london HUG

well I just got back from the 1st meeting of v2 of the london haskell users group (apparently it used to exist before; and the ghosts of its former incarnation floated around the room in the form of code kata people)
dude (derek) gave a talk on why do monads matter? - a brave thing to do, given how many have tried their own take on a monad tutorial (myself included). nothing spectacular but certainly another take on monads, and it did tickle my brain in the right areas enough into realising that <$> only needs a Functor so it certainly paid off in the OH! sense; even though that leap was personal to me and wouldn't be apparent if you were at the talk - there was no mention of functors at all, really
Turnout was better than the average dutchhug turnout (sorry Shaun)
It also turns out theres a regular Haskell coding dojo in London, hoodlums, already happening (apparently a spinoff or somehow related to v1 of the london hug)
Went to pub afterwards. room booked (or at least some upstairs space that was otherwise empty). chocolate orange beer, which was less disgusting than it sounds. it was cool to meet a bunch of people using haskell for $ (although I count myself in their ranks these days).
after rapidly throwing down a few of those chocolate orange beers (hence the incoherency and lack of case), I shouted out suggestions for future talks on: agda; quickcheck; parsec; and functors/monads/arrows/applicative (turned out some fucker already had a talk on that...)
next meeting 28th nov 2012. i'll probably be there.
ps also at the pub I met another programmer also called Ben - I asked him if he's going to BenConf but although he'd heard of it, it hadn't suckered him in.

mifi vs ipv6

today's ipv6 bug: mifi internal nameserver that redirects you to a "mifi not connected" web page when its not connected ... returns some really random shit when you ask it for AAAA. at least sometimes - I don't think it always does that, but maybe? I don't have it switched on but disconnected much.

yield zipper

Oleg wrote about converting an arbitrary traversable into a zipper.

His code uses delimited continuations, and I puzzled a while (years...) before starting to understand what was going on.

I just read Yield: mainstream delimited continuations.

It looked to me like I could easily change Oleg's zipper can be expressed using "yield" which gives a different view, that I think I might have understood more easily - because I know yield from other languages, and don't properly have my head around continuations (which is basically the point of the "Yield" paper, I think)

So then, my altered version of the zipper on Oleg's page, using yield:

>  import Data.Traversable as T


>  type Zipper t a = Iterator (Maybe a) a (t a)

>  make_zipper :: T.Traversable t => t a -> Zipper t a
>  make_zipper t = run $ T.mapM f t
>   where
>   f a = do
>     r <- yield a
>     return $ maybe a id r

This is run and yield pretty much as defined on page 10 of the yield paper:

>  data Iterator i o r = Result r | Susp o (i -> Iterator i o r)
>  yield x = shift (\k -> return $ Susp x k)
>  run x = reset $ x >>= return . Result

and some test code:

>  sample = [1,2,3]

>  main = do
>    let (Susp a1 k1) = make_zipper sample
>    print a1
>    let (Susp a2 k2) = k1 Nothing
>    print a2
>    let (Susp a3 k3) = k2 $ Just 100
>    print a3
>    let (Result end) = k3 Nothing
>    print end

and below, to make this posting properly executable, here's Oleg's library code for shift/reset:

> -- The Cont monad for delimited continuations, implemented here to avoid
> -- importing conflicting monad transformer libraries

>  newtype Cont r a = Cont{runCont :: (a -> r) -> r}


>  instance Monad (Cont r) where
>     return x = Cont $ \k -> k x
>     m >>= f  = Cont $ \k -> runCont m (\v -> runCont (f v) k)

>  reset :: Cont r r -> r
>  reset m = runCont m id

>  shift :: ((a -> r) -> Cont r r) -> Cont r a
>  shift e = Cont (\k -> reset (e k))

Update 1: Changed types from Oleg's Z | ZDone to the yield paper's Susp | Result

cd

# cd
bash: cd: write error: Success