December 2016 - Haskell-Cafe

ANN: Free hands-on Haskell workshop in Augsburg, Germany on 2012-12-17 [with German videos]
by Ingo Blechschmidt 08 Dec '16

08 Dec '16

Dear lovers of generalized abstract nonsense, this mail is directed at Haskell neophytes, novices, and advanced learners living near Augsburg, Germany; and at Haskell aficionados living all over the world but speaking German. On 2012-12-17, we -- the Curry Club Augsburg -- will organize a free all-day hands-on workshop on Haskell. Join us in the morning without having seen a single line of Haskell code and leave with working knowledge of Haskell's basics. If you already know a bit of Haskell, then learn your ways with Haskell's vast library ecosystem, how to structure bigger programs, how to check your code for correctness, and how to write parsers/websites/numerical simulations/user interfaces/two- and three-dimensional games. So that we don't exceed our capacity for individual mentoring during the workshop, a mandatory but nonbinding registration is necessary. [1] In case that you want to dive deeper into Haskell and other functional languages after having participated in the workshop, you are invited to join us at our monthly meetings. There are typically two to three talks on a wide range of topics of interest to Haskell programmers, such as latest news from the Kmettiverse, glimpses into formal logic and proof assistants like Isabelle, and introductions to the category-theoretic background of freer monads or to infinite-time Turing machines and the effective topos. All of our talks are recorded [2]. Our workshop documents are CC-licensed [3]. Feel free to copy them, modify them, and host Haskell workshops at other places! Cheers, Ingo of Augsburg's Curry Club [1] http://curry-club-augsburg.de/posts/2016-12-06-dritter-workshop.html [2] https://www.youtube.com/channel/UCKV6TVS9DWaEix6l3_l457Q [3] https://rawgit.com/curry-club-aux/haskell-workshop/gh-pages/uebung.pdf https://rawgit.com/curry-club-aux/haskell-workshop/gh-pages/uebung2.pdf http://curry-club-augsburg.de/posts/2015-10-27-monaden-in-haskell.html (All hail the monad tutorial fallacy. English translation by Harry Garrood at https://gist.github.com/iblech/429652a31b05e408262b.)

1 0

Injective type family on a sub domain
by Guillaume Bouchard 08 Dec '16

08 Dec '16

Hi. I have the following GADT : ----------------------------- type family Or a b where Or 'False 'False = 'False Or _ _ = 'True data Expr t where Add :: Expr t -> Expr t' -> Expr (Or t t') Lit :: Int -> Expr 'False Var :: Expr 'True ---------------------- The idea is that if the `Expr` contains a sub `Var`, its type is `Expr 'True`, else it is `Expr 'False`. I now want to evaluate my expression, something like that : -------------- eval :: Expr t -> Int eval (Lit i) = i eval (Add a b) = eval a + eval b eval Var = error "Cannot evaluate expression with variable" ---------------- Using the GADT I previously defined, I'm tempted to remove the impossible "Var" case with : --------------- eval' :: Expr 'False -> Int eval' (Lit i) = i eval' (Add a b) = eval' a + eval' b ---------------- However this does not typecheck because GHC cannot deduce that `a` and `b` are `~ Expr 'False`. Because the type family `Or` is not injective. The wiki https://ghc.haskell.org/trac/ghc/wiki/InjectiveTypeFamilies classifies injectives types families in three categories, but I don't think my `Or` appears in any of them. Actually `Or` is injective only if `Or t t' ~ 'False` and in this case, we can deduce that `t ~ 'False` and `t' ~ 'False`. I qualify it as a "partialy injective type family". The type checker does not know that, hence my code does not compile. Is there a solution, other than writing a custom type checker plugin? Is there a way to provide the inverse type family function, something such as: type family InverseOr a where InverseOr 'False = ('False, 'False) Thank you. -- G.

