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Happy parsing wierdness
by Mike Menzel 02 Dec '07

02 Dec '07

Hi, I'm using the Happy parser, and I've threaded a Success/Failure monad-like thing through it so that if the parse succeeds, Success AST (AST is the datatype I want to turn my tokens into) is returned and if it fails, Fail String is returned (in a similar manner to how such threading is described in the Happy Documentation). That is, data SorF a = Succeed a | Fail String. Now, if the input is valid, I get the correct result (namely Success AST). If invalid input is given (ie the wrong token is encountered), happyError is triggered correctly, and and a Fail String is returned. However, if incomplete info is given (let's say a rule is expecting two tokens, and only one is read before the end of input), the parser generated by happy throws error("Reading EOF"). I have defined happyError on both the empty list of tokens, and the non-empty list as returning a Fail String. I'm stumped as to why I get this error. Any advice? Best, Mike.

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Modelling a mutable variable store
by Kannan Goundan 02 Dec '07

02 Dec '07

I'm implementing an interpreter for the lambda calculus augmented with mutable variables. I'm having problems doing the mutable state stuff in Haskell. Here's what I have so far: type Expr = ... terms in the language ... type Value = ... values in the language ... type Ident = String type VarId = String type Env = Map Ident Target data Target = TValue Value | TVar VarId -- idents might refer to values or vars in the store. type Store = Map VarId (Maybe Value) -- Using "Maybe" because vars start out uninitialized. eval :: (Env, Store, Expr) -> (Store, Value) -- The Store is threaded through the evaluator. One problem is that the link between an identifier and it's entry in the store is via VarIds, which are just strings. For example, if I see the identifier "x", I first look it up in the Env, and if it is a "TVar", I then look it up in the store. I'd like to get something stronger (in Java, I would use a pointer). Another problem is that entries in my Store never get garbage collected. Again, if I were using pointers in Java, this wouldn't be an issue. There's also the issue of finding a more elegant way of threading the Store through my evaluator, but I'm not concerned too much about that at this point. I can probably define a state-carrying monad like Parsec. My real concerns are the first two issues. Thanks. - Kannan

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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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