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Illegal type def
by PR Stanley 02 Dec '07

02 Dec '07

Hi > type assoc k v = [(k, v)] works beautifully and everything makes sense. > type Assoc v = (Ord k) => [(k, v)] This doesn't work. Is there any wayof defining k as an element of type Ordinal. I could redefine k by putting Char or Int in its place. Why can't I be more general? Thanks, Paul

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Re: [Haskell-cafe] Sequencing Parsers: a Simple Example
by PR Stanley 01 Dec '07

01 Dec '07

> > PRS: (>>=) :: Parser a -> Parser b -> Parser b > > p >>= f = \inp -> > > case p inp of > > [] -> [] > > [(v, out)] -> parse (f v) out > >You probably want: > >(>>=) :: Parser a -> (a -> Parser b) -> Parser b >p >>= f = \inp -> case parse p inp of > [] -> [] > [(v,out)] -> parse (f v) out > >Assuming that you're following Graham Hutton's book. > >Note that this definition won't actually compile; you probably need a >Monad instance and a newtype to get this to work properly (see >http://www.cs.nott.ac.uk/~gmh/Parsing.lhs for a working version of the >same code). PRS: That explains it then. [(v, out)] -> parse (f v) out caused a type mismatch. Cheers, Paul

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Sequencing Parsers: a Simple Example
by PR Stanley 01 Dec '07

01 Dec '07

Hi (>>=) :: Parser a -> Parser b -> Parser b p >>= f = \inp -> case p inp of [] -> [] [(v, out)] -> parse (f v) out based on a lot of guesswork, after the mess created by the OCR, I managed to get the above example to work syntactically but is it semantically correct? Thanks, Paul

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trouble building "unix-2.2.0.0" on cygwin
by Galchin Vasili 01 Dec '07

01 Dec '07

Hello, 0) All work being done on cygwin. Version 6.8.1 of ghc. 1) I ran "runhaskell Setup.lhs configure" and did a "tail -f config.log" in order to follow the config process. 2) Next I did the build "runhaskell Setup.lhs build" but there were many include files referenced in HsUnix.h that couldn't be found, e.g. sys/times.h, sys/resources.h, sys/wait.h, .... 3) I went back through the file config.log and all of the so-called missing include files had supposedly been found during the config process. 4) Next I went to c:/cygwin/usr/include/sys and found all of the so-called missing include files. I am trying to get my confidence level up with respect to the config/build/install (and along with darcs and haddock) process up high so I can make a significant contribution to the Haskell effort. please .. any help will be appreciated. Kind regards, Vasya

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Categories list in French (off-topic?)
by Maurício 01 Dec '07

01 Dec '07

Hi, I'm learning about categories using Saunders Mac Lane book. I'm also learning French. Do you guys know of a nice French mailing list, or forum, where people discuss about categories (and where beginners are accepted)? Sorry to ask that in a Haskell list, but you guys are the ones I know with interest in category theory. Thanks, Maurício

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Editorial error or something meaningful?
by PR Stanley 01 Dec '07

01 Dec '07

Hi taken from ch.8.3 in the Hutton book: "Whereas return v always succeeds, the dual parser failure always fails regardless of the contents of the input string:" The dual parser failure? Cheers, Paul

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Design of a Physics Interface
by Luke Palmer 01 Dec '07

01 Dec '07

I'm currently working on idioms for game programming using FRP. After going through several representations of physics as arrows[1] I decided that physics objects must not be implemented as arrows, because introducing new arrows in the middle of a computation[2] leads to ugly pain. So far the best approach I have is to represent the physics world as a single object World, with a function > integrate :: TimeStep -> World -> World But I can't figure out a good way to represent bodies in this world. I considered: > newBody :: (Position,Velocity) -> World -> (Body,World) Where Body is an ADT with an internal representation of an Integer or something. The problem with this is that (1) there is no way to guarantee that a Body actually exists in a World (which is a minor but still annoying issue), and (2) that there's a possibility that you could make a Body in one world and use it in another and there would be no way to detect the error. (2) could be solved using an internal representation of Data.Unique, but (1) still remains a problem. And as long as the function > deleteBody :: Body -> World -> World exists, it will always remain a problem. Are there any clever idioms I can use for this interface or implementation? Is there a way to store the data for bodies along with the Body object instead of with the world in a way that the relationships between them respect different generations of the World (through integrate)? Any other ideas? Thanks, Luke [1] Once as SF () (Position,Velocity), and again as SF PhysIn PhysOut where PhysIn = (Impulse,Momentum) and PhysOut = (Position,Velocity). [2] Using the arrow joinSF :: SF (Event (SF [a] a)) [a], and other similar style functions taking streams of SF events...

