Send Beginners mailing list submissions to beginners@haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/beginners or, via email, send a message with subject or body 'help' to beginners-requ...@haskell.org
You can reach the person managing the list at beginners-ow...@haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Beginners digest..." Today's Topics: 1. parse String -> Expression (John Moore) 2. Re: parse String -> Expression (Felipe Lessa) 3. Re: parse String -> Expression (Felipe Lessa) 4. Re: parse String -> Expression (Daniel Fischer) 5. Re: Installing packages in Ubuntu (Nathan M. Holden) 6. Re: Installing packages in Ubuntu (Maur??cio CA) 7. Simple haskell problem ! Help please (Denis Firsov) 8. Re: Simple haskell problem ! Help please (Joe Fredette) 9. Re: Simple haskell problem ! Help please (Tim Attwood) ---------------------------------------------------------------------- Message: 1 Date: Sun, 8 Nov 2009 17:53:48 +0000 From: John Moore <john.moor...@gmail.com> Subject: [Haskell-beginners] parse String -> Expression To: beginners@haskell.org Message-ID: <4f7ad1ad0911080953t17ffd10co237995d6a6ce5...@mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" Hi, I'm trying to find a way to parseRPN (Reverse Polish Numbers) to expressions rather than to just numbers. e.g. I want the answer to be in the form (Multiply (Val 2) (Val 3)) rather than just the answer. Are these anyway near the steps parseRPN :: String->Expression This is a lot more complicated then I thought.!!! First do we have to read in a string is this (IsString) fromString :: String -> a Then this goes on a stack pushStack :: a -> Stack -> Stack (Takes a value and puts in on a stack) Later we pop it off popStack :: Stack -> (a,Stack) -- takes the value of the stack and leaves the stack Do we also have to define taking off the stack such as head(popstack) or fst(popstack) if we do we would probably have one for putting it onto a stack. Do we then turn the value into an Expression.? -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.haskell.org/pipermail/beginners/attachments/20091108/4fec4429/attachment-0001.html ------------------------------ Message: 2 Date: Sun, 8 Nov 2009 16:49:37 -0200 From: Felipe Lessa <felipe.le...@gmail.com> Subject: Re: [Haskell-beginners] parse String -> Expression To: beginners@haskell.org Message-ID: <20091108184937.ga22...@kira.casa> Content-Type: text/plain; charset=us-ascii On Sun, Nov 08, 2009 at 05:53:48PM +0000, John Moore wrote: > Hi, > I'm trying to find a way to parseRPN (Reverse Polish Numbers) to > expressions rather than to just numbers. e.g. I want the answer to be in the > form > > (Multiply (Val 2) (Val 3)) rather than just the answer. You don't need to code all the parser by hand. You can use, for example, Parsec parsers. Spoilers ahead!!! If your data type is data Expr a = Multiply (Expr a) (Expr a) | Val a then you may write something like expression :: Parser a -> Parser (Expr a) expression valParser = spaces >> (mult <|> val) where expr = expression valParser mult = char "*" >> (Multiply <$> expr <*> expr) val = Val <$> valParser using Parsec for a concrete example. HTH, -- Felipe. ------------------------------ Message: 3 Date: Sun, 8 Nov 2009 16:53:00 -0200 From: Felipe Lessa <felipe.le...@gmail.com> Subject: Re: [Haskell-beginners] parse String -> Expression To: beginners@haskell.org Message-ID: <20091108185300.gb22...@kira.casa> Content-Type: text/plain; charset=us-ascii On Sun, Nov 08, 2009 at 04:49:37PM -0200, Felipe Lessa wrote: > You don't need to code all the parser by hand. You can use, for > example, Parsec parsers. IOW, you may use the language's implicit stack instead of building your own. -- Felipe. ------------------------------ Message: 4 Date: Sun, 8 Nov 2009 20:00:10 +0100 From: Daniel Fischer <daniel.is.fisc...@web.de> Subject: Re: [Haskell-beginners] parse String -> Expression To: beginners@haskell.org Message-ID: <200911082000.11449.daniel.is.fisc...@web.de> Content-Type: text/plain; charset="iso-8859-15" Am Sonntag 08 November 2009 18:53:48 schrieb John Moore: > Hi, > I'm trying to find a way to parseRPN (Reverse Polish Numbers) to > expressions rather than to just numbers. e.g. I want the answer to be in > the form > > (Multiply (Val 2) (Val 3)) rather than just the answer. > I'd suggest using something like type Stack = [Expression] parseRPN :: Parser Expression parseRPN = rpn [] parseVal :: Parser Expression parseVal = do num <- parseNumber return (Val num) rpn :: Stack -> Parser Expression rpn stack = (do char '+' case stack of (x:y:ts) -> rpn (Add x y:ts) _ -> parsecFail "BinOp requires two values on Stack") <|> (do char '*' case stack of (x:y:ts) -> rpn (Multiply x y:ts) _ -> parsecFail "BinOp requires two values on Stack") <|> ... <|> (do v <- parseVal rpn (v:stack)) <|> (do eof case stack of [x] -> return x _ -> parsecFail "No parse") -- You could also use the userstate of Parsec for the stack > > > Are these anyway near the steps > parseRPN :: String->Expression > > This is a lot more complicated then I thought.!!! > > First do we have to read in a string is this (IsString) > > fromString :: String -> a > > Then this goes on a stack > > pushStack :: a -> Stack -> Stack (Takes a value and puts in on a stack) > > Later we pop it off > > popStack :: Stack -> (a,Stack) -- takes the value of the stack and leaves > the stack > > Do we also have to define taking off the stack such as head(popstack) or > fst(popstack) if we do we would probably have one for putting it onto a > stack. > > Do we then turn the value into an Expression.? ------------------------------ Message: 5 Date: Sun, 8 Nov 2009 14:14:06 -0500 From: "Nathan M. Holden" <nathanmhol...