- Beginners - Haskell.org

system and FilePath with space
by PICCA Frederic-Emmanuel 26 Jan '17

26 Jan '17

Hello I try to execute a command using system. my code is like this system (unwords ["python", p]) where p :: FilePath p = "/my super/script.py" has you can see this path contain a space. When I try to run it I get an error python: can't open file '/my': So the system command is wrong. I understand that I need to escape the space in the command, like this p = "/my\ super/script.py" now is there a function in haskell for this. a sort of sanitizer for FilePath when used by system. Thanks Frederic

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Memory Profiling Graph Explanation!
by Gilberto Melfe 24 Jan '17

24 Jan '17

Hello all! I'm trying to understand the memory allocation charts produced by the Haskell GHC profiler. My doubt at this moment is this: the graph shown at [1], is what is called an Area Graph; but I can't find information on what sort of area graph it is, is it an overlapped area graph or a stacked area graph? or put another way, is the higher point (more or less horizontally centered) value read from the x (seconds) axis (from 0K) or from the top "line" for the lesser area plots (namely from around 4k) So, is the highest value an absolute value taken from the y axis, or that value minus the higher value for the 2nd greatest area plot in the graph? Thank You Very Much in advance for any help! Gilberto Melfe [1] https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/profiling.h…

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(no subject)
by PICCA Frederic-Emmanuel 24 Jan '17

24 Jan '17

Hello, Here a reduction of my problem values :: IO [IO (Maybe Int)] values = do let v = [Just 1, Just 2, Just 3, Nothing, Just 5, Nothing, Just 7] :: [Maybe Int] return $ map return v main :: IO () main = do vs <- values nvs <- mapM_ go vs print nvs where go :: IO (Maybe Int) -> IO Int go v' = do Just v <- v' return v when I run this script, I get a runtime error picca@diffabs6:~/tmp$ runhaskell test.hs test.hs: user error (Pattern match failure in do expression at test.hs:13:10-15) What I want is a go method which skip silently the (IO Nothing) values. so when used in the mapM_ it return only the values which are returned by the IO (Maybe Int) (stored in the values) Thanks for your help Frédéric Indeed

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What is wrong with my types signatures.
by PICCA Frederic-Emmanuel 23 Jan '17

23 Jan '17

Hello here a snipset code class Shape sh => FrameND t sh where shapeND :: t -> MaybeT IO sh rowND :: t -> sh -> MaybeT IO (XrdMeshFrame sh) I have an instance of FrameND then I created a function which use this class framesND :: (Shape sh, FrameND a sh) => Pipe a (XrdMeshFrame sh) IO () framesND = do d <- await sh' <- lift $ runMaybeT $ shapeND d let n = size sh' forM_ [0..n-1] (\i' -> do f <- lift $ runMaybeT $ rowND d (fromIndex sh' i') when (isJust f) (yield (fromJust f))) But when I compile it; I get this error message src/Hkl/XRD.hs:613:55: Could not deduce (sh ~ Maybe a0) from the context (Shape sh, FrameND a sh) bound by the type signature for framesND :: (Shape sh, FrameND a sh) => Pipe a (XrdMeshFrame sh) IO () at src/Hkl/XRD.hs:606:13-70 `sh' is a rigid type variable bound by the type signature for framesND :: (Shape sh, FrameND a sh) => Pipe a (XrdMeshFrame sh) IO () at src/Hkl/XRD.hs:606:13 Expected type: Maybe (XrdMeshFrame sh) Actual type: Maybe (XrdMeshFrame (Maybe a0)) In the first argument of `fromJust', namely `f' In the first argument of `yield', namely `(fromJust f)' In the second argument of `when', namely `(yield (fromJust f))' So I do not undestand why the typse system guess Maybe a0 instead of sh in f If I read this f <- lift $ runMaybeT $ rowND d (fromIndex sh' i') runMaybeT return a (Maybe (XrdMeshFrame sh)) and then I lift it into the Pipe. So What is wrong ? thanks Frederic

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how to skip pattern match error when applying a mapM_
by PICCA Frederic-Emmanuel 18 Jan '17

18 Jan '17

Sorry I forgot to put a subject ________________________________________ De : Beginners [beginners-bounces(a)haskell.org] de la part de PICCA Frederic-Emmanuel Envoyé : mardi 17 janvier 2017 11:15 À : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Objet : [Haskell-beginners] (no subject) Hello, Here a reduction of my problem values :: IO [IO (Maybe Int)] values = do let v = [Just 1, Just 2, Just 3, Nothing, Just 5, Nothing, Just 7] :: [Maybe Int] return $ map return v main :: IO () main = do vs <- values nvs <- mapM_ go vs print nvs where go :: IO (Maybe Int) -> IO Int go v' = do Just v <- v' return v when I run this script, I get a runtime error picca@diffabs6:~/tmp$ runhaskell test.hs test.hs: user error (Pattern match failure in do expression at test.hs:13:10-15) What I want is a go method which skip silently the (IO Nothing) values. so when used in the mapM_ it return only the values which are returned by the IO (Maybe Int) (stored in the values) Thanks for your help Frédéric Indeed _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Feedback needed - CLI app + guide
by Carlo Matteo Scalzo 16 Jan '17

