- Beginners - Haskell.org

Could not get parser ready
by Marcus Manning 06 Nov '17

06 Nov '17

Hello, I follow the instructions of script [1] in order to set up a parser functionality. But I' get into problems at page 202 with the code: p :: Parser (Char,Char) p = do x ← item item y ← item return (x,y) ghci and ghc throw errors: Prelude> let p:: Parser (Char,Char); p = do {x <- item; item; y <- item; return (x,y)} <interactive>:10:65: error: • Couldn't match type ‘[(Char, String)]’ with ‘Char’ Expected type: String -> [((Char, Char), String)] Actual type: Parser ([(Char, String)], [(Char, String)]) • In a stmt of a 'do' block: return (x, y) In the expression: do x <- item item y <- item return (x, y) In an equation for ‘p’: p = do x <- item item y <- item .... Did the semantics of do expr changed? [1] https://userpages.uni-koblenz.de/~laemmel/paradigms1011/resources/pdf/haske… Cheers, iconfly.

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Problem with Parser combinators
by Baa 03 Nov '17

03 Nov '17

Hello, all! I try this parser for `Read` instance: import qualified Text.ParserCombinators.ReadP as P import qualified Text.ParserCombinators.ReadPrec as RP import Data.Text as T instance Read RV where readPrec = parens $ do RP.lift P.skipSpaces s <- RP.look guard (not $ null s) (RP.+++) (RV1 <$> readPrec::RP.ReadPrec RV) (pure $ RV2 $ T.pack s) And see that `+++` (or <|>) works in strange way (I don't understand what happens). IMHO if first parser fails (i.e. `RV1 <$> readPrec::RP.ReadPrec RV`) then second should return `RV2 $ T.pack s`). But seems that if 1st fails then all fail too! I tried `+++`, `<++`, `<|>` - the same result. How is it possible?! I need to return something if parsing fails. Best is to return another parser but initial problem was that alternative parser must return the same type as first one. So, I decided to return "constant" - here it is `RV2 $ T.pack s`. Guarding from empty string is needed for this alternative value which I'll return when 1st parser fails. But... seems something is wrong. === Best regards, Paul

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How to write Read instance
by Baa 02 Nov '17

02 Nov '17

Hello all! I found some errors in the reading of previously shown big and complex record. Reason is: some of fields are reading wrongly. So, is there any useful documents how to write `Read` instance correctly (because I can't find good tutorial in the Web and often hit errors with `Read` instance)? May be tutorial/examples/any good info... === Best regards, Paul

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What's wrong with Haddock?!
by Baa 26 Oct '17

26 Oct '17

I often hit such errors: parse error on input ‘-- ^ my doc-string is here’ when generate Haddock documentation. This happens on line like: instance Something UTCTime where some a = blahBlah -- ^ my doc-string is here Sometimes such doc-strings are passing, sometimes - not. I can understand what is the reason. Is there some rule how to format such kind of doc-strings? === Cheers, Paul

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Re: [Haskell-beginners] Is it possible such Monad? (Baa)
by 鲍凯文 26 Oct '17

26 Oct '17

Hi, 1. I think your data type is isomorphic to data Allpass w m a = Pass Bool (m a) w 2. Maybe first try to write a Functor instance? Then you can find out what constraints are necessary for 'w' and 'm' to write it. 3. What does this data type intend to represent? Best, toz On Wed, Oct 25, 2017 at 5:32 AM <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. Is it possible such Monad? (Baa) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 25 Oct 2017 13:56:31 +0300 > From: Baa <aquagnu(a)gmail.com> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] Is it possible such Monad? > Message-ID: <20171025135631.1f9913d2@Pavel> > Content-Type: text/plain; charset=US-ASCII > > Hello All! > > Is it possible to write Monad for such type: > > data Allpass w m a = Nopass (m a) w | Allpass (m a) w > > I can write (>>=), IMHO such type can not be Monad due to `w` is not > under `m` monad, right? > > > === > Best regards, Paul > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 112, Issue 22 > ****************************************** >

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Is it possible such Monad?
by Baa 25 Oct '17

25 Oct '17

Hello All! Is it possible to write Monad for such type: data Allpass w m a = Nopass (m a) w | Allpass (m a) w I can write (>>=), IMHO such type can not be Monad due to `w` is not under `m` monad, right? === Best regards, Paul

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Manipulate list, one element at a time
by martin 24 Oct '17

24 Oct '17

Hello all, How can I do something to each element of a list, but only modify one element at at time, so the result is a list of lists, where each sublist is the original list with one element altered. Something like type Each a = (a->a) -> [a] -> [[a]] I came up with: each :: Each a each f [] = [] each f (x:xs) = (f x : xs) : (map (x:) $ each f xs) λ> each (*10) [1..3] [[10,2,3],[1,20,3],[1,2,30]] but I wonder if there is a more standard way of doing this

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runState (liftM (+100) pop) [1, 2, 3, 4] (Example from LYH)
by Olumide 24 Oct '17

