- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Coroutines
by Ryan Ingram 25 Dec '08

25 Dec '08

On Thu, Dec 18, 2008 at 2:00 AM, Nicolas Pouillard <nicolas.pouillard(a)gmail.com> wrote: > I don't see why one would need session types, channels... to express that. > I maybe need a more complicated coroutines (ruby) example that would require > using this system. OK, how would you type these routines in Haskell? def simple yield "hello" yield 1 yield (lambda { |x| x + 1 }) end def useSimple state = 0 result = nil simple { |x| if (state == 0) then result = x else if (state == 1) then result += (x * 4).toString else if (state == 2) then result += x.call(10).toString state = state + 1 } result end I know it's a bit contrived, but you get the idea. In Haskell using Control.Coroutine: simple :: forall rest. Session (String :!: Int :!: (Int -> Int) :!: rest) rest () simple = do put "hello" put 1 put (\x -> x + 1) useSimple :: forall rest. Session (String :?: Int :?: (Int -> Int) :?: rest) rest String useSimple = do string <- get int <- get func <- get return (string ++ show (int * 4) ++ show (func 10)) result :: String result = snd $ connects simple useSimple -- result = "hello411"

7 8

Stupid question #374: why is MaybeT not in the standard library?
by Brian Hurt 25 Dec '08

25 Dec '08

I wrote my own implementation of MaybeT (which was a usefull exercise), but a quick google showed: http://www.haskell.org/haskellwiki/New_monads/MaybeT But I'm wondering why it's not in the standard library. The standards committee just hasn't gotten around to it yet? Or was there some discussion of this in the past on some (public) maillist, that my admittedly shallow googling failed to uncover, that someone could point me at? Thanks. Brian

6 9

ANN: Control.Monad.IfElse
by Jeff Heard 25 Dec '08

25 Dec '08

Provides useful anaphoric and monadic versions of if-else and when, as well as infix operators for the actions to allow things like this: -- Allow the clause to be in m a whenM (Gtk.widgetIsFocus win) $ do this that -- anaphoric if. If the condition is nonempty, pass the conditional value to the "then" clause, otherwise do an action that requires no parameter. aif (lookup name list) (\file -> Right `liftM` hGetLine file) (do f <- openFile name l <- hGetLine file return $ Right (f,l)) -- infix version of Control.Monad.When (x>5) >>? putStrLn x -- infix version of anaphoric when lookup name list >>=? \value -> putStrLn value -- infix version of anaphoric whenM Gtk.widgetIntersect win rect >>=>>? \intersectingRectangle' -> Gtk.invalidate intersectingRectangle'

2 1

ANNOUNCE: Cryptol now freely available
by Don Stewart 25 Dec '08

25 Dec '08

Galois is pleased to announce that Cryptol, the language of cryptography, is now available to the public! Cryptol is a domain specific language for the design, implementation and verification of cryptographic algorithms, developed over the past decade by Galois for the United States National Security Agency. It has been used successfully in a number of projects, and is also in use at Rockwell Collins, Inc. Cryptol is implemented in Haskell. Domain-specific languages (DSLs) allow subject-matter experts to design solutions in using familiar concepts and constructs. Cryptol, as a DSL, allows domain experts in cryptography to design and implement cryptographic algorithms with a high degree of assurance in the correctness of their design, and at the same time, producing a high performance implementation of their algorithms. In this release, Galois has made a implementation of the Cryptol language available free of charge for non-commercial uses. More information on the blog: http://www.galois.com/blog/2008/12/24/cryptol-the-language-of-cryptography-… And at http://www.cryptol.net Enjoy!