4 7

Re: [Haskell-cafe] Gitter Haskell Community
by Ben Spencer 08 Dec '16

08 Dec '16

I think it's important to recognize that users have different tastes and needs. Additionally, Gitter allows you to view the chat channel if you wish. It's also something that's publicly viewable and searchable from Gitter explore. It doesn't require the odd signup process that Slack has implemented. I can actually see my chat history for more than 2 days! But beyond that if you like Slack and its community then more power to you. We can certainly have a difference of opinion and preferences. -Ben On Dec 07, 2016, at 12:09 PM, Michael Walker <mike(a)barrucadu.co.uk> wrote: The Haskell slack seems to be working well, so regardless of the suitability of slack to open communities or not, I don't really see the advantage of fragmenting the community even further. Why would someone pick gitter over the two existing platforms that have thousands of users? They would be intentionally limiting the help they can get. On 7 December 2016 at 17:07, Ben Spencer <ben.richard.spencer(a)me.com> wrote: Hey Chris, As I've mentioned Slack isn't a good choice for open communities. It's really designed for businesses. On Dec 07, 2016, at 11:52 AM, Christopher Allen <cma(a)bitemyapp.com> wrote: For that, there's http://fpchat.com/ which is an established Slack community. The #haskell channel alone has 1,208 people in it right now. On Wed, Dec 7, 2016 at 10:50 AM, Tomas Carnecky <tomas.carnecky(a)gmail.com> wrote: Usability matters. It's easier to tell people to open a browser window and point them at a URL than tell them to download an IRC chat client and how to connect to the server and... On Wed, Dec 7, 2016 at 5:41 PM William Yager <will.yager(a)gmail.com> wrote: What are the advantages of this over the #haskell IRC on freenode? It's very active, usually with over 1500 nicks at any given time. I generally prefer IRC to any of these hip web chat solutions because IRC is client-agnostic and very rugged against companies folding or deciding they don't want to host a project any more. Basically the only way to kill an IRC channel is through social attrition, whereas any social value built up in hosted chat services might disappear overnight. The one major advantage of hosted chats over IRC is that they work better with mobile users, but I don't think that's very relevant for haskell dev. Will On Wed, Dec 7, 2016 at 7:34 AM, Ben Spencer <ben.richard.spencer(a)me.com> wrote: Why Gitter you might ask? _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. -- Chris Allen Currently working on http://haskellbook.com _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. -- Michael Walker (http://www.barrucadu.co.uk)

9 11

Re: [Haskell-cafe] Gitter Haskell Community
by Justin Wood 07 Dec '16

07 Dec '16

Ben, Please make sure that you use reply all in the future so that the message does not go just to me. For everyone else on the list, this was the message > That comes back to usability. The client functions but it is not appealing. Personal taste, I suppose. I agree that freenode webchat is not the most appealing piece of software, but there are plenty of web based IRC clients. I'm sure there are some that are appealing. On Wed, Dec 7, 2016 at 11:56 AM, Ben Spencer <ben.richard.spencer(a)me.com> wrote: > That comes back to usability. The client functions but it is not > appealing. Personal taste, I suppose. > > On Dec 07, 2016, at 11:55 AM, Justin Wood <justin.k.wood(a)gmail.com> wrote: > > > It's easier to tell people to open a browser window and point them at a > URL > > You could always point someone to https://webchat.freenode.net/ or any > other web based IRC client. > > On Wed, Dec 7, 2016 at 11:51 AM, Christopher Allen <cma(a)bitemyapp.com> > wrote: > >> For that, there's http://fpchat.com/ which is an established Slack >> community. The #haskell channel alone has 1,208 people in it right >> now. >> >> On Wed, Dec 7, 2016 at 10:50 AM, Tomas Carnecky >> <tomas.carnecky(a)gmail.com> wrote: >> > Usability matters. It's easier to tell people to open a browser window >> and >> > point them at a URL than tell them to download an IRC chat client and >> how to >> > connect to the server and... >> > >> > On Wed, Dec 7, 2016 at 5:41 PM William Yager <will.yager(a)gmail.com> >> wrote: >> >> >> >> What are the advantages of this over the #haskell IRC on freenode? It's >> >> very active, usually with over 1500 nicks at any given time. >> >> >> >> I generally prefer IRC to any of these hip web chat solutions because >> IRC >> >> is client-agnostic and very rugged against companies folding or >> deciding >> >> they don't want to host a project any more. Basically the only way to >> kill >> >> an IRC channel is through social attrition, whereas any social value >> built >> >> up in hosted chat services might disappear overnight. >> >> >> >> The one major advantage of hosted chats over IRC is that they work >> better >> >> with mobile users, but I don't think that's very relevant for haskell >> dev. >> >> >> >> Will >> >> >> >> On Wed, Dec 7, 2016 at 7:34 AM, Ben Spencer < >> ben.richard.spencer(a)me.com> >> >> wrote: >> >>> >> >>> Why Gitter you might ask? >> >> >> >> _______________________________________________ >> >> Haskell-Cafe mailing list >> >> To (un)subscribe, modify options or view archives go to: >> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> >> Only members subscribed via the mailman list are allowed to post. >> > >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > To (un)subscribe, modify options or view archives go to: >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > Only members subscribed via the mailman list are allowed to post. >> >> >> >> -- >> Chris Allen >> Currently working on http://haskellbook.com >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > >

