two or three on the most popular two or three platforms - most people don't care about "correct".. I've had people - including employees of the FSF, who I would expect otherwise from - tell me that their HTML was tested on some limited set of browser/platform pairs when I pointed out it was incorrect and hence broke on some obscure browser/platform combination. Or even on one of those browser/platform pairs with a configuration change. This also seems to be true in the Haskell community. The web platforms help catching the obvious problems you find testing against a browser. But - like most popular web platforms - break the tools that help with the creation of valid HTML Validating HTML against a DTD is much easier than doing so against browsers, but my clients never did it unless I set it up for them. Doing so helped find more than one really obscure bug, and on one occasion kept my clients web site working when a major browser tweaked how it handled broken code and broke a lot of web sites. I think this may be related to why ADTs are a hard sell. While they help a lot writing correct programs, but for just writing working programs not so much.

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[ANN] Haskell FFI Tutorial
by Alex Petrov 13 Nov '14

13 Nov '14

Hi Everyone, I've recently started working on Haskell FFI Tutorial [1]. The repository already contains code that explains how to: * (expressively) represent C `struct` in Haskell code * call C code from Haskell * call Haskell code from C * operate on nested `struct`s * operate on `struct` arrays * decode `unions` * read and write C fixed-length strings and Pointer-type Strings * how to import functions from, for example, stdlib I'm still working on more story-like writeups, but the first one (about how to call Haskell from C) is already available [2]. If you have any feedback / comments / questions, know how to improve it or would like to see more things covered, just ping me. Thanks [1] https://github.com/ifesdjeen/haskell-ffi-tutorial [2] https://github.com/ifesdjeen/haskell-ffi-tutorial#calling-haskell-from-c -- Alex https://twitter.com/ifesdjeen http://clojurewerkz.org/

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xs not in scope
by Roelof Wobben 13 Nov '14

13 Nov '14

Hello, Im trying to find several answers to problem1 of the 99 haskell problems. Find the last item in a list. Now im trying to find a guarded solution. So far I have this: last3::[a]-> Maybe a; last3 a | [] = Nothing | ([a]) = Just a | (_:xs) = last3 xs But I see this error messages : src/Main.hs@10:8-10:10Not in scope: xs How to solve this ? Roelof

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Using multiple versions of the Haskell Platform on Windows
by Yitzchak Gale 13 Nov '14

13 Nov '14

The win-hp-path project provides the use-hp command, which makes it easy to switch between different versions of Haskell Platform on Windows. https://github.com/ygale/win-hp-path We are using it for running cabal and GHC in command prompt windows. In particular, we can also use it in build scripts used by the build management team who are not Haskell programmers. Please let me know if you can use this in more complex dev environments, or if you have suggestions about how it could be enhanced to do that. Pull requests are welcome. Thanks, Yitz

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How is maintaining the iCalendar package?
by Alfredo Di Napoli 13 Nov '14

13 Nov '14

Hello fellow Haskellers, anybody knows who is maintaining https://github.com/tingtun/iCalendar ? I have tried to email the maintainer listed on Github, but his activity seems quite low. Posting it here hoping to get a bit more resonance. Alfredo

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open-source project looking for novice help?
by Richard Eisenberg 13 Nov '14

13 Nov '14

I'm teaching an introductory Haskell course this semester (http://cis.upenn.edu/~cis194/fall14/) and am about to assign students their final project. Is there anyone out there who would welcome getting some novice help on an open-source project? You certainly don't need to commit to accept their patch(es), but I know it would be a great experience for some of the students to contribute to the "real" world of Haskell, instead of just doing exercises. You can get an accurate summary of what we've covered by looking at the lecture titles at http://cis.upenn.edu/~cis194/fall14/lectures.html Students will be choosing projects starting next week, and will need to have them completed by Dec. 15 or so. A good project is about 20 hours of work. Thanks! Richard

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