@gmail.com> Subject: [Haskell-beginners] Re: Installing packages in Ubuntu To: streb...@hotmail.com Cc: beginners@haskell.org Message-ID: <200911081414.06456.nathanmhol...@gmail.com> Content-Type: Text/Plain; charset="us-ascii" I can't say that you NEED to be using Cabal. I have it, and I'm also an Ubuntu newbie. You can download the bootstrap installer, you'll need to install (via runghc Setup configure, runghc Setup build, runghc Install) the most recent HTML package, and parsec 2.x to get it to run. Then just add an alias (or, I did) so you don't have to write a super-long command to run Cabal. Running Kubuntu 9.10 On Sunday 08 November 2009 08:28:24 am beginners-requ...@haskell.org wrote: > Message: 6 > Date: Sat, 7 Nov 2009 22:05:34 +0000 (UTC) > From: Glurk <streb...@hotmail.com> > Subject: [Haskell-beginners] Installing packages in Ubuntu > To: beginners@haskell.org > Message-ID: <loom.20091107t230222-...@post.gmane.org> > Content-Type: text/plain; charset=us-ascii > > Hi, > > I'm a beginner to both Cabal and Linux. > I'm a bit confused as to whether I need to be using Cabal when installng > packages. > > I'm running Ubuntu 9.10, and from the Synaptic Package Manager, I can > select various Haskell packages to install - will this install the package > correctly, or is there something I need to do in Cabal as well ? > > Thanks ! :) ------------------------------ Message: 6 Date: Sun, 08 Nov 2009 18:19:39 -0200 From: Maur??cio CA <mauricio.antu...@gmail.com> Subject: [Haskell-beginners] Re: Installing packages in Ubuntu To: beginners@haskell.org Message-ID: <hd794s$3i...@ger.gmane.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed > I'm a beginner to both Cabal and Linux. > I'm a bit confused as to whether I need to be using Cabal when installng > packages. There's no real confront between then, as cabal will default to install its packages in your home directory. Follow this rule in Ubuntu: install what's available from Ubuntu repositories (using Synaptic) and, whenever you want, need or feel like doing it, install from cabal. If something goes wrong for any reason, just rm ~/.cabal and you get back to what you have in a fresh install. (If you are a begginer in Haskell: also do use cabal system to package your own programs. This will give you a good understandment on how the package system work, and let you more confortable to experiment with advanced language features.) Best, MaurĂcio ------------------------------ Message: 7 Date: Sun, 8 Nov 2009 00:01:46 +0200 From: Denis Firsov <denis.fir...@gmail.com> Subject: [Haskell-beginners] Simple haskell problem ! Help please To: beginners@haskell.org Message-ID: <5874e6730911071401t280bc718m4e94514f58e2a...@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Hi ! I am beginner in Haskell and have problems with this problem: compress :: Eq a => [a] -> [(a, Int)] If you have string "AAABCCC" it transforms it to : {A, 3} {B,1} {C,3} Could you help me with it ? Thank you in advance ! ------------------------------ Message: 8 Date: Sun, 8 Nov 2009 23:56:37 -0500 From: Joe Fredette <jfred...@gmail.com> Subject: Re: [Haskell-beginners] Simple haskell problem ! Help please To: Denis Firsov <denis.fir...@gmail.com> Cc: beginners@haskell.org Message-ID: <c9c4b3f2-3cf7-4927-bcf2-e3f0a017e...@gmail.com> Content-Type: text/plain; charset=US-ASCII; format=flowed; delsp=yes Here are some hints to consider: 1. There are two ways to think about this -- pattern matching on a list, or higher-order functions 2. The former will require thinking about several cases, a. What is a "compressed" empty list? b. How many elements do we need to look at at once? c. What happens if the first and the next element are the same? d. What if they're different? 3. Higher order functions over the list might be easier, consider the `takeWhile` and `dropWhile` functions, as well as the `group` function, there are several ways to use these functions to solve the problem. HTH /Joe On Nov 7, 2009, at 5:01 PM, Denis Firsov wrote: > Hi ! I am beginner in Haskell and have problems with this problem: > compress :: Eq a => [a] -> [(a, Int)] > If you have string "AAABCCC" it transforms it to : {A, 3} {B,1} {C,3} > > Could you help me with it ? > Thank you in advance ! > _______________________________________________ > Beginners mailing list > Beginners@haskell.org > http://www.haskell.org/mailman/listinfo/beginners ------------------------------ Message: 9 Date: Mon, 9 Nov 2009 00:07:38 -0800 From: "Tim Attwood" <timoth...@comcast.net> Subject: [Haskell-beginners] Re: Simple haskell problem ! Help please To: beginners@haskell.org Message-ID: <hd8ike$u9...@ger.gmane.org> Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original > Hi ! I am beginner in Haskell and have problems with this problem: > compress :: Eq a => [a] -> [(a, Int)] > If you have string "AAABCCC" it transforms it to : {A, 3} {B,1} {C,3} > > Could you help me with it ? > Thank you in advance ! This is straightforward with a list comprehension and the group function. import Data.List compress s = [(head g, length g) | g <- group s] ------------------------------ _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners End of Beginners Digest, Vol 17, Issue 11 *****************************************