1 0

Re: [Haskell-cafe] Gitter Haskell Community
by Ben Spencer 07 Dec '16

07 Dec '16

Hey Chris, As I've mentioned Slack isn't a good choice for open communities. It's really designed for businesses. On Dec 07, 2016, at 11:52 AM, Christopher Allen <cma(a)bitemyapp.com> wrote: For that, there's http://fpchat.com/ which is an established Slack community. The #haskell channel alone has 1,208 people in it right now. On Wed, Dec 7, 2016 at 10:50 AM, Tomas Carnecky <tomas.carnecky(a)gmail.com> wrote: Usability matters. It's easier to tell people to open a browser window and point them at a URL than tell them to download an IRC chat client and how to connect to the server and... On Wed, Dec 7, 2016 at 5:41 PM William Yager <will.yager(a)gmail.com> wrote: What are the advantages of this over the #haskell IRC on freenode? It's very active, usually with over 1500 nicks at any given time. I generally prefer IRC to any of these hip web chat solutions because IRC is client-agnostic and very rugged against companies folding or deciding they don't want to host a project any more. Basically the only way to kill an IRC channel is through social attrition, whereas any social value built up in hosted chat services might disappear overnight. The one major advantage of hosted chats over IRC is that they work better with mobile users, but I don't think that's very relevant for haskell dev. Will On Wed, Dec 7, 2016 at 7:34 AM, Ben Spencer <ben.richard.spencer(a)me.com> wrote: Why Gitter you might ask? _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. -- Chris Allen Currently working on http://haskellbook.com

2 1

Re: [Haskell-cafe] Gitter Haskell Community
by Ben Spencer 07 Dec '16

07 Dec '16

Hey William, I'll speak from my experience. I've noticed that the developers I work with (mostly younger developers) have embraced tech like Github, Slack, and Gitter. When I first tried Slack I didn't like it much but now I understand the appeal. I remember trying IRC a long time ago. It seemed to work for the time but for whatever reason I didn't take to it. This part is my opinion, but if a community like Haskell wants to gain wider appeal we have to meet new people where they are at. Most people know about Github and like Github. A solution like Gitter integrates nicely. Slack isn't a good option because it's designed for businesses in mind. The chat history is limited and you have to pay for extra features. IRC is a bit aged. It doesn't make it bad but newer developers are interested in the new shiny technology. I've also seen a few posts that in order to get it to work well (ie having a chat history) you have to have a hosted service. That's overhead I rather not have. But more importantly, we should meet the community where they're at and that's on Github. Best, Ben Spencer On Dec 07, 2016, at 11:41 AM, William Yager <will.yager(a)gmail.com> wrote: What are the advantages of this over the #haskell IRC on freenode? It's very active, usually with over 1500 nicks at any given time. I generally prefer IRC to any of these hip web chat solutions because IRC is client-agnostic and very rugged against companies folding or deciding they don't want to host a project any more. Basically the only way to kill an IRC channel is through social attrition, whereas any social value built up in hosted chat services might disappear overnight. The one major advantage of hosted chats over IRC is that they work better with mobile users, but I don't think that's very relevant for haskell dev. Will On Wed, Dec 7, 2016 at 7:34 AM, Ben Spencer <ben.richard.spencer(a)me.com> wrote: Why Gitter you might ask?