16 Jan '17

Hi all, I'm new to the community here, so please do let me know if I'm doing something wrong :-) I've been learning Haskell over the last year or so, reading books etc., but I couldn't find a simple, real world example, so I decided to to write a simple command-line application in Haskell. The application and the code are free, and I've also written a short guide that uses the application as a use case to talk about Haskell and its features. Can you guys please download the guide (for free, of course) , and let me know if you found it useful? You can find it here: https://fifohaskell.com Please let me have your feedback (my personal email is on the website, if you don't want to reply here) - and thank you very much for your help! Carlo

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safe versions of pred and succ?
by Graham Gill 16 Jan '17

16 Jan '17

Suppose I have a function f :: (Enum a) => a -> ... f x ... = ...pred x ... If a is also an instance of Bounded, then I get a runtime error if pred minBound is evaluated during evaluation of f. If I try to protect the use of pred x with a check using minBound, then I have to add a Bounded constraint on type a, which would mean, for example, that I can no longer use f with type Integer. Do I need two different versions of f, one for Bounded a and one for non-Bounded a? Is there a more elegant way to take care of this problem? I don't know much about all of the type magic available in GHC. For example, from another list message, groupConsecutive :: (Enum a,Eq a) => [a] -> [[a]] groupConsecutive = foldr go [] where go x ls@(hd@(y:_):yss) | x == y || x == pred y = (x:hd):yss | otherwise = [x]:ls go x [] = [[x]] > groupConsecutive [1,2,3,7,8,10,11,12] [[1,2,3],[7,8],[10,11,12]] > groupConsecutive ([1,0,1,2]::[Word]) *** Exception: Enum.pred{Word}: tried to take `pred' of minBound In the first go case, if type a is also Bounded, y == minBound and x /= y, then we know already that x /= pred y, we want the guard to fail and that we pass to the otherwise guard to start a new sublist. But how to implement it without making the function unavailable for un-Bounded types? Graham

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Re: [Haskell-beginners] Beginners Digest, Vol 103, Issue 8
by Saqib Shamsi 16 Jan '17

16 Jan '17

Hello Francesco, Sorry for the late reply. Yes, your solution is elegant and does what I wanted. This is what I did -- zipped is the list of tuples formed as you mentioned λ> groupBy (\x y -> (snd x) == (snd y)) zipped [[(1,0),(2,0),(3,0)],[(7,3),(8,3)],[(10,4),(11,4),(12,4)]] λ> [[fst tpl | tpl <- x] | x <- it] [[1,2,3],[7,8],[10,11,12]] This works perfectly fine. Thank you. Regards, Saqib Shamsi On 14 January 2017 at 00:27, <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Better Code (Saqib Shamsi) > 2. Re: Better Code (Francesco Ariis) > 3. Re: Better Code (Joel Neely) > 4. Re: Better Code (Joel Neely) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 13 Jan 2017 21:35:54 +0530 > From: Saqib Shamsi <shamsi.saqib(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Better Code > Message-ID: > <CAKpYsCGKeWoHttqpZAF7jAkhe++PqLSfwZA820tA=Je8hFiu8g@mail. > gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hi, > > The problem that I wish to solve is to divide a (sored) list of integers > into sublists such that each sublist contains numbers in consecutive > sequence. > > For example, > *Input:* [1,2,3,7,8,10,11,12] > *Output:* [[1,2,3],[7,8],[10,11,12]] > > I have written the following code and it does the trick. > > -- Take a list and divide it at first point of non-consecutive number > encounter > divide :: [Int] -> [Int] -> ([Int], [Int]) > divide first [] = (first, []) > divide first second = if (last first) /= firstSecond - 1 then (first, > second) > else divide (first ++ [firstSecond]) (tail second) > where firstSecond = head second > > > -- Helper for breaking a list of numbers into consecutive sublists > breakIntoConsecsHelper :: [Int] -> [[Int]] -> [[Int]] > breakIntoConsecsHelper [] [[]] = [[]] > breakIntoConsecsHelper lst ans = if two == [] then ans ++ [one] > else ans ++ [one] ++ > breakIntoConsecsHelper two [] > where > firstElem = head lst > remaining = tail lst > (one, two) = divide [firstElem] > remaining > > > -- Break the list into sublists of consective numbers > breakIntoConsecs :: [Int] -> [[Int]] > breakIntoConsecs lst = breakIntoConsecsHelper lst [[]] > > -- Take the tail of the result given by the function above to get the > required list of lists. > > However, I was wondering if there was a better way of doing this. Any help > would be highly appreciated. > > Thank you. > Best Regards, > Saqib Shamsi >

