March 2012 - Beginners

populating a bloom filter; stymied by ST monad
by Joey Hess 14 Mar '12

14 Mar '12

I have a potentially very large number of values to feed into a bloom filter. They're coming from IO: getValues :: (v -> k -> k) -> k -> IO k getValues update initial = go initial =<< gen where go v [] = return v go v (f:fs) = do x <- val f go (maybe v (`update` v) x) fs gen = undefined val f = undefined This streams lazily, but if I build a list (getValues (:) []), the laziness is lost; the whole list is constructed and returned before any of it can be used. Which uses too much memory of course. So I can't use Data.BloomFilter.fromListB. Seems I need to do something like this: let filter = newMB (cheapHashes 13) 33554432 --- sized for 1 million items filter' <- unsafeFreezeMB <$> getValues insertMB filter Except this won't work, the mutable bloom filter uses the ST monad. And I cannot see a way to combine the three "MB" functions into a single computation in the same ST monad. Perhaps stToIO to is what I need, but I can't work out how to use it. Help? -- see shy jo

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NCurses program cannot be interpreted?
by Aditya Manthramurthy 13 Mar '12

13 Mar '12

Hi, I am writing an ncurses based haskell program. I am unable to run it via ghci - I get an error message saying that UI.NCurses could not be found. If I run it via "runhaskell", the terminal gets messed up. The only way that I found works is to compile it using "ghc source.hs" and running the binary. I can guess that not being able to interpret it has something to do with the terminal like capabilities of ghci (as ncurses needs a terminal I suppose). But why does runhaskell not work either? I am just trying to understand how the three ways to run the program are different. Thanks, Aditya

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Class instance for a type without a parameter
by Peter Hall 12 Mar '12

12 Mar '12

Can someone remind me why I can't do this: data Digit = D0 | D1 | D2 | D3 | D4 | D5 | D6 | D7 | D8 | D9 deriving (Eq, Ord, Show) instance Num [Digit] where ... -- This isn't allowed I could make Num a => [a] an instance of Num, but I can't make Digit also an instance of Num because D8 + D5 is not a digit. So then I could introduce a ConvertableToNum class, but I feel like I'd be heading down a path with a lot of unnecessary extra type classes. Thanks, Peter

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Re: [Haskell-beginners] Type Tree variables
by Ozgur Akgun 12 Mar '12

12 Mar '12

You need to provide a Show instance for your data type. The easiest way is to add 'deriving Show' at the end of your data type declaration. The errors you get aren't to do with the function f. On 11 March 2012 16:01, bahadýr altan <doaltan(a)yahoo.co.uk> wrote: > These are the ones I get, they are the same but they have different inputs > : > > Main> f (Branch 12 (Branch 7 (Branch 3 Empty Empty) (Branch 10 Empty > Empty)) (Branch 24 Empty Empty)) > ERROR - Cannot find "show" function for: > *** Expression : f (Branch 12 (Branch 7 (Branch 3 Empty Empty) (Branch 10 > Empty Empty)) (Branch 24 Empty Empty)) > *** Of type : (Integer,Integer,Tree,Tree,Integer,Tree,Tree) > > > Main> f (Branch 13 (Branch 2 Empty Empty) (Branch 48 Empty Empty)) > ERROR - Cannot find "show" function for: > *** Expression : f (Branch 13 (Branch 2 Empty Empty) (Branch 48 Empty > Empty)) > *** Of type : (Integer,Integer,Tree,Tree,Integer,Tree,Tree) > > To be clear, I need to get a tree as an input. I mean the output should be > like this : > > f (Branch a (Branch b (Branch d Empty Empty) (Branch e Empty Empty)) > (Branch c Empty Empty)) = Branch b (Branch d Empty Empty) (Branch a (Branch > e Empty Empty) (Branch c Empty Empty)) > > > Thank you very much for helping me I really appreciate it :) > > ------------------------------ > *From:* Ozgur Akgun <ozgurakgun(a)gmail.com> > *To:* bahadýr altan <doaltan(a)yahoo.co.uk>; Haskell Beginners < > beginners(a)haskell.org> > *Sent:* Sunday, 11 March 2012, 17:50 > > *Subject:* Re: [Haskell-beginners] Type Tree variables > > What's the error you get? > > On 11 March 2012 15:37, bahadýr altan <doaltan(a)yahoo.co.uk> wrote: > > I tried "f" function but it always give errors. Thanks anyway.. > > > Ozgur > > > -- Ozgur Akgun

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Re: [Haskell-beginners] Type Tree variables
by Ozgur Akgun 12 Mar '12

12 Mar '12

What's the error you get? On 11 March 2012 15:37, bahadýr altan <doaltan(a)yahoo.co.uk> wrote: > I tried "f" function but it always give errors. Thanks anyway.. > Ozgur

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Fw: Type Tree variables
by bahadýr altan 12 Mar '12