1 0

Re: [Haskell-cafe] Gitter Haskell Community
by Ben Spencer 07 Dec '16

07 Dec '16

Hello Tomas, I agree usability does matter. On Dec 07, 2016, at 11:55 AM, Ben Spencer <ben.richard.spencer(a)me.com> wrote: Hello Tomas, I agree usability does matter. On Dec 07, 2016, at 11:51 AM, Tomas Carnecky <tomas.carnecky(a)gmail.com> wrote: Usability matters. It's easier to tell people to open a browser window and point them at a URL than tell them to download an IRC chat client and how to connect to the server and... On Wed, Dec 7, 2016 at 5:41 PM William Yager <will.yager(a)gmail.com> wrote: What are the advantages of this over the #haskell IRC on freenode? It's very active, usually with over 1500 nicks at any given time. I generally prefer IRC to any of these hip web chat solutions because IRC is client-agnostic and very rugged against companies folding or deciding they don't want to host a project any more. Basically the only way to kill an IRC channel is through social attrition, whereas any social value built up in hosted chat services might disappear overnight. The one major advantage of hosted chats over IRC is that they work better with mobile users, but I don't think that's very relevant for haskell dev. Will On Wed, Dec 7, 2016 at 7:34 AM, Ben Spencer <ben.richard.spencer(a)me.com> wrote: Why Gitter you might ask? _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

1 0

State of records in Haskell
by Øystein Kolsrud 07 Dec '16

07 Dec '16

Hi! I recently had a look at a talk by Simon Peyton Jones on lenses (https://skillsmatter.com/skillscasts/4251-lenses-compositional-data-access-…) and happened to notice that, at the very end, he mentions that there were changes going into GHC 7.10 that would make it possible to use the same field name for multiple record types. I was searching around a little for more information on this, but had a hard time finding what I was looking for and was hoping that someone here could give me a pointer. What does GHC support in this domain at the moment, and are there any particular flags I need to use in order to use that functionality? Best regards, Øystein Kolsrud

2 1

CFP Bx 17: 6th International Workshop on Bidirectional Transformations
by Meng Wang 07 Dec '16

