February 2014 - Haskell-Cafe

ANN: aivika-1.1 (the simulation library for DES and SD)
by David Sorokin 16 Feb '14

16 Feb '14

Hi Cafe, Aivika is a multi-paradigm simulation library with a strong emphasis on the Discrete Event Simulation (DES) in the first order and System Dynamics (SD) in the second one. A new version introduces an infinite stream of simulation data. This stream is based on the process-oriented computation of DES (this computation in its turn works on top of the event queue and uses the continuations). So, a random input of arrivals distributed exponentially can be described as such a stream in just one line of code! There is also a processor that just a function that takes an input stream and then returns a resulting stream after the processing. The processor is a Haskell arrow, which means that we can use the proc-notation in case of need, although there is a lot of predefined combinators such as that one which unites parallel processors into one processor. There are finite and infinite queues. The library collects the statistics for theses queues. We can either work with the queue directly, or create a buffer processor for the queue (with loop-back or not). There is also so called the server that provides its own processor. It suits for modeling working places. As before, the library collects the statistics too. In my personal opinion the library was powerful enough already earlier but now it can also allow the modeler to define some models using the mentioned above streams and processors directly in a very high-level Haskell code in such a way as he/she would draw the model on the diagram like those ones which are used by some visual simulation software tools. At the same time, all parts of the library are very well integrated. Actually, all them are inter-connected, where more high-level monads are just based on more low-level ones. Now the top is the mentioned above processor (the Haskell arrow). Unfortunately, the PDF document included in the package is outdated. Moreover, it doesn't cover even 10% of the features of my simulation library. But there are example models that illustrate the most important features. Also I use them for my tests. Best regards, David Sorokin [1] http://hackage.haskell.org/package/aivika [2] http://hackage.haskell.org/package/aivika-experiment [3] http://hackage.haskell.org/package/aivika-experiment-chart

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C++ exception (from *.so) catchable by Haskell??
by Nick Rudnick 16 Feb '14

16 Feb '14

Dear all, after trying around a while this seems to be a good question for Haskell-cafe to me: Is somewhere a place (e.g. some Hackage package) where intercepting exceptions from C++ library code is done, so one can look *how* it is done? Generally, it is the following: 1) Injecting code that throws a an exception in C++ library code, e.g., throw runtime_error("OOOPS..."); 2) then, I tried out everything in reach (Haskell docs, Hackage interfaces to C++ considered to be well-done) to get it caught (try, catch, bracket, finally; various exception types; forkOS). 3) To my surprise, every time the output stays the same: terminate called after throwing an instance of 'std::runtime_error' what(): OOOPS... Is it possible at all to get a grip of this? Is there some place where it is done? Does the terminate call already happen with the C++ execution? Any comment welcome, cheers, Nick

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GHC 7.6.3 (and others) hashes
by Alexander Solla 16 Feb '14

16 Feb '14

Hi everybody, I'm building up a body of SaltStack "states" for developing with and running Haskell web-apps. Since I am hoping my project will go big (don't we all?) I'm hoping to do things right, the first time (up to the constraints of marginal costs...) So, to that end, I'm trying to go a little overboard on the systems security front, since the costs are so low. Is there any where I can find SHA hashes for the official GHC builds? In particular, I'm looking for a hash for the file located at: https://www.haskell.org/ghc/dist/7.6.3/ghc-7.6.3-x86_64-unknown-linux.tar.b… I suppose downloading via an encrypted http connection goes a long way to mitigate MITM, etc. And I realize that long term, I'll want to host my own copy (and I will). But that is quickly sliding up the cost curve. Plus, I won't know if the copy I host will be okay unless I get the hash from a trusted source. Is there some place to get that hash? Thanks!

