# News aggregator

### Compilation a la Carte given at Universiteit Utrecht, Computing | Laurence E. Day - Academia.edu

del.icio.us/haskell - Sat, 12/28/2013 - 4:17pm
Categories: Offsite Blogs

### Types ala carte

del.icio.us/haskell - Sat, 12/28/2013 - 4:16pm
Categories: Offsite Blogs

### Roman Cheplyaka: Resources in Tasty (update)

Planet Haskell - Sat, 12/28/2013 - 4:00pm

In a recent article I described how resources were introduced to the Tasty test framework, as well as some alternative approaches. This article describes the new API, introduced in Tasty 0.7.

To recap, there was a function, withResource, that handled creation/acquisition and disposal of resources, but if you needed to access the resource directly in the tests, you had to store the resource in an IORef (or similar) as part of the initialization routine.

At the time it seemed acceptable, but later I discovered that when the number of resources was bigger than one or two, or even not known in advance (when tests are generated rather than just written down), this was inconvenient enough to start looking for a different solution.

One of the major problems with tests receiving the resource value directly, as in

withResource :: IO a -> (a -> IO ()) -> (a -> TestTree) -> TestTree

… was that the resource could be used not only in the tests themselves, but to construct the tests, which is bad/wrong for a number of reasons. For instance, we don’t want to create the resources when we’re not running tests, but we still want to know which tests we have.

The solution I found is to pass not the value of the resource, but an IO action yielding the resource.

withResource :: IO a -> (a -> IO ()) -> (IO a -> TestTree) -> TestTree

Even though it’s an IO action, it doesn’t acquire the resource, because such a resource wouldn’t be shared across multiple tests, which is the semantics we’re after. Instead, it returns the resource which has been acquired (think: reads from an IORef or MVar). But thanks to it being an IO action, it can only be used inside a test, and not to construct or alter tests based on the resource value.

Here’s a modified example from the last article which works with this new API:

import Test.Tasty import Test.Tasty.HUnit -- assumed defintions data Foo acquire :: IO Foo release :: Foo -> IO () testWithFoo :: Foo -> Assertion (acquire, release, testWithFoo) = undefined main = do defaultMain $withResource acquire release tests tests :: IO Foo -> TestTree tests getResource = testGroup "Tests" [ testCase "x"$ getResource >>= testWithFoo ]
Categories: Offsite Blogs

### Segmentation fault/access violation when CPLUS_INCLUDE_PATH not set correctly

haskell-cafe - Sat, 12/28/2013 - 3:30pm
L.S., When I compile an application against an experimental version of wxHaskell*, the application always ends with the message: Segmentation fault/access violation in generated code , unless I set the environment variable CPLUS_INCLUDE_PATH to C:\Program Files\Haskell Platform\2013.2.0.0\mingw\lib\gcc\mingw32\4.5.2\include\c++\ , which is the same value as used when compiling wxHaskell. If I copy the contents of C:\Programs\Haskell Platform\2013.2.0.0\ to some other location and let CPLUS_INCLUDE_PATH point to the new location of directory c++, the segmentation fault message reappears. Why is this and how can create an application that also works on another computer? Regards, Henk-Jan van Tuyl * wxHaskell is a binding to the wxWidgets GUI library, which is written in C++
Categories: Offsite Discussion

### Just wrote a doge spell corrector in Haskell based on Norvig's algorithm. I will now correct your every meme.

Haskell on Reddit - Sat, 12/28/2013 - 2:59pm
Categories: Incoming News

### Why Dataflow is not Popular

Haskell on Reddit - Sat, 12/28/2013 - 2:51pm
Categories: Incoming News

### Consistency issue with type level numeric literals

haskell-cafe - Sat, 12/28/2013 - 10:54am
GHC 7.6.3 has quite convenient type-level numeric literals. We can use numbers like 1 and 2 in types. However, using the type level numeral has been quite a bit of a challenge, illustrated, for example, by the following SO question. http://stackoverflow.com/questions/20809998/type-level-nats-with-literals-and-an-injective-successor It seems the challenge has the reason: the type level numerals and their operations provided in GHC.TypeLits have a consistency issue. The following code demonstrates the issue: it constructs two distinct values of the type Sing 2. Singletons aren't singletons. {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE PolyKinds #-} module NotSing where import GHC.TypeLits
Categories: Offsite Discussion

### www.fpcomplete.com

del.icio.us/haskell - Sat, 12/28/2013 - 9:25am
Categories: Offsite Blogs

### cs.mcgill.ca

del.icio.us/haskell - Sat, 12/28/2013 - 7:35am
Categories: Offsite Blogs

### hackage.haskell.org

del.icio.us/haskell - Sat, 12/28/2013 - 6:26am
Categories: Offsite Blogs

### gist.github.com

del.icio.us/haskell - Sat, 12/28/2013 - 4:26am
Categories: Offsite Blogs

### Audrey Tang: Perl 6：冒險的降臨

Planet Haskell - Sat, 12/28/2013 - 1:23am
<article lang="zh-Hant">

(這是 Day 24 – Advent Ventures 的中譯，作者是 Larry Wall。)

