November 2017 - Beginners

Non-exhaustive patterns
by Jim 18 Nov '17

18 Nov '17

Hey guys, Thiis is my function below: notThe :: String -> Maybe String notThe word | word /= "the" = Just word | word == [] = Just [] | otherwise = Nothing replaceThe :: String -> String replaceThe word = go (words word) where go (x:xs) | notThe x == Just [] = [] | notThe x == Just word = word ++ go xs | notThe word == Nothing = " a " ++ go xs > replaceThe "what" "what*** Exception: chap12.hs:(13,13)-(16,55): Non-exhaustive patterns in function go I thought I covered all the potential patterns in my replaceThe function. I'm not sure what pattern I've missed, any thoughts? Best regards, Jim

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argument counts...
by Gregory Guthrie 17 Nov '17

17 Nov '17

I have a simple function: revFn (x:xs) = (revFn xs) . (x:) Of course GHCi correctly infers the type as: revFn :: [a] -> [a] -> c Adding the base case: revFn [] xs = xs Now gives an error; "Equations for 'revF' have different numbers of arguments" Of course this can be "fixed" by either adding the cancelled argument to the first clause, or converting the base case to only have one explicit argument, and a RHS of a lambda or identity function. But since the interpreter already correctly inferred that the first clause has two arguments (with only one explicit), why does it then ignore this and give an error when the second clause shows two explicit arguments? The types are all correct in either case - why require explicit arguments? Or, perhaps I am missing something simple?

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Re: [Haskell-beginners] Relation between Effects, Indexed monads, Free monads (PY)
by 鲍凯文 16 Nov '17

16 Nov '17

Hi, Disclaimer: I am not a lawyer. Nor do I claim to understand all this stuff. The enforcement of sequential ordering is sort of baked into monads; the haskell (read: do-notation) way of combining monadic computations is using bind (i.e. in the "fancy list of instructions with a way to bind results"). This is only for the structure of the computation, though. Every monadic computation is still a pure function, so in the end the RTS, well, executes parts of those at a time, just as pure functions. The ordering guarantee that more corresponds to imperative programming is most obvious (to me) with ST, IO, and State; data dependencies ensure that things "later" in the monadic chain cannot be executed until everything "before". But those effects all involve (mutable) state. Monadic chaining, i.e. combining effectful computations, is separate from the concept of mutable state, etc. It seems more like just an algebra for effects. If you're talking about marked monads as in those with extra type information or phantom params, like when used in tandem with the rank-2 type trick (used in ST, region-based IO, etc.), I think that's a little different, or at least beyond what monads are. Those were created to handle things like guaranteeing that state is only modified single-threadedly (ST), or to ensure that you can't do things like forget to close a handle or leak a ref to a closed one (regions). I think Free monads and (Kiselyov's) Eff monad are still different. They still appeal to the "monads as sequence of instructions" view, but uh, especially with the Eff monad, they're pretty fancy. As in at least personally, for "normal" programming, I haven't felt the need to delve into them. I think the important upshot is: what are you working on? Do monadic computations model what you want to do naturally? If they do, do you need something stronger than what they provide? It's hard to answer the latter questions without answering the first. Sorry for rambling, toz On Wed, Nov 15, 2017 at 4:00 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. Relation between Effects, Indexed monads, Free monads (PY) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Wed, 15 Nov 2017 14:15:13 +0200 > From: PY <aquagnu(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Relation between Effects, Indexed monads, > Free monads > Message-ID: <f34c805e-bad4-845e-d09b-43d9a9baea13(a)gmail.com> > Content-Type: text/plain; charset=utf-8; format=flowed > > Hello, All! > > I'm not sure is it question for Cafe or Beginners List, so first I'll > try here. > > There are kind of errors related to wrong execution sequence. They are > good known in OOP: object keeps state internally and check it before to > execute got message - to prevent wrong execution sequence. Best > implementation is: state machine, also can be used Markov's algorithm, > more complex can be done with Petri nets, etc. > > Example: if I have complex inserting into DB (consisting of several > inserts), I can check done of previous step before to start next, etc. > After read of different Haskell resources I found solution for it: > Indexed Monads. They (if I got it rightly) are monads with additional > type parameter which "marks" monad, gives it more specific > qualification, for example instead of monad "Opening" (action) we have > monad "Opening-of-closed", exactly what I'm talking: type-level check of > correct actions sequence (allowed transition). > > After more research I found Free Monads and Effects and something like > "free monad can be used to proof your program". How?! Free monads make > "active" (executable) code into "passive" code (data instead of code), > which can be interpreted separately, so program becomes like Lisp > program: code is a data (so, can be modified, checked, etc, etc) and > what exactly will be executing - decides interpreter of such code. But > do they allow to proof right sequence of actions in such data-like code? > > What are the Effects I don't understand yet. > > Somewhere I find something like: Eff = Free Monad + indexed monad (may > be I'm not right here). So my questions are: > > - how Effects, Free Monad, Indexed Monads are related? > > - can effects replace indexed monads? > > - are indexed monad yet usable/modern concept or they are abandoned and > replaced by Effects or something else? Do you use indexed monads in real > projects? > > - can I (and how) to use both of them? Or: can I use only FreeMonads / > Effects to solve the same problem (control of correct sequence of > actions) like with indexed monads help? > > > === > > 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 113, Issue 10 > ****************************************** >

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ANNOUNCE: Haskell Communities and Activities Report (33rd ed., November 2017)
by Mihai Maruseac 15 Nov '17

15 Nov '17

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (33rd edition, November 2017) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2017/report.pdf http://haskell.org/communities/11-2017/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of February 2018: target deadline for contributions to the May 2018 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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Relation between Effects, Indexed monads, Free monads
by PY 15 Nov '17