24 Oct '17

Dear List, Apologies for the awkward question title -- its the best I can do ATM. I'm still working my way through chapter 13 of LYH (title for a few monads more) and I came across the following two beasts ghci> runState (liftM (+100) pop) [1,2,3,4] (101,[2,3,4]) ghci> runState (fmap (+100) pop) [1,2,3,4] (101,[2,3,4]) See http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions Even though I'm still struggling to wrap my mind around monads, I sort of understand what's going on here. The problem is that I can't explain why the function (+100) is applied to _only_ the value 1 in (1,[2,3,4]). Regards, - Olumide

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Where clause indentation
by Wink Saville 21 Oct '17

21 Oct '17

I created a file with the dividedBy example from Chapter 8.5 of "Haskell Programming from first principles" : dividedBy :: Integral a => a -> a -> (a, a) dividedBy num denom = go num denom 0 where go n d count | n < d = (count, n) | otherwise = go (n - d) d (count + 1) I get the following error when I load into ghci: $ ghci chapter8_5-IntegralDivision.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/wink/.ghci [1 of 1] Compiling Main ( chapter8_5-IntegralDivision.hs, interpreted ) chapter8_5-IntegralDivision.hs:4:7: error: parse error (possibly incorrect indentation or mismatched brackets) | 4 | | n < d = (count, n) | ^ Failed, 0 modules loaded. λ> But if I put the "go" function on its own line: dividedBy :: Integral a => a -> a -> (a, a) dividedBy num denom = go num denom 0 where go n d count | n < d = (count, n) | otherwise = go (n - d) d (count + 1) It does compile: $ ghci chapter8_5-IntegralDivision.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/wink/.ghci [1 of 1] Compiling IntegralDivision ( chapter8_5-IntegralDivision.hs, interpreted ) Ok, 1 module loaded. Or I can put the "where" on the previous line: dividedBy :: Integral a => a -> a -> (a, a) dividedBy num denom = go num denom 0 where go n d count | n < d = (count, n) | otherwise = go (n - d) d (count + 1) it also compiles: $ ghci chapter8_5-IntegralDivision.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/wink/.ghci [1 of 1] Compiling Main ( chapter8_5-IntegralDivision.hs, interpreted ) Ok, 1 module loaded. λ> Can someone shed light on what I've done wrong? -- Wink

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Hack “Importify” by Serokell During #Hacktoberfest!
by Gints Dreimanis 18 Oct '17

18 Oct '17

All of us love to read explicit import lists (especially in an unknown module), but we all hate to write and maintain them. >From our point of view, it is obligatory for any code that goes into production to have only explicit imports. That way, the code is easier to comprehend – no one is confused about the location of a stray type or identifier, even when reading from GitHub or a simple editor. There are, however, usability and maintainability issues with explicit imports, which make enforcing explicit imports in style guides a topic for discussion. To enjoy the benefits of explicit imports while keeping the drawbacks in check, we have implemented *Importify*. *Importify* is a tool developed by Serokell tooling team led by Dmitry Kovalnikov. It is a refactoring tool that helps import management in Haskell code. We’re getting close to releasing it on Hackage, but it requires some polishing. You can contribute to Importify <https://github.com/serokell/importify/issues?q=is%3Aissue+is%3Aopen+label%3…> and Serokell’s own custom Prelude Universum <https://github.com/serokell/universum/issues?q=is%3Aissue+is%3Aopen+label%3…> during Hacktoberfest <https://github.com/blog/2433-celebrate-open-source-this-october-with-hackto…> . *Importify* is all about decluttering the import blocks. It can remove: - unused symbols in explicit import lists - unused qualified imports - unused implicit imports - unnecessarily hidden symbols Highlights of the functionality of *Importify* are that it works deeply with explicit import lists (it can, for instance, remove unused constructors), and it works with hiding imports. The latter feature isn’t present in any open-source refactoring tool that is known to us. It is also compatible with autoexporter <https://hackage.haskell.org/package/autoexporter>. *Importify *works extremely well with large Haskell projects — we use it to great success to clean up Cardano SL <https://github.com/input-output-hk/cardano-sl/>. Importify only parses source code instead of building projects. For a project of the size of Cardano SL it takes only two minutes to build Importify cache while building it would take forty minutes on the same machine. In the future, we plan for *Importify *to be able to add used symbols to imports automatically. To speed up *Importify *runs, we will implement a cache server that caches each Hackage package. You will be able to upload caches for your projects there as well. Next on the roadmap is functionality to convert imports between *implicit* and *explicit*, and between *qualified* and *unqualified* forms. Finally, we will be adding functionality to resolve merge conflicts in import section automatically. If you notice that managing imports becomes a slog, feel free to try this tool out yourself. Spotted a bug? Contact us. <bugs(a)serokell.io> To those, who are in the mood for some hacking this Autumn, we’re looking forward to your #Hacktoberfest contributions!

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