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Better Code
by Saqib Shamsi 13 Jan '17

13 Jan '17

Hi, The problem that I wish to solve is to divide a (sored) list of integers into sublists such that each sublist contains numbers in consecutive sequence. For example, *Input:* [1,2,3,7,8,10,11,12] *Output:* [[1,2,3],[7,8],[10,11,12]] I have written the following code and it does the trick. -- Take a list and divide it at first point of non-consecutive number encounter divide :: [Int] -> [Int] -> ([Int], [Int]) divide first [] = (first, []) divide first second = if (last first) /= firstSecond - 1 then (first, second) else divide (first ++ [firstSecond]) (tail second) where firstSecond = head second -- Helper for breaking a list of numbers into consecutive sublists breakIntoConsecsHelper :: [Int] -> [[Int]] -> [[Int]] breakIntoConsecsHelper [] [[]] = [[]] breakIntoConsecsHelper lst ans = if two == [] then ans ++ [one] else ans ++ [one] ++ breakIntoConsecsHelper two [] where firstElem = head lst remaining = tail lst (one, two) = divide [firstElem] remaining -- Break the list into sublists of consective numbers breakIntoConsecs :: [Int] -> [[Int]] breakIntoConsecs lst = breakIntoConsecsHelper lst [[]] -- Take the tail of the result given by the function above to get the required list of lists. However, I was wondering if there was a better way of doing this. Any help would be highly appreciated. Thank you. Best Regards, Saqib Shamsi

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Re: [Haskell-beginners] Better Code
by Carlo Matteo Scalzo 13 Jan '17

13 Jan '17

Hi Saqib, perhaps something like this? divide :: [Int] -> [[Int]] divide [] = [] divide (x:xs) = divideAux xs [] [x] x divideAux :: [Int] -> [[Int]] -> [Int] -> Int -> [[Int]] divideAux [] result current max = result ++ [current] divideAux (x:xs) result current max = if x - max > 1 then divideAux xs (result ++ [current]) [x] x else divideAux xs result (current ++ [x]) x This can of course be generalised to other types (as per Joel's requirement) just by replacing the check on line 7, i.e., 'x - max > 1'. Happy to explain/talk about the code if you'd like me to :-) Cheers, Carlo On Fri, Jan 13, 2017 at 7:14 PM, Joel Neely <joel.neely(a)gmail.com> wrote: > Had a chance to chat with ghci, so earlier conjecture not confirmed: > > Prelude Data.List> groupBy (\x y -> x == y-1) [1,2,3,7,8,10,11,12] > > [[1,2],[3],[7,8],[10,11],[12]] > > So close but no cigar. > > On Fri, Jan 13, 2017 at 10:05 AM, Saqib Shamsi <shamsi.saqib(a)gmail.com> > wrote: > >> Hi, >> >> The problem that I wish to solve is to divide a (sored) list of integers >> into sublists such that each sublist contains numbers in consecutive >> sequence. >> >> For example, >> *Input:* [1,2,3,7,8,10,11,12] >> *Output:* [[1,2,3],[7,8],[10,11,12]] >> >> I have written the following code and it does the trick. >> >> -- Take a list and divide it at first point of non-consecutive number >> encounter >> divide :: [Int] -> [Int] -> ([Int], [Int]) >> divide first [] = (first, []) >> divide first second = if (last first) /= firstSecond - 1 then (first, >> second) >> else divide (first ++ [firstSecond]) (tail second) >> where firstSecond = head second >> >> >> -- Helper for breaking a list of numbers into consecutive sublists >> breakIntoConsecsHelper :: [Int] -> [[Int]] -> [[Int]] >> breakIntoConsecsHelper [] [[]] = [[]] >> breakIntoConsecsHelper lst ans = if two == [] then ans ++ [one] >> else ans ++ [one] ++ >> breakIntoConsecsHelper two [] >> where >> firstElem = head lst >> remaining = tail lst >> (one, two) = divide [firstElem] >> remaining >> >> >> -- Break the list into sublists of consective numbers >> breakIntoConsecs :: [Int] -> [[Int]] >> breakIntoConsecs lst = breakIntoConsecsHelper lst [[]] >> >> -- Take the tail of the result given by the function above to get the >> required list of lists. >> >> However, I was wondering if there was a better way of doing this. Any >> help would be highly appreciated. >> >> Thank you. >> Best Regards, >> Saqib Shamsi >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> >> > > > -- > Beauty of style and harmony and grace and good rhythm depend on > simplicity. - Plato > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > -- Carlo Matteo Scalzo Mobile: +44 (0) 7934 583582 E-mail: cmscalzo(a)gmail.com

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