15 Nov '17

Hello, All! I'm not sure is it question for Cafe or Beginners List, so first I'll try here. There are kind of errors related to wrong execution sequence. They are good known in OOP: object keeps state internally and check it before to execute got message - to prevent wrong execution sequence. Best implementation is: state machine, also can be used Markov's algorithm, more complex can be done with Petri nets, etc. Example: if I have complex inserting into DB (consisting of several inserts), I can check done of previous step before to start next, etc. After read of different Haskell resources I found solution for it: Indexed Monads. They (if I got it rightly) are monads with additional type parameter which "marks" monad, gives it more specific qualification, for example instead of monad "Opening" (action) we have monad "Opening-of-closed", exactly what I'm talking: type-level check of correct actions sequence (allowed transition). After more research I found Free Monads and Effects and something like "free monad can be used to proof your program". How?! Free monads make "active" (executable) code into "passive" code (data instead of code), which can be interpreted separately, so program becomes like Lisp program: code is a data (so, can be modified, checked, etc, etc) and what exactly will be executing - decides interpreter of such code. But do they allow to proof right sequence of actions in such data-like code? What are the Effects I don't understand yet. Somewhere I find something like: Eff = Free Monad + indexed monad (may be I'm not right here). So my questions are: - how Effects, Free Monad, Indexed Monads are related? - can effects replace indexed monads? - are indexed monad yet usable/modern concept or they are abandoned and replaced by Effects or something else? Do you use indexed monads in real projects? - can I (and how) to use both of them? Or: can I use only FreeMonads / Effects to solve the same problem (control of correct sequence of actions) like with indexed monads help? === Best regards, Paul

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Re: [Haskell-beginners] Beginners Digest, Vol 113, Issue 8
by nikhil kaushik 14 Nov '17

14 Nov '17

Thank you all for the guidance. ~ Nikhil On Tue, Nov 14, 2017 at 5:30 PM, <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. Re: Resource-recommendation (Frerich Raabe) > 2. Re: Resource-recommendation (Steven Leiva) > 3. Re: Resource-recommendation (Fabien R) > > > ---------- Forwarded message ---------- > From: Frerich Raabe <raabe(a)froglogic.com> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Cc: > Bcc: > Date: Mon, 13 Nov 2017 13:50:09 +0100 > Subject: Re: [Haskell-beginners] Resource-recommendation > On 2017-11-13 12:00, nikhil kaushik wrote: > >> Please recommended some books considering my situation. >> > > I very much recommend Graham Hutton's "Programming in Haskell", see > > http://www.cs.nott.ac.uk/~pszgmh/pih.html > > -- > Frerich Raabe - raabe(a)froglogic.com > www.froglogic.com - Multi-Platform GUI Testing > > > > ---------- Forwarded message ---------- > From: Steven Leiva <leiva.steven(a)gmail.com> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Cc: > Bcc: > Date: Mon, 13 Nov 2017 14:25:58 +0000 > Subject: Re: [Haskell-beginners] Resource-recommendation > Chris Allen (co-author of Haskell From First Principles) has a post > <https://github.com/bitemyapp/learnhaskell> regarding exactly this > question. > > Don't get too bogged down in the details. The only reason I bring it up is > because he suggests the spring 13 > <https://www.seas.upenn.edu/~cis194/spring13/> version of CIS 194. > Haven't taken either though (and maybe he just hasn't seen the new one??). > > - Steven > > > > On Mon, Nov 13, 2017 7:50 AM, Frerich Raabe raabe(a)froglogic.com wrote: > >> On 2017-11-13 12:00, nikhil kaushik wrote: >> >> > Please recommended some books considering my situation. >> >> >> I very much recommend Graham Hutton's "Programming in Haskell", see >> >> >> http://www.cs.nott.ac.uk/~pszgmh/pih.html >> >> >> -- >> >> Frerich Raabe - raabe(a)froglogic.com >> >> www.froglogic.com - Multi-Platform GUI Testing >> >> _______________________________________________ >> >> Beginners mailing list >> >> Beginners(a)haskell.org >> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> >> >> > > Steven Leiva > 305.528.6038 > leiva.steven(a)gmail.com > http://www.linkedin.com/in/stevenleiva > > > ---------- Forwarded message ---------- > From: Fabien R <theedge456(a)free.fr> > To: beginners(a)haskell.org > Cc: > Bcc: > Date: Tue, 14 Nov 2017 08:25:55 +0100 > Subject: Re: [Haskell-beginners] Resource-recommendation > On 13/11/2017 12:00, nikhil kaushik wrote: > > Hello, > > I have just started learning Haskell and I am really enjoying it. As I do > > not have a good mathematical background and a little experience with > > programming in general, > Since you have a math background, this book may suit: > The Haskell Road to Logic, Math and Programming > > -- > Fabien > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > -- ~Nikhil Kaushik

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Resource-recommendation
by nikhil kaushik 14 Nov '17

14 Nov '17

Hello, I have just started learning Haskell and I am really enjoying it. As I do not have a good mathematical background and a little experience with programming in general, which book is recommended for me? I started learning haskell from LYAH website and I completed 6 chapters but there are no exercises in that book so I want a book which is beginner friendly and has exercises. Please recommended some books considering my situation. Thank you -- ~Nikhil Kaushik

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Type constructor
by mike h 08 Nov '17

08 Nov '17

Hi, I’m modelling fractions with a view to looking at continued fractions and I have this recursive structure. type Numerator = Integer data Fraction = Numbr Integer | F Numerator Fraction in ghci I do λ-> :t F 1 (Numbr 2) F 1 (Numbr 2) :: Fraction which is fine. But what surprised me is that it also works without using Numbr e.g. λ-> :t F 1 2 F 1 2 :: Fraction why is this? Thanks Mike

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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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