September 2021 - Haskell-Cafe

Pattern synonyms and explicit forall.
by coot＠coot.me 18 Sep '21

18 Sep '21

Consider the following existential type: ``` data Some where Some :: a -> Some ``` The following (bidirectional) pattern synonym is accepted: ``` pattern Any :: a -> Some pattern Any a = Some a ``` However using an explicit `forall` results in a type error: ``` pattern Any :: forall a. a -> Some pattern Any a = Some ``` ``` Patterns.hs:63:23: error: • Couldn't match expected type ‘a’ with actual type ‘a1’ ‘a1’ is a rigid type variable bound by a pattern with constructor: Some :: forall a. a -> Some, in a pattern synonym declaration at Patterns.hs:63:18-23 ‘a’ is a rigid type variable bound by the signature for pattern synonym ‘Any’ at Patterns.hs:62:23 • In the declaration for pattern synonym ‘Any’ • Relevant bindings include a :: a1 (bound at Patterns.hs:63:23) | 63 | pattern Any a <- Some a | ^ ``` This is indpendent whether `ExplicitForAll` or `ScopedTypedVariables` extensions are enabled or not. Is there a way around it, or is this a missing feature? Best regards, Marcin Szamotulski Sent with ProtonMail Secure Email.

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 217, Issue 16
by Anthony Clayden 18 Sep '21

18 Sep '21

>>* Haskell's big problem right now is adoption. * Heh heh, this sentence is multi-ways ambiguous. As in the 'problem' is that there's too many adopters/the wrong sort of people trying to adopt Haskell. The 'problem' is that Haskell is not keeping to its objective to "avoid success at all costs". > I'm not quite sure this is a problem. Haskell and its libraries are designed around math concepts, as opposed to CS concepts (semigroup vs IConcatenable). Learning it all but requires learning some algebra as well, thus a lower adoption rate is expected. Thank you for saying that out loud. I've been feeling for some time that GHC is getting harder to use. You're telling me that because I don't get Category Theory you want to throw me out of using Haskell. People who are allergic to Lambda-calculus are already excluded. So the ostensible difficulty on these threads: that the documentation is unhelpful, is not the problem at all. In fact it's deliberate that the docos are exclusionary. As part of this grand plan, Viktor is deliberately making the Libraries docos impenetrable. (By which I mean: the style is clear, but the material is unlearnable. Apparently I need to know what is an endomorphism before I can write a Foldable instance) And that explains a paradox: I learnt Haskell over a decade ago. There was far less intro material; and certainly no store of StackOverflow questions/answers. It should have been more difficult to learn than today. But I don't remember learning being as difficult as the claims on this thread. (Memory can deceive, I guess.) Back then Haskell was a much smaller, simpler language. There was less that could go wrong at the type level; and the error messages didn't keep recommending to switch on bamboozling features that (as it turned out) made the difficulty harder. Don't go claiming that if I switch off those features today it's as if I was still using H2010. Simply not true. I know that's not true, because I can still go back and use Hugs, or use GHC v7.10 (vintage ~2015), and have a much more pleasant/less bamboozling experience. Then "Haskell's big problem right now" is (depending on your point of view) either GHC or too many of the wrong adopters. And with Dependent Types coming along GHC is going to get a lot worse. (Maybe GHC will eventually get through the fog, throw out a load of cruft and become on average as usable as v7.10. I'm not prepared to go through the pain for 5 years to find out. I'll stick with v7.10.)

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Re: [Haskell-cafe] Haskell's "historical futurism" needs better writing, not better tools
by Douglas McIlroy 18 Sep '21

18 Sep '21

> Lambda calculus is an excellent place for beginners to start. > What could be easier to learn? It's certainly easier than grokking > a Turing machine; and much easier than Haskell: less than a > handful of primitives yet can compute anything computable. There's a category error (in the sense of logic, not algebra) here. Learning a language is not comparable to understanding a program. The concept of lambda calculus is far more subtle than that of Turing machine. But if you know both, a program is likely to be much easier to understand when presented in lambda calculus than when presented as a Turing machine. Doug

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 217, Issue 20
by Michael Turner 17 Sep '21

17 Sep '21

On Thu, Sep 16, 2021 at 11:57 PM Michael Turner < michael.eugene.turner(a)gmail.com> wrote: > Wow, someone on this thread said of me, from what I wrote on the > beginner's list, "He's trying to translate Haskell to C/C++." > Brandon Allbery: "Actually no, they were talking about someone on StackOverflow." I'm afraid Anthony Clayden actually WAS talking about me: https://mail.haskell.org/pipermail/haskell-cafe/2021-September/134468.html (Or am I missing some Haskeller in-joke about StackOverflow?) Regards, Michael Turner Executive Director Project Persephone 1-25-33 Takadanobaba Shinjuku-ku Tokyo 169-0075 Mobile: +81 (90) 5203-8682 turner(a)projectpersephone.org Understand - http://www.projectpersephone.org/ Join - http://www.facebook.com/groups/ProjectPersephone/ Donate - http://www.patreon.com/ProjectPersephone Volunteer - https://github.com/ProjectPersephone "Love does not consist in gazing at each other, but in looking outward together in the same direction." -- Antoine de Saint-Exupéry On Fri, Sep 17, 2021 at 9:08 PM <haskell-cafe-request(a)haskell.org> wrote: > > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Re: Haskell's "historical futurism" needs better writing, not > better tools (Tom Ellis) > 2. Re: Haskell-Cafe Digest, Vol 217, Issue 17 (Brandon Allbery) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 17 Sep 2021 11:15:05 +0100 > From: Tom Ellis <tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Haskell's "historical futurism" needs > better writing, not better tools > Message-ID: <20210917101505.GA3781@cloudinit-builder> > Content-Type: text/plain; charset=us-ascii > > On Fri, Sep 17, 2021 at 06:54:50PM +0900, Michael Turner wrote: > > On Fri, Sep 17, 2021 at 4:58 PM Tom Ellis > > <tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> wrote: > > > Michael, I have an offer for you (in fact two): > > > > > > 1. I will collaborate with you to produce the guide to Haskell's > > > evaluation that *you* would want to read. > > > > > > 2. I will collaborate with you to write the NLP tool that you want to > > > write in Haskell. > > > > I think if I write anything long, it would first have to be my > > "Haskell in Plain English: A Guide for Lexical Semanticists." > > Fair enough. > > > > I can't do these without collaborating with someone like you. I > > > simply don't know what someone else wants to read. > > > > So much of good writing is just figuring out an audience. As I wrote > > to Viktor above, the section he presented to me in case I had trouble > > with it could be absolutely ideal for someone with more grounding. > > They could admire the sentence I struggled with, for how it > > encapsulates their understanding while refreshing their memory. > > Yes indeed. The audience that I write for is myself since that's the > audience I know. I am interested in writing for a more general > audience but I don't have motivation at the moment to do so without a > member of that general audience on the team. > > > I'm not sure that gearing a piece toward what /I/ would like to read > > is a much bigger audience > > I suspect the group of people who are interested in yet frustrated by > Haskell is *far* bigger than the group of people who are already > familiar with Haskell! > > > If I had to suggest an approach you could try on your own, it would > > be this: > > Thank you for the suggestion. I will bear it in mind if I decide to > tackle this in the future. > > Tom > > > ------------------------------ > > Message: 2 > Date: Fri, 17 Sep 2021 08:02:13 -0400 > From: Brandon Allbery <allbery.b(a)gmail.com