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best way to code this
by briand＠aracnet.com 03 Jun '15

03 Jun '15

Hi, A simple example of something I was trying to do but it had some questions along the lines of "the best way to do this". Given a list l = [a] and an a-list alist = [ (a,b) ] the idea is to find all the items in l which are in alist and then create a list [b] so the overall function should be [a] -> [ (a,b) ] -> [b] the solution is straightforward: l1 = filter (\x -> isJust (lookup x alist)) l l2 = map (\x -> fromJust (lookup x alist)) l1 `fromJust` used in the construction of l2 won't fail, because only the elements for which the lookup succeeded are in l1. This would be something called `filterMap` but I couldn't find such a function in the list library, but it seems like there just has to be one defined in a library somewhere. the above seems clumsy, i'm wondering how to make it "more pretty". generally i was also wondering if the above construction is as inefficient as it looks because of the double-lookup, or would the compiler actually be able to optimize that code into something more efficient ? this code is not being used on large sets of data so efficiency doesn't matter, I'm just curious. Thanks, Brian

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Re: [Haskell-beginners] [Haskell-cafe] I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this?
by Christopher Allen 01 Jun '15

01 Jun '15

Strongly agreed with ok and Tikhon. I've tried these weird pidgins and localized fabrications wrapped around Haskell before, it's only a stumbling block. Haskell is simple, you don't have to lie - just encourage them not to draw false analogies. On Mon, Jun 1, 2015 at 7:52 PM, Tikhon Jelvis <tikhon(a)jelv.is> wrote: > If you teach them about how operators are normal functions in Haskell, > using an operator to index into an array makes the indexing operation less > magical—a big pedagogical boon, in my view. The fewer special cases in the > language you're teaching, the better, and looking similar to other > languages is not a good reason for a special case. > > Especially since the similarity could be misleading!) > > On Mon, Jun 1, 2015 at 5:48 PM, <ok(a)cs.otago.ac.nz> wrote: > >> > I'm a part time tutor even though I don't look Elizabethan >> > >> > I was trying to lower the learning curve for students >> >> Using square brackets for array indexing in Haskell >> would be more a case of putting a stumbling block in >> their way than lowering the learning curve. >> >> Fortran uses A(I), not A[I], and has for the last fifty-some >> years. The official definition of Simula 67 uses A(I) as >> well, despite its predecessor Algol 60 using a[i]. COBOL >> uses A(I), and has done so nearly as long as Fortran. PL/I >> (yes, it still exists) uses A(I), not A[I]. BASIC uses >> A(I), this is still so in Visual Basic.NET. If memory >> serves me correctly, MINITAB uses parentheses, not brackets. >> >> Is a pattern beginning to emerge? >> >> Lying to the students by implicitly telling them "all programming >> languages use square brackets for array indexing" will be doing >> them no favour. Even lying about Haskell is no kindness. >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-beginners] [Haskell-cafe] I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this?
by KC 01 Jun '15

01 Jun '15

On Jun 1, 2015 5:52 PM, "Tikhon Jelvis" <tikhon(a)jelv.is> wrote: If you teach them about how operators are normal functions in Haskell, using an operator to index into an array makes the indexing operation less magical—a big pedagogical boon, in my view. The fewer special cases in the language you're teaching, the better, and looking similar to other languages is not a good reason for a special case. Especially since the similarity could be misleading!) Good point (s) -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Re: [Haskell-beginners] [Haskell-cafe] I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this?
by KC 01 Jun '15

01 Jun '15

Then a tuple might work better as input to a function to index into an array e.g. myArray (5) -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey On Jun 1, 2015 1:21 PM, "Tikhon Jelvis" <tikhon(a)jelv.is> wrote: > You could make myArray a function that takes a list as an input. Of > course, all your other array functions have to account for this too. A > potential advantage is that this approach leaves the underlying array type > abstract, so you could mix and match different data structure on the > backend. (IntMap, Array, Vector… etc.) > > A disadvantage is that this is non-standard usage which could be confusing > to people and there's no way I know of to statically ensure the list passed > in always had one element. That is, myArray [1, 2] would be legal and > result in a runtime error. > > I don't know of a way to do it while using a normal array type directly. > > On Mon, Jun 1, 2015 at 1:17 PM, KC <kc1956(a)gmail.com> wrote: > >> I think someone had a complicated program to use brackets for array >> indexing - is it possible to use a DSL for this? >> >> That is, to use myArray [5] and have a DSL convert it to standard Haskell >> syntax >> >> -- >> -- >> >> Sent from an expensive device which will be obsolete in a few months! :D >> >> Casey >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> >> >

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I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this?
by KC 01 Jun '15

01 Jun '15

I think someone had a complicated program to use brackets for array indexing - is it possible to use a DSL for this? That is, to use myArray [5] and have a DSL convert it to standard Haskell syntax -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Command line interface programming
by Dananji Liyanage 01 Jun '15

01 Jun '15

Hi All, I'm in the process of learning Haskell, and I need to write a command line interface program which can interact with the user. Right now, I have a sample program written using 'System.Console.Haskeline' package. Can someone please give me some additional material, which I can read on with respect to this? Thanks in advance! -- Regards, Dananji Liyanage

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ANNOUNCE: Haskell Communities and Activities Report (28th ed., May 2015)
by Mihai Maruseac 29 May '15

29 May '15

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (28th edition, May 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2015/report.pdf http://haskell.org/communities/05-2015/html/report.html 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 September 2015: target deadline for contributions to the November 2015 edition of the HC&A 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 October (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 and Alejandro Serrano Mena <hcar at haskell.org> -- 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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Re: [Haskell-beginners] Continuations
by Shishir Srivastava 29 May '15

29 May '15

Hi Amy Thanks for explanation. Yes I can now see that equivalence relation between your example and mine. It does sometime feel alien to see variables representing partially applied functions (e.g k in this case) when you are coming from the imperative land. Cheers, Shishir Srivastava

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Continuations
by Shishir Srivastava 29 May '15

29 May '15

Hi, Reading on continuation I've came across this new style of creating the functions which I guess is not very clear in how it works --------------- Prelude> let add_cps x y = \k -> k (x+y) Prelude> add_cps 3 4 $ print 7 --------------- I have some questions as to 1) what is the role of variable 'k' and what eventually happens to it. 2) How does print work after the $ because there is clearly no parameter being passed to it. Thanks, Shishir Srivastava

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Data.Vector in Haskell
by Dananji Liyanage 28 May '15

28 May '15

Hi All, I'm trying to extract elements from a Vector using 'head' and 'tail' function in Data.Vector package, but it gives me the following error now. <interactive>:41:18: Couldn't match expected type ‘Data.Vector.Vector a’ with actual type ‘vector-0.10.9.1:Data.Vector.Vector Int’ NB: ‘Data.Vector.Vector’ is defined in ‘Data.Vector’ in package ‘vector-0.10.12.3’ ‘vector-0.10.9.1:Data.Vector.Vector’ is defined in ‘Data.Vector’ in package ‘vector-0.10.9.1’ Relevant bindings include it :: a (bound at <interactive>:41:1) In the first argument of ‘Data.Vector.head’, namely ‘it’ In the expression: Data.Vector.head it It was working fine before. Any help would be appreciated. Thanks in advance! -- Regards, Dananji Liyanage

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