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Protecting against main thread exit
by Roman Cheplyaka 15 Feb '14

15 Feb '14

When using forkFinally or bracket, I like to have the guarantee that the cleanup action will be executed regardless of how the action or thread is terminated — normally or via an exception. Of course, all guarantees are off when dealing with SIGKILL or power reset, but there's one other annoying exception (no pun intended) — when the main thread terminates, all other threads are silently shut down, without any chance to clean up. Why is it done that way? Why not kill them with an async exception instead? That would be a more uniform behavior. Are there any good ways to protect again the main thread exit? (Note that I'm talking about a library, so I can't have direct control over what's going on in the main thread.) Roman

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[ANN] anonymous-sums
by Omari Norman 15 Feb '14

15 Feb '14

Someone once wondered why there are no anonymous sum types in Haskell: https://groups.google.com/forum/#!topic/haskell-cafe/cIxfLdcJ7aI Wonder no more: http://hackage.haskell.org/package/anonymous-sums Occasionally I have needed this so I split it off into a package. If there is already something like this out there (or some elegant way to do this that doesn't involve hairy extensions), please let me know and I will be all too happy to deprecate this. Omari

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Naming scheme for partial functions
by Michael Snoyman 15 Feb '14

15 Feb '14

Greg Weber and I have been discussing some changes to mono-traversable[1]. One of the modules we provide is Data.NonNull, which provides total versions of functions like `last`. A change we're looking at would require having a partial version of `last` defined in a separate typeclass (IsSequence), which would allowing for more optimized implementations of the total `last` function for datatypes which support it (e.g., strict ByteStrings). But what should we name it? I'm sure everyone's familiar with the `unsafe` naming convention, but that's not appropriate here: standard usage shows `unsafe` meaning a function which can cause a segfault. I initially named it `partialLast`, but partial can also imply partial function application. Greg brought up the idea of suffixing the function with something like `Throws` or `Errors`, which I think I'm a bit partial to myself[2]. So my questions are: * Is there some already used naming scheme out there for partial functions which I've missed? * Do people have any ideas to throw into the mix? Michael [1] https://github.com/snoyberg/mono-traversable/pull/21 [2] Pardon the pun.

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HGamer3D 0.3.0 on Github
by Peter Althainz 14 Feb '14

14 Feb '14

Dear All, HGamer3D version 0.3.0 is finalized and on Github. www.hgamer3d.org www.github.com/urs-of-the-backwoods/HGamer3D News: runs on Linux and Windows, example Games/Cuboid2.hs with FRP regards Peter Althainz

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ANN: asynchronous-exceptions
by Roman Cheplyaka 13 Feb '14

13 Feb '14

It is often useful to distinguish between synchronous and asynchronous exceptions. The common idiom is to run a user-supplied computation catching any synchronous exceptions but allowing asynchronous exceptions (such as user interrupt) pass through. base 4.7 (shipped with GHC 7.8) will have SomeAsyncException type that solves this problem. asynchronous-exceptions is a new package that serves two purposes: * provide compatibility with older `base` versions that lack the `SomeAsyncException` type * define convenient functions for catching only synchronous exceptions Roman

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Lisp Style Restarts in Haskell
by Christopher Howard 13 Feb '14

13 Feb '14

Hi. I was curious if Haskell has something like "restarts" in Common Lisp. My search engines didn't give relevant results for "Haskell restarts". However, there were several results for "Haskell coroutines" and, from what I can gather, restarts and coroutines are a similar or related subject. -- http://www.lugod.org/presentations/pgp/why.html

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ghci in gallery
by Duncan Rowland 13 Feb '14

13 Feb '14

Dear All, new to haskell, so this might be obvious, but... I'm setting up a gallery installation (OSX) that exposes a command line haskell to the visitors. The process is run with user 'nobody', so hopefully that will be a bit safer, i.e. cat visitor_commands_pipe | sudo -u nobody ghci -XOverloadedStrings But how question are: 1) what sort of damage could a malicious user do to my system by entering commands into the pipe? 2) what should I do to protect my system? I can filter the user commands as I like, but I do not want to run 'in a webpage'. Thanks in advance, -Duncan.

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