07 Dec '16

CFP Bx 17: 6th International Workshop on Bidirectional Transformations CALL FOR PAPERS Sixth International Workshop on Bidirectional Transformations (Bx 2017) Uppsala, Sweden (Saturday April 29th, as part of ETAPS 2017) http://bx-community.wikidot.com/bx2017:home Bidirectional transformations (bx) are a mechanism for maintaining the consistency of at least two related sources of information. Such sources can be relational databases, software models and code, or any other document following standard or ad-hoc formats. Bx are an emerging topic in a wide range of research areas, with prominent presence at top conferences in several different fields (namely databases, programming languages, software engineering, and graph transformation), but with results in one field often getting limited exposure in the others. Bx 2017 is a dedicated venue for bx in all relevant fields, and is part of a workshop series that was created in order to promote cross-disciplinary research and awareness in the area. As such, since its beginning in 2012, the workshop has rotated between venues in different fields. IMPORTANT DATES - Abstract submission: 20 Jan 2017 - Paper submission: 27 Jan 2017 - Author notification: 17 Feb 2017 - Camera-ready version: 1 March 2017 - Workshop date: 29 April 2017 AIMS AND TOPICS The aim of the workshop is to bring together researchers and practitioners, established and new, interested in bx from different perspectives, including but not limited to: - data and model synchronization - view updating - inter-model consistency analysis and repair - data/schema (or model/metamodel) co-evolution - coupled software/model transformations - inversion of transformations and data exchange mappings - domain-specific languages for bx - analysis and classification of requirements for bx - bridging the gap between formal concepts and application scenarios - analysis of efficiency of transformation algorithms and benchmarks - survey and comparison of bx technologies - case studies and tool support PAPER CATEGORIES Submissions to Bx 2017 can be: - Regular papers (up to 15 pages) - in-depth presentations of novel concepts and results - applications of bx to new domains - survey papers providing novel comparisons between existing bx technologies and approaches - case studies - Tool papers (up to 8 pages) - guideline papers presenting best practices for employing a specific bx approach (with a specific tool) - presentation of new tools or substantial improvements to existing ones - qualitative and/or quantitative comparisons of applying different bx approaches and tools - Short papers (up to 4 pages) - work in progress - small focused contributions - position papers and research perspectives - critical questions and challenges for bx - Talk proposals (up to 2 pages) - proposed lectures about topics of interest for bx - existing work representing relevant contributions for bx - promising contributions that are not mature enough to be proposed as papers of the other categories All papers are expected to be self-contained and well-written. Tool papers are not expected to present novel scientific results, but to document artifacts of interest and share bx experience/best practices with the community. Short papers should primarily provoke interesting discussion at the workshop and will not be held to the same standard of maturity as regular papers. Talk proposals are expected to present works of particular interest for the community and that are worth a talk slot at the workshop. We strongly encourage authors to ensure that any (variants of) examples are present in the bx example repository at the time of submission, and for tool papers, to allow for reproducibility with minimal effort, either via a virtual machine (e.g. via Share - http://share20.eu) or a dedicated website with relevant artifacts and tool access. SUBMISSION GUIDELINES Submitted papers must follow the CEUR one column style available at http://ceur-ws.org/Vol-XXX/samplestyles/. Papers must be submitted via the EasyChair system: https://www.easychair.org/conferences/?conf=bx2017. Submissions not complying with the above guidelines may be excluded from the reviewing process without further notice. If a paper is accepted, one author of the paper is expected to participate in the workshop to present it. Authors of accepted tool papers are also expected to be available to demonstrate their tool at the event. PROCEEDINGS AND SPECIAL ISSUE The workshop proceedings, including all accepted papers (except talk proposals), will be published electronically by CEUR (http://ceur-ws.org) Authors of accepted papers (of all categories except talk proposal) that have high-quality and the potential to be extended into journal articles will be invited to submit a revised and extended version of their paper to an expected special issue of the Journal of Object Technology (http://www.jot.fm) these papers will then be subject to a careful reviewing and selection process according to the scientific standards of the Journal of Object Technology. PROGRAM CO-CHAIRS - Romina Eramo, University of L'Aquila, Italy - Michael Johnson, Macquarie University, Australia PROGRAM COMMITTEE MEMBERS - Anthony Anjorin, University of Paderborn, Germany - Soichiro Hidaka, Hosei University, Japan - Max E. Kramer, Karlsruhe Institute of Technology, Germany - James McKinna, University of Edinburgh, UK - Hugo Pacheco, University of Minho, Portugal - Alfonso Pierantonio, University of L'Aquila, Italy - Andy Schürr, Technical University of Darmstadt, Germany - Daniel Strüber, Universität Koblenz-Landau, Germany - James Terwilliger, Microsoft, USA - Meng Wang, University of Kent, UK - Bernhard Westfechtel, Universität Bayreuth, Germany - Manuel Wimmer, Vienna University of Technology, Austria +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Dr. Meng Wang | Lecturer in Computer Science School of Computing | University of Kent | Canterbury, Kent, CT2 7NF, UK m.w.wang(a)kent.ac.uk<mailto:m.w.wang@kent.ac.uk> | www.cs.kent.ac.uk/~mw516<http://www.cs.kent.ac.uk/~mw516> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++

1 0

Qualified imports and REPL output in GHCi 8.0.1
by Atrudyjane 07 Dec '16

07 Dec '16

When I do a qualified import such as: λ> import qualified Data.Bool then query a type, this is the output. λ> :t Data.Bool.bool Data.Bool.bool :: a -> a -> Bool -> a Was expecting: λ> :t Data.Bool.bool Data.Bool.bool :: a -> a -> Data.Bool.bool -> a I experimented with changing the -fno-implicit-import-qualified flag and the result was the same. Is there an option that needs to be set to show the expected behavior? Regards, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.

2 2