12 Mar '12

----- Forwarded Message ----- From: bahadýr altan <doaltan(a)yahoo.co.uk> To: Ozgur Akgun <ozgurakgun(a)gmail.com> Sent: Sunday, 11 March 2012, 16:42 Subject: Re: [Haskell-beginners] Type Tree variables I'm sorry if I mislead you, I think I didn't mention my problem clearly there. I'm actually stuck in the position of declaring subtrees, I couldn't find a solution to that. My function is like f :: Tree -> Tree And my tree structure is like data Tree = Empty | Branch Integer Tree Tree And When I write this which I learned from you : (c1, c2, d1, d2, e1, e2 :: Tree) I get error. If you tell me How should I solve that I'd appreciate. Thanks, Bahadir ________________________________ From: Ozgur Akgun <ozgurakgun(a)gmail.com> To: bahadýr altan <doaltan(a)yahoo.co.uk>; Haskell Beginners <beginners(a)haskell.org> Sent: Sunday, 11 March 2012, 16:23 Subject: Re: [Haskell-beginners] Type Tree variables (Replying to the list again) On 11 March 2012 14:15, bahadýr altan <doaltan(a)yahoo.co.uk> wrote: Hi, >Thanks for your reply. I tried your solution but my function becomes like this and it gives an error on the right side : function (Node a (Node b _ _) (Node c _ _)) = (Node c (Node a _ _) (Node b _ _)) This is not what you originally said though. If you need to refer to the subtrees, you just need to name them. (Instead of using _) [ not a very useful function, but still: ] f :: Tree a -> Tree a f (Node x (Node y y1 y2) (Node z z1 z2)) = Node x (Node y y1 y2) (Node z z1 z2) -- also think about the types: (x, y, z :: a) (y1, y2, z1, z2 :: Tree a) The thing is I don't need to process the subtrees but I still have to return them as an output. I still can't do that. And I read a few chapters in LYAH, it was pretty good, I learned haskell from there. However, I think the answer of this question isn't in LYAH. >Thanks.. >Bahadir -- Ozgur Akgun

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Re: [Haskell-beginners] Type Tree variables
by Ozgur Akgun 12 Mar '12

12 Mar '12

Hi, Please reply to the list so others can contribute. On 11 March 2012 14:42, bahadýr altan <doaltan(a)yahoo.co.uk> wrote: > I'm sorry if I mislead you, I think I didn't mention my problem clearly > there. I'm actually stuck in the position of declaring subtrees, I couldn't > find a solution to that. > > My function is like > > f :: Tree -> Tree > > And my tree structure is like > > data Tree = Empty | Branch Integer Tree Tree > > And When I write this which I learned from you : > (c1, c2, d1, d2, e1, e2 :: Tree) > > I get error. If you tell me How should I solve that I'd appreciate. > Hence I was suggesting to think about the types of the pattern variables. f (Branch x (Branch y y1 y2) (Branch z z1 z2)) = (x,y,y1,y2,z,z1,z2) This definition will work, and return a 7-tuple given a Tree. Try to load this into ghci and see what the type of f is (:t f). One hint: y and y1 aren't of the same type. Ozgur

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Re: [Haskell-beginners] Type Tree variables
by Ozgur Akgun 12 Mar '12

12 Mar '12

(Replying to the list again) On 11 March 2012 14:15, bahadýr altan <doaltan(a)yahoo.co.uk> wrote: > Hi, > Thanks for your reply. I tried your solution but my function becomes > like this and it gives an error on the right side : > function (Node a (Node b _ _) (Node c _ _)) = (Node c (Node a _ _) (Node b > _ _)) > This is not what you originally said though. If you need to refer to the subtrees, you just need to name them. (Instead of using _) [ not a very useful function, but still: ] f :: Tree a -> Tree a f (Node x (Node y y1 y2) (Node z z1 z2)) = Node x (Node y y1 y2) (Node z z1 z2) -- also think about the types: (x, y, z :: a) (y1, y2, z1, z2 :: Tree a) > The thing is I don't need to process the subtrees but I still have to > return them as an output. I still can't do that. And I read a few chapters > in LYAH, it was pretty good, I learned haskell from there. However, I think > the answer of this question isn't in LYAH. > Thanks.. > Bahadir > -- Ozgur Akgun

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Type Tree variables
by bahadýr altan 12 Mar '12

12 Mar '12

Hello everyone, I'm trying to write a generic function which works on binary trees. For example my function works on just first three nodes of a binary tree, but the tree is bigger, I don't care the other nodes. Like this : function Node a (Node b Subtree Subtree) (Node c Subtree Subtree) The problem is I couldn't find a way to represent "Subtree" structures. A guy here told me to use variables of type Tree, but I don't know how to create them. Can you tell me how to solve this problem? Thanks a lot.

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Parsec, parsing 'free text'
by Franco 11 Mar '12

11 Mar '12

I am using Parsec as my parsing library and quite liking it: though, I am unsure on how to properly tackle a 'free element' problem. Let me explain: In my file, there are some elements which look like this: this is some <red - formatted> text. I call this a FormatString. They can be found inside many elements, like: [ someconditions | this is some <red - formatted> text.] + this is some <red - formatted> text. -> somenumberhere To parse a FormatString I call 'generalText', which looks like this: 56 -- TEXT -- 57 58 generalText = many anyText >>= \fss -> 59 return (foldl (+:+%) (toFString "" []) fss) 60 61 anyText = try ( formattedText ) 62 <|> plainText 63 64 formattedText = char '<' >> 65 sepEndBy1 format spacebar >>= \fs -> -- format parser 66 string "- " >> 67 manyTill anyChar (char '>') >>= \cs -> 68 return (toFString cs fs) 69 70 plainText = many1 (noneOf "<") >>= \xs -> 71 return (toFString xs []) It works, but causes quite some headaches in using it: - I would like to call a "read with generalText until you find a ']' character. - or a "read with generalText until you find a " -> " (do not consume it) And the like, but I do not know how. If it were a plain string I would call manyTill1 anyChar myP. I thought of first parsing a raw string searching for a terminator and then feeding it to generalText. But isn't that cluncky codewise? Thanks for your help -F

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