1 0

When using functional dependencies to combine...
by Martijn van Steenbergen 24 Dec '08

24 Dec '08

Good evening everyone, My program reads: > module Boom where > > import Control.Monad.State > > type SucParser s = StateT [s] [] > > newtype WithUnit s a = WithUnit (SucParser s (a, ())) > > foo :: SucParser s [s] > foo = get > > bar :: WithUnit s [s] > bar = WithUnit get The compiler complains: Boom.hs:13:0: Couldn't match expected type `([s], ())' against inferred type `[s]' When using functional dependencies to combine MonadState s (StateT s m), arising from the instance declaration at <no location info> MonadState ([s], ()) (StateT [s] []), arising from a use of `get' at Boom.hs:13:15-17 When generalising the type(s) for `bar' I'm wondering if I'm making a silly mistake or if there's something less trivial going on here. Could someone please explain the error and give a hint on how to fix it? Thanks much. :-) Martijn.

2 3

Comparing on multiple criteria
by Martijn van Steenbergen 24 Dec '08

24 Dec '08

Hello all, Data.Ord has a handy function called comparing, and its documentation shows an example of its use. But what if you want to sort a list of values based on multiple criteria? It turns out there is a neat way to do this: compareTuple = mconcat [comparing fst, comparing snd] The default Monoid instances for Ordering and functions work exactly as required here. (Thanks to vixey in #haskell for the hint to look at monoids!) To reverse the order of a criterion or a set of criteria, flip can be used: compareTuple' = mconcat [comparing fst, flip $ comparing snd] I think it would be really neat if these two use cases were also described in comparing's documentation. Who is the right person to ask this of? Thanks in advance, Martijn.

5 5

The Applicative Functor Monad
by Jeremy Shaw 24 Dec '08

24 Dec '08

Hello, I want to make a Monad which is almost exactly like the Writer monad, except instead of using mappend to glue Monoids together, it uses <*> to glue applicative functors together. Here is the code: import Control.Applicative import Data.Monoid -- * Sample Implementation of the Writer Monad data Writer w a = Writer { runWriter :: (w, a) } instance (Monoid w) => Monad (Writer w) where return a = Writer (mempty, a) (>>=) = bindWriter bindWriter :: (Monoid w) => Writer w a -> (a -> Writer w b) -> Writer w b bindWriter (Writer (w,a)) f = let (Writer (w', b)) = f a in Writer (w `mappend` w', b) -- * Sample Implementation of the Applicative Functor Monad data AF af a = AF { runAF :: (af, a) } bindAF :: (Applicative f) => AF (f (a -> b)) x -> (x -> AF (f a) y) -> AF (f b) y bindAF (AF (f, x)) g = let (AF (a, y)) = g x in AF (f <*> a, y) -- instance (Applicative f) => Monad (AF (f ... As you can see, the similarity is striking. Alas, AF and bindAF do not quite have the right type signatures to be used for an instance of the Monad class. Is there some clever work-around I missing? (Aside from, -fno-implicit-prelude). Thanks! - jeremy

3 3

shortest paths, prioritiy queues
by Johannes Waldmann 24 Dec '08

24 Dec '08

Hello. I am looking for a single-source shortest-path implementation (Dijkstra's algorithm, cf. Chapter 25 in Cormen,Leiserson,Rivest). An efficient implementation would use Binomial or Fibonacci heaps (Chap.s 20 + 21). The problem seems to be "(fib-)heap-decrease-key" which assumes that we have, with the key, a pointer to its position in the heap, because that is where there will be a destructive update. How is this dealt with in Okasaki's book + library? - Thanks, J.W.

4 5

Renaming a package on Hackage
by Jeff Heard 24 Dec '08

24 Dec '08

How does one go about renaming a package on Hackage? I finally figured out a name for Thingie, and I'd like to change the package name to reflect that, so that future versions look to the new package and not the old one.

3 2

Time for a new logo?
by Don Stewart 24 Dec '08

24 Dec '08

I noticed a new haskell logo idea on a tshirt today, http://image.spreadshirt.net/image-server/image/configuration/13215127/prod… Simple, clean and *pure*. Instead of the "we got lots going on" of the current logo. Any graphic designers want to try some variations on this theme of purity? A new year, and a new mature logo... -- Don

57 113