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Haskell Platform web page is out of date
by Paul Johnson 09 Mar '11

09 Mar '11

The Haskell Platform web page at http://hackage.haskell.org/platform// seems to need updating. (Incidentally, that double slash at the end doesn't look right). * The "next release" is promised in Jan 2011. * The "Release Timetable" schedules the next release for 5 March 2011. I just worry that this is one of the first things someone investigating Haskell sees, and it creates a bad first impression. Paul.

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Haskell Platform 2011.2
by tsuraan 09 Mar '11

09 Mar '11

The wiki page for Haskell Platform is still listing March 5 (today) as the planned release date. Is this still the plan, or should the page be updated?

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Overlaping Parsec rules
by Hauschild, Klaus (EXT) 09 Mar '11

09 Mar '11

Hi, to solve this ICFP task http://www.cs.cornell.edu/icfp/task.htm I'm currnetly working on the parser. With the hint from Thu (reading Phillip Wadlers monadic parser paper) and consulting http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours/Parsing I produce a first working version of the parser. After this great moment I completed the token definition and near all parsing rules. For the complete code have a look at http://code.google.com/p/hgmltracer/source/browse/#svn%2Ftrunk%2FhGmlTracer data GmlToken = -- structures FunctionToken TokenSequence | ArrayToken TokenSequence | -- control operators ApplyToken | IfToken | -- number operators AddiToken | AddfToken | ACosToken | ASinToken | ClampfToken | CosToken | DiviToken | DivfToken | EqiToken | EqfToken | FloorToken | FracToken | LessiToken | LessfToken | ModiToken | MuliToken | MulfToken | NegiToken | NegfToken | ToRealToken | SinToken | SqrtToken | SubiToken | SubfToken | -- points GetXToken | GetYToken | GetZToken | PointToken | -- arrays GetToken | LengthToken | -- environment IdentifierToken String | BinderToken String | -- types BoolToken Bool | IntToken Int | RealToken Double | StringToken String deriving Show And parseGml :: String -> [GmlToken] parseGml input = case parse parseList "gml" input of Left err -> error ("Parse error: " ++ (show err)) Right gml -> gml parseList = sepBy parseGml' spaces parseGml' = -- structures -- parseFunction -- <|> parseArray -- control operators parseControlOperators -- number operators <|> parseNumberOperators -- points <|> parsePointOperators -- arrays <|> parseArrayOperators -- types <|> parseBool <|> parseInteger <|> parseString -- environment <|> parseIdentifier <|> parseBinder parseArray = parseSequence '[' ']' parseFunction = parseSequence '{' '}' parseSequence start end = do char start array <- parseList char end return $ ArrayToken array parseControlOperators = parseApply <|> parseIf parseApply = do string "apply" return $ ApplyToken parseIf = do string "if" return $ IfToken parseNumberOperators = do string "addi" return $ AddiToken <|> do string "addf" return $ AddfToken <|> do string "acos" return $ ACosToken <|> do string "asind" return $ ASinToken <|> do string "clampf" return $ ClampfToken <|> do string "cos" return $ CosToken <|> do string "divi" return $ DiviToken <|> do string "divf" return $ DivfToken <|> do string "eqi" return $ EqiToken <|> do string "eqf" return $ EqfToken <|> do string "floor" return $ FloorToken <|> do string "frac" return $ FracToken <|> do string "lessi" return $ LessiToken <|> do string "lessf" return $ LessfToken <|> do string "modi" return $ ModiToken <|> do string "muli" return $ MuliToken <|> do string "mulf" return $ MulfToken <|> do string "negi" return $ NegiToken <|> do string "negf" return $ NegfToken <|> do string "real" return $ ToRealToken <|> do string "sin" return $ SinToken <|> do string "Sqrt" return $ SqrtToken <|> do string "subi" return $ SubiToken <|> do string "subf" return $ SubfToken parsePointOperators = do string "getx" return $ GetXToken <|> do string "gety" return $ GetYToken <|> do string "getz" return $ GetZToken <|> do string "point" return $ PointToken parseArrayOperators = do string "get" return $ GetToken <|> do string "length" return $ LengthToken parseInteger = liftM (IntToken . read) $ many1 digit parseBool = do string "true" return $ BoolToken True <|> do string "false" return $ BoolToken False parseString = do char '"' string <- many (noneOf "\"") char '"' return $ StringToken string parseIdentifier = do identifier <- many (noneOf " ") return $ IdentifierToken identifier parseBinder = do char '/' binder <- many (noneOf " ") return $ BinderToken binder parseComment = do char '%' many (noneOf "") newline return $ () spaces = skipMany1 space After gluing all this together in my mind all worked well. But it doesn't. The test file for parsing looks like: { /self /n n 2 lessi { 1 } { n 1 subi self self apply n muli } if } /fact 12 fact fact apply * I think there is a problem with overlaping rules. There is a parser rule consuming "negi" and resulting in the NegiToken. A single "n" is a valid identifier. For the example file my parser says: unexpected " ", expecting "negi" * I think the same problem is present for "parseInteger" and "parseReal" (currently no in code but looks like "parseReal = do a <- many1 digit \n char '.' \n b <- many1 digit \n return $ RealToken (read (a ++ "." ++ b)" * Something with "parseFunction" is going really wrong. * All parsig rules are designed with the condition that each "construct" is separated by whitespaces. For this before parsing the input will be preprocessed: insert spaces, removing whitespaces and so on. Now in the parsed result appears (IdentifierToken ""). I think my version is not the best way to parse a identifer: parseIdentifier = do identifier <- many (noneOf " ") return $ IdentifierToken identifier Please help me. Klaus