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cabal-install
by Ben Franksen 01 Dec '07

01 Dec '07

Just thought I install the latest version (0.4.0) from hackage and test it. Build and install went fine, but then it gets strange: ben@sarun: .../haskell/cabal-install-0.4.0 > sudo runhaskell Setup.lhs install Installing: /usr/local/bin ben@sarun: .../haskell/cabal-install-0.4.0 > rehash ben@sarun: .../haskell/cabal-install-0.4.0 > cabal --help Usage: cabal [GLOBAL FLAGS] or: cabal COMMAND [FLAGS] [...etc...] Commands: fetch Downloads packages for later installation or study. install Installs a list of packages. update Updates list of known packages clean Removes downloaded files list List available packages on the server. info Emit some info ben@sarun: .../haskell/cabal-install-0.4.0 > cd .. ben@sarun: .../software/haskell > cabal info Config file /home/ben/.cabal/config not found. Writing default configuration to /home/ben/.cabal/config. cabal: dist/Conftest.c: openFile: does not exist (No such file or directory) When later I tried the 'install' command, it reminds me to 'cabal update' first, which I do (no response, so probably worked), however next 'cabal install' gives the same funny answer about a missing dist/Conftest.c. Schade. Cheers Ben

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Rigid type-var unification failure in existentials used with parametrically polymorphic functions
by Pablo Nogueira 30 Nov '07

30 Nov '07

A question about existential quantification: Given the existential type: data Box = forall a. B a in other words: -- data Box = B (exists a.a) -- B :: (exists a.a) -> Box I cannot type-check the function: mapBox :: forall a b. (a -> b) -> Box -> Box -- :: forall a b. (a -> b) -> (exists a.a) -> (exists a.a) mapBox f (B x) = B (f x) Nor can I type check: mapBox :: forall a. (a -> a) -> Box -> Box -- :: forall a. (a -> a) -> (exists a.a) -> (exists a.a) mapBox f (B x) = B (f x) The compiler tells me that in both functions, when it encounters the expression |B (f x)|, it cannot unify the universally quantified |a| (which generates a rigid type var) with the existentially quatified |a| (which generates a different rigid type var) -- or so I interpret the error message. However, at first sight |f| is polymorphic so it could be applied to any value, included the value hidden in |Box|. Of course, this works: mapBox :: (forall a b. a -> b) -> Box -> Box mapBox f (B x) = B (f x) Because it's a tautology: given a proof of a -> b for any a or b I can prove Box -> Box. However the only value of type forall a b. a -> b is bottom. This also type-checks: mapBox :: (forall a. a -> a) -> Box -> Box mapBox f (B x) = B (f x) When trying to give an explanation about why one works and the other doesn't, I see that, logically, we have: forall a. P(a) => Q implies (forall a. P(a)) => Q when a does not occur in Q. The proof in our scenario is trivial: p :: (forall a. (a -> a) -> (Box -> Box)) -> (forall a. a -> a) -> Box -> Box p mapBox f b = mapBox f b However, the converse is not true. Yet, could somebody tell me the logical reason for the type-checking error? In other words, how does the unification failure translate logically. Should the first mapBox actually type-check? Isn't the code for mapBox :: forall a. (a -> a) -> Box -> Box encoding the proof: Assume forall a. a -> a Assume exists a.a unpack the existential, x :: a = T for some T apply f to x, we get (f x) :: a pack into existential, B (f x) :: exists a.a Discharge first assumption Discharge second assumption The error must be in step 3. Sorry if this is obvious, it's not to me right now.

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Re: [Haskell-cafe] A tale of Project Euler
by Sebastian Sylvan 30 Nov '07

30 Nov '07

On Nov 28, 2007 12:12 PM, Kalman Noel <kalman.noel(a)bluebottle.com> wrote: > Sebastian Sylvan: > > primes :: [Integer] > > primes = 2 : filter (null . primeFactors) [3,5..] > > > > primeFactors :: Integer-> [Integer] > > primeFactors n = factor n primes > > where > > factor m (p:ps) | p*p > m = [] > > | m `mod` p == 0 = p : factor (m `div` p) (p:ps) > > | otherwise = factor m ps > > Your definition gives a strange meaning to primeFactors. I'd want that for all > n, product (primeFactors n) == n. I think this law holds for the code posted > by Olivier. Yes you're right. That is property should clearly hold. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862

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