</article> <article lang="zh-Hant">

（包括 13 歲的天才小妹）

（沒人能為暫時的邂逅、永遠的告別做好準備。）

<hruby><rb annotation="blogger">部落客</rb></hruby>聽到雲端天使彈出視窗、放聲歡唱？

【……現在你有成千上萬的問題……】

（大聲）<hruby><rb annotation="sanity test">健全測試</rb></hruby> #1 合格了嗎？結果如何？

TimToady 得到賜福，開始指揮 Perl 朝聖者的合唱。】

</article>
Categories: Offsite Blogs

### www.glc.us.es

del.icio.us/haskell - Fri, 12/27/2013 - 11:45pm
Categories: Offsite Blogs

### [Fp-nl] Call for participation: Dutch FunctionalProgramming Day 2014

haskell-cafe - Fri, 12/27/2013 - 11:32pm
Dear all, The next Netherlands Functional Programming day (NL-FP 2014) will take place on Friday, January 10, 2014 at the University of Amsterdam at Amsterdam Science Park. You are all cordially invited to participate and, of course, to give a presentation. The day will largely follow the pattern of the previous NL-FP days with a moderately timed start, a day of enjoyable talks in between lunch and coffee breaks and ending with a joint dinner in a nearby restaurant. All further details can be found on the NL-FP 2014 site(s): http://staff.science.uva.nl/~grelck/nl-fp-day-2014.html http://staff.science.uva.nl/~grelck/nl-fp-dag-2014.html Hope to see you all in Amsterdam in January! Best regards, Clemens Grelck
Categories: Offsite Discussion

### A glimpse into a new general purpose programming language under development at Microsoft

Lambda the Ultimate - Fri, 12/27/2013 - 6:59pm

Microsoft's Joe Duffy and team have been (quietly) working on a new programming language, based on C# (for productivity, safety), but leveraging C++ features (for performance). I think it's fair to say - and agree with Joe - that a nirvana for a modern general purpose language would be one that satisfies high productivity (ease of use, intuitive, high level) AND guaranteed (type)safety AND high execution performance. As Joe outlines in his blog post (not video!):

At a high level, I classify the language features into six primary categories:

1) Lifetime understanding. C++ has RAII, deterministic destruction, and efficient allocation of objects. C# and Java both coax developers into relying too heavily on the GC heap, and offers only “loose” support for deterministic destruction via IDisposable. Part of what my team does is regularly convert C# programs to this new language, and it’s not uncommon for us to encounter 30-50% time spent in GC. For servers, this kills throughput; for clients, it degrades the experience, by injecting latency into the interaction. We’ve stolen a page from C++ — in areas like rvalue references, move semantics, destruction, references / borrowing — and yet retained the necessary elements of safety, and merged them with ideas from functional languages. This allows us to aggressively stack allocate objects, deterministically destruct, and more.

2) Side-effects understanding. This is the evolution of what we published in OOPSLA 2012, giving you elements of C++ const (but again with safety), along with first class immutability and isolation.

3) Async programming at scale. The community has been ’round and ’round on this one, namely whether to use continuation-passing or lightweight blocking coroutines. This includes C# but also pretty much every other language on the planet. The key innovation here is a composable type-system that is agnostic to the execution model, and can map efficiently to either one. It would be arrogant to claim we’ve got the one right way to expose this stuff, but having experience with many other approaches, I love where we landed.

4) Type-safe systems programming. It’s commonly claimed that with type-safety comes an inherent loss of performance. It is true that bounds checking is non-negotiable, and that we prefer overflow checking by default. It’s surprising what a good optimizing compiler can do here, versus JIT compiling. (And one only needs to casually audit some recent security bulletins to see why these features have merit.) Other areas include allowing you to do more without allocating. Like having lambda-based APIs that can be called with zero allocations (rather than the usual two: one for the delegate, one for the display). And being able to easily carve out sub-arrays and sub-strings without allocating.

5) Modern error model. This is another one that the community disagrees about. We have picked what I believe to be the sweet spot: contracts everywhere (preconditions, postconditions, invariants, assertions, etc), fail-fast as the default policy, exceptions for the rare dynamic failure (parsing, I/O, etc), and typed exceptions only when you absolutely need rich exceptions. All integrated into the type system in a 1st class way, so that you get all the proper subtyping behavior necessary to make it safe and sound.

6) Modern frameworks. This is a catch-all bucket that covers things like async LINQ, improved enumerator support that competes with C++ iterators in performance and doesn’t demand double-interface dispatch to extract elements, etc. To be entirely honest, this is the area we have the biggest list of “designed but not yet implemented features”, spanning things like void-as-a-1st-class-type, non-null types, traits, 1st class effect typing, and more. I expect us to have a few of these in our mid-2014 checkpoint, but not all of them.

What do you think?

Categories: Offsite Discussion

### Looking for collaborators on a social/web/science project

Haskell on Reddit - Fri, 12/27/2013 - 6:20pm
Categories: Incoming News

### Deblogger: a small project to play with Haskell and data structures

Haskell on Reddit - Fri, 12/27/2013 - 5:48pm
Categories: Incoming News

### Guide to GHC Extensions - School of Haskell | FP Complete

del.icio.us/haskell - Fri, 12/27/2013 - 4:25pm
Categories: Offsite Blogs

### Free monads in category theory (part 3)

del.icio.us/haskell - Fri, 12/27/2013 - 2:52pm
Categories: Offsite Blogs

### FP Complete's guide to GHC extensions -- a gentler documentation than the GHC docs.

Haskell on Reddit - Fri, 12/27/2013 - 2:03pm
Categories: Incoming News