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operations on lists with continuations
by Evan Laforge 09 Mar '11

09 Mar '11

I have a few functions for operating on lists that take continuations: -- | Like takeWhile but with a continuation, so you can chain takes without -- copying. takeWhileThen :: (a -> Bool) -> ([a] -> [a]) -> [a] -> [a] takeWhileThen _ _ [] = [] takeWhileThen f cont (x:xs) | f x = x : takeWhileThen f cont xs | otherwise = cont (x:xs) But of course this isn't enough, then I start wanting a takeThen. And then a filterThen. That makes me think plain explicit recursion would be clearer, but the problem I have with that is that it has an imperative feel. What I mean by that is that the result is a product of the changing state of the recursing function, and it's non-composable. E.g. filterUntil start end msgs = go where go [] = [] go (m:ms) | msg_start m >= start = takeWhile ((<end) . msg_start) (m : ms) | condition m = m : go ms | otherwise = go ms Whereas with filterThen I can write: filterUntil start end = filterThen condition ((>=start) . msg_start) . takeWhile ((<end) . msg_start) This looks much more functional to me, and can be composed: filterUntilPlus1 start end = filterThen condition ((>=start) . msg_start) $ takeWhileThen ((<end) . msg_start) $ take 1 So my question is, am I about to discover this already exists in the Prelude, or is unnecessary because of something I'm overlooking? I imagine it must be a special case of some kind of generalization, right? Surely someone has thought of this and taken it much further than I have. I suppose this starts to look like maybe a CPS version of a parser... but it seems lighter-weight for simple tasks.

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cabal install / upgrade (was: Re: some problem with Cabal)
by Evan Laforge 09 Mar '11

09 Mar '11

> Generally, it shouldn't (and doesn't), but here some packages have to be > reinstalled because they have to be built against different dependencies. > > I'd prefer cabal install to refuse here (overridable with a --force flag) > because reinstalling boot libs (containers, filepath, directory, process, > time) tends to be a bad idea. Seconded! I've had my setup screwed up because some package wanted to recompile base against a tiny version bump on the random package for whatever reason. The presence of two bases with the same version, one in the ghc dir and one in /usr/local/lib proceded to screw up all kinds of things. It took a long time to figure out because I wasn't expecting two copies of base with the same version, and it somehow managed to overwrite the package file in package.conf.d so the original metadata was gone. Eventually I gave up and reinstalled ghc. And all I did was accidentally run cabal upgrade somepackage when I wanted to install a newer version of somepackage. By the time I realized it was apparently still aliased to cabal break-everything and hit ^C it was too late. And it's not like I haven't been warned away from cabal upgrade either... but it made me think, if the command is so dangerous, should it have a scarier name?

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Tagless interpreter, expression problem and zipper
by Roel van Dijk 09 Mar '11

