# News aggregator

### Why is this algorithm so much slower in Haskell than in C?

Code in haskell: http://pastebin.com/29wRDiU5 compiled with ghc -O2 filename Code in C: http://pastebin.com/L33n9S88 compiled with gcc -O3 --std=c99 -lm filename I need to generate fifty 40-bit numbers where each bit is set with probability p (in my examples p is set to 0.001). In haskell code it's represented by fractions of 10000 for speedup, so sorry if it's a bit unclear. It's a part of a bigger algorithm, but according to a profiler the slowest one. My question is why is Haskell version so much slower than C? (~4sec vs ~0.25sec)

submitted by Envielox[link] [34 comments]

### Syntax highlighted markdown in this subreddit

Any chance of getting codeblocks syntax highlighted as Haskell in this subreddit? The standard way these days is you write

``` haskell x = foo `bar` mu where y = 2 ```E.g. this works on Github and Hakyll. Right now in this subreddit, if you write the above, you get:

haskell x = foo `bar` mu where y = 2

Which generates:

<p><code>haskell x = foo `bar` mu where y = 2 </code></p>Meaning some nifty JavaScript could find such elements identified by <p><code>…</code></p> and then haskell followed by a newline to be highlighted as Haskell code. If reddit ever explicitly supports code fences, then we can just turn this off.

Question is: can moderators add scripts to a subreddit due to the security issue? Maybe if a reddit admin approved it?

submitted by cookedbacon[link] [10 comments]

### Using lift recursively? (joke)

### Danny Gratzer: Bargain Priced Coroutines

The other day I was reading the 19th issue of the Monad.Reader and there was a fascinating post on coroutines.

While reading some of the code I noticed that it, like most things in Haskell, can be reduced to 5 lines with a library that Edward Kmett has written.

Consider the type of a trampoline as described in this article

newtype Trampoline m a = Tramp {runTramp :: m (Either (Tramp m a) a)}So a trampoline is a monadic computation of some sort returning either a result, a, or another computation to run to get the rest.

Now this looks strikingly familiar. A computation returning Trampoline m a is really a computation returning a tree of Tramp m a’s terminating in a pure value.

This sounds like a free monad!

import Control.Monad.Trans.Free import Control.Monad.Identity type Trampoline = FreeT IdentityRecall that FreeT is defined as

data FreeF f a b = Pure a | Free (f b) data FreeT f m a = FreeT (m (FreeF f a (FreeT f m a)))This is isomorphic to what we where looking at before. As an added bonus, we’ve saved the tedium of defining our own monad and applicative instance for Trampoline.

We can now implement bounce and pause to define our trampolines. bounce must take a computation and unwrap it by one level, leaving either a value or another computation.

This is just a matter of rejiggering the FreeF into an Either

bounce :: Functor m => Trampoline m a -> m (Either (Trampoline m a) a) bounce = fmap toEither . runFreeT where toEither (Pure a) = Right a toEither (Free m) = Left $ runIdentity mpause requires some thought, the trick is to realize that if we wrap a computation in one layer of Free when unwrapped by bounce we’ll get the rest of the computation.

Therefore,

pause :: Monad m => Trampoline m () pause = FreeT $ return (Free . Identity $ return ())So that’s 6 lines of code for trampolines. Let’s move on to generators.

A generator doesn’t yield just another computation, it yields a pair of a computation and a freshly generated value. We can account for this by changing that Identity functor.

type Generator c = FreeT ((,) c)Again we get free functor, applicative and monad instances. We two functions, yield and runGen. Yield is going to take one value and stick it into the first element of the pair.

yield :: Monad m => g -> Generator g m () yield g = FreeT . return $ Free (g, return ())This just sticks a good old boring m () in the second element of the pair.

Now runGen should take a generator and produce a m (Maybe c, Generator c m a). This can be done again by pattern matching on the underlying FreeF.

runGen :: (Monad m, Functor m) => Generator g m a -> m (Maybe g, Generator g m a) runGen = fmap toTuple . runFreeT where toTuple (Pure a) = (Nothing, return a) toTuple (Free (g, rest)) = (Just g, rest)Now, last but not least, let’s build consumers. These wait for a value rather than generating one, so -> looks like the right functor.

type Consumer c = FreeT ((->) c)Now we want await and runCon. await to wait for a value and runCon to supply one. These are both fairly mechanical.

runConsumer :: Monad m => c -> Consumer c m a -> m a runConsumer c = (>>= go) . runFreeT where go (Pure a) = return a go (Free f) = runConsumer c $ f c runCon :: (Monad m, Functor m) => Maybe c -> Consumer c m a -> m (Either a (Consumer c m a)) runCon food c = runFreeT c >>= go where go (Pure a) = return . Left $ a go (Free f) = do result <- runFreeT $ f food return $ case result of Pure a -> Left $ a free -> Right . FreeT . return $ freerunCon is a bit more complex than I’d like. This is to essentially ensure that if we had some code like

Just a <- await lift $ do foo bar baz Just b <- awaitWe want foo, bar, and baz to run with just one await. You’d expect that we’d run as much as possible with each call to runCon. Thus we unwrap not one, but two layers of our FreeT and run them, then rewrap the lower layer. The trick is that we make sure never to duplicate side effects by using good old return.

We can sleep easy that this is sound since return a >>= f is f a by the monad laws. Thus, our call to return can’t do anything detectable or too interesting.

While this is arguably more intuitive, I don’t particularly like it so we can instead write

runCon :: (Monad m, Functor m) => Maybe c -> Consumer c m a -> m (Either a (Consumer c m a)) runCon food = fmap go . runFreeT where go (Pure a) = Left a go (Free f) = Right (f food)Much simpler, but now our above example wouldn’t run foo and friends until the *second* call of runCon.

Now we can join generators to consumers in a pretty naive way,

(>~>) :: (Functor m, Monad m) => Generator c m () -> Consumer c m a -> m a gen >~> con = do (cMay, rest) <- runGen gen case cMay of Nothing -> starve con Just c -> runCon c con >>= use rest where use _ (Left a) = return a use rest (Right c) = rest >~> cAnd now we can use it!

addGen :: Generator Int IO () addGen = do lift $ putStrLn "Yielding 1" yield 1 lift $ putStrLn "Yielding 2" yield 2 addCon :: Consumer Int IO () addCon = do lift $ putStrLn "Waiting for a..." Just a <- await lift $ putStrLn "Waiting for b..." Just b <- await lift . print $ a + b main = addGen >~> addConWhen run this prints

Yielding 1 Waiting for a... Yielding 2 Waiting for b... 3Now, this all falls out of playing with what functor we give to FreeT. So far, we’ve gotten trampolines out of Identity, generators out of (,) a, and consumers out of (->) a.

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### New gtk2hs 0.12.4 release

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!

~d