October 2012 - Haskell-Cafe

Compiler jobs
by Tom Hawkins 01 Oct '12

01 Oct '12

Hi, BAE Systems has some open positions for principal level Haskell programmers with compiler experience. Our DARPA project[1] is building a compiler for a new programming language with dynamic information flow control, targeting a custom ISA with hardware enforced information security. On the project, we are partnered with academics from the University of Pennsylvania, Harvard, and Northeastern University. Send a resume If interested. Thanks! -Tom [1] http://www.crash-safe.org/

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ANN: monadLib-3.7.1 on Hackage
by Iavor Diatchki 01 Oct '12

01 Oct '12

Hello, I am pleased to announce the availability of monadLib-3.7.1 on Hackage. MonadLib is a library intended to help programmers to quickly and easily construct various monads. The library has support for a wide range of effects: threading state, read-only variables, collecting output, exception handling, back-tracking computation, and working with explicit continuations. It provides similar functionality to libraries such as MTL, but it has a simpler interface and improved functionality for a number of the transformers. The main change in this version is improved support for working with continuations: now we support invoking the same continuation in contexts that have different types. Happy hacking! -Iavor

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Is inspectable recursion in Arrows possible?
by Alessandro Vermeulen 01 Oct '12

01 Oct '12

Dear all, I am trying to find a way to translate normal recursive notation such as the |fib| function below to an arrow, retaining as much of the structure of the recursive notation as possible. In addition I would like to inspect the arrow. For this I created a datatype containing a constructor for each Arrow{..} class: Fib: > fib 0 = 0 > fib 1 = 1 > fib n = fib (n-2) + fib (n-1) My R datatype, the instances for this datatype consist of the mapping to the appropriate constructor: > data R x y where > -- Category > Id :: R a a > Comp :: R b c -> R a b -> R a c > -- Arrow > Arr :: (a -> b) -> R a b > Split :: R b c -> R b' c' -> R (b,b') (c,c') > Cache :: (a -> a -> Bool) -> R a a > -- ArrowChoice > Choice :: R b c -> R b' c' -> R (Either b b') (Either c c') > -- ArrowLoop > Loop :: R (b, d) (c, d) -> R b c > -- ArrowApply > Apply :: R (R b c, b) c Translating the |fib| function from above first resulted in the following definition. It is not allowed however due to the proc n on the RHS of the declaration for |fibz|. I know that the grammar of the Arrow Notation prevents this, but what is the underlying reason for this? > fib' :: (ArrowChoice r, ArrowLoop r) => r Int Int > fib' = proc x -> do > rec fibz <- proc n -> case n of > 0 -> returnA -< 0 > 1 -> returnA -< 1 > n' -> do l <- fibz -< (n'-2) > r <- fibz -< (n'-1) > returnA -< (l+r) > fibz -<< x Rewriting the function above to use a let statement compiles. However, here my second problem arises. I want to be able to inspect the recursion where it happens. However, in this case |fibz| is an infinite tree. I would like to capture the recursion into fibz, I hoped the rec would help me with that in combination with |loop| but maybe I am wrong? > fib'' :: (ArrowChoice r, ArrowLoop r, ArrowApply r) => r Int Int > fib'' = proc x -> do > let fibz = proc n -> case n of > 0 -> returnA -< 0 > 1 -> returnA -< 1 > n' -> do l <- fibz -< (n'-2) > r <- fibz -< (n'-1) > returnA -< (l+r) > fibz -<< x Basically, is it possible to observe this kind of recursion? (Perhaps even within the boundaries of Arrow Notation) I could perhaps add another constructor like fix. Any thoughts on this? Cheers, Alessandro Vermeulen

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One of the new buzz phrases is "Event-Sourcing"; is Haskell suitable for this?
by KC 30 Sep '12

30 Sep '12

http://martinfowler.com/eaaDev/EventSourcing.html http://martinfowler.com/articles/lmax.html -- -- Regards, KC

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