09 Mar '11

Hello everyone, I am stuck rewriting some code in the tagless style. My problem can be thought of as an interpreter for a very simple language: > data Exp = Lit Integer > | Add Exp Exp > | Mul Exp Exp > deriving (Show) But some complexity is added by the fact that my interpreter also needs to know the context in which literals appear in an expression. So given the example > t1 :: Exp > t1 = (Lit 3 `Add` Lit 4) `Mul` Lit 2 I want to have access to the fact that the literal 3 appears on the left side of an addition. Therefore I created a type: > data Ctx = Empty > | AddL Exp Ctx > | AddR Exp Ctx > | MulL Exp Ctx > | MulR Exp Ctx > deriving (Show) This is almost a zipper over the Exp type. I left out a context for literals themselves because it wasn't necessary. Now I can create a simple evaluation function: > eval :: (a -> a -> a) > -> (a -> a -> a) > -> (Integer -> Ctx -> a) > -> Exp > -> a > eval add mul lit = go Empty > where > go ctx (Lit n) = lit n ctx > go ctx (Add x y) = let x' = go (AddL y ctx) x > y' = go (AddR x ctx) y > in x' `add` y' > go ctx (Mul x y) = let x' = go (MulL y ctx) x > y' = go (MulR x ctx) y > in x' `mul` y' This function can evaluate an expression to any type 'a', but it needs some additional functions to accomplish that: add, mul and lit. Notice how the 'lit' function has access to a literal's context. To interpret an Exp as an Integer I use: > evalInt :: Exp -> Integer > evalInt = eval (+) (*) const Or as a String: > evalStr1 :: Exp -> String > evalStr1 = eval addStr mulStr (const . show) > addStr x y = "(" ++ x ++ " + " ++ y ++ ")" > mulStr x y = "(" ++ x ++ " * " ++ y ++ ")" Or a silly example which uses the context: > evalStr2 :: Exp -> String > evalStr2 = eval addStr mulStr lit > where > lit _ (AddL (Lit 4) (MulL _ _)) = "Foo!" > lit n _ = show n The silly example replaces a literal with "Foo!" if it appears on the left side of an addition with 4, and the whole addition is the left side of a multiplication, like in (x + 4) * ?. All of this works. But I want to be able to add additional constructors to the Exp language without changing existing code. This looks like the expression problem. Ignoring the zipper-like context I came up with the following: First define the language: > class Lit a where lit :: Integer -> a > class Add a where add :: a -> a -> a > class Mul a where mul :: a -> a -> a Integer interpreter: > instance Lit Integer where lit = fromInteger > instance Add Integer where add = (+) > instance Mul Integer where mul = (*) String interpreter, using a newtype so I don't need TypeSynonymInstances: > newtype Str = Str {unS :: String} > instance Show Str where show = show . unS > instance Lit Str where lit = Str . show > instance Add Str where add x y = Str $ addStr (unS x) (unS y) > instance Mul Str where mul x y = Str $ mulStr (unS x) (unS y) Same example expression, now polymorphic: > t1 :: (Lit a, Add a, Mul a) => a > t1 = (lit 3 `add` lit 4) `mul` lit 2 This expression can now be interpreted in multiple ways: > t1 :: Integer >> 14 > t1 :: Str >> "((3 + 4) * 2)" This solves the expression problem. I can add new structures to my language in other modules without having to change existing code. The necessary parts are mentioned in the type constraint (Lit a, Add a, Mul a). But I lost the power of the context! How do I get it back? Attached are the two approaches, WithTags.hs and Tagless.hs (but without context). Any help would be greatly appreciated!

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some problem with Cabal
by Qi Qi 09 Mar '11

09 Mar '11

Hi, When I tried the haskell platform 2011.2 source, from here http://code.galois.com/darcs/haskell-platform/download-website/linux.html . Configuration passed successfully. But when making, it gives the following error: scripts/build.sh ************************************************** Scanning system for any installed Haskell Platform components... Found:None. New packages to install: None! All done. ************************************************** Building "/usr/local/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.10.1.0" <command line>: cannot satisfy -package Cabal-1.10.1.0: Cabal-1.10.1.0-64e8f05722ea5bbdf07df2e5a3491105 is unusable due to missing or recursive dependencies: directory-1.1.0.0-85d1d0f9d96dffdacf64f3cc6fba6f2f process-1.0.1.5-4cab1bf0666275ac101dd48c7565b64c (use -v for more information) Error: Compiling the Setup script failed make: *** [build.stamp] Error 2 I think this is the same problem as I got during installing some other packages via cabal such as hmatrix, ghc-mod, happy and etc. Does anyone have any idea about how to solve it? Thanks! -- Qi Qi

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have anyone tried GHC 7.0.2
by Qi Qi 08 Mar '11

08 Mar '11

Hi, After I installed GHC 7.0.2 and cabal-install 0.10.0 using Cabal library 1.10.1, I found that some package failed to install. For example like Happy, I got the following error: cabal install happy Resolving dependencies... <command line>: cannot satisfy -package Cabal-1.10.1.0: Cabal-1.10.1.0-64e8f05722ea5bbdf07df2e5a3491105 is unusable due to missing or recursive dependencies: directory-1.1.0.0-85d1d0f9d96dffdacf64f3cc6fba6f2f process-1.0.1.5-4cab1bf0666275ac101dd48c7565b64c (use -v for more information) cabal: Error: some packages failed to install: happy-1.18.6 failed during the configure step. The exception was: ExitFailure 1 Besides happy package, some other packages like ghc-mod, hmatrix also failed to install. Does anyone else have the same issue after installed ghc 7.0.2? By the way, does anyone hear anything about Haskell-platform 2011.2 ? Thanks! -- Qi Qi

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Error recovery in Happy
by Tom 08 Mar '11

08 Mar '11

Hey guys, I was wondering if there were possiblities to ignore certain errors during parsing. I tried using the error token, but that didn't seem to work. I looked at the following topic http://haskell.1045720.n5.nabble.com/Error-detection-in-GLR-Happy-grammar-t… For my parser I need to be able to parse a certain token at any place, but only in some places it is part of a rule. In other cases this token should be ignored after which the parser just procedes to the next token. Is this possible with Happy? Should I use the monadic parser to achieve this? More concretely, I have tokenized some sourcecode which contains comments, these should be ignored except when they are located above a function, in which case I want them to parse. Any ideas about this issue are greatly appreciated! Thanks, Tom

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Haskell IDE
by Hauschild, Klaus (EXT) 08 Mar '11

08 Mar '11

Hi Haskellers, whats your Haskell IDE of choise? Currently I use leksah. Is the EclipseFP Plugin for Eclipse a real alternative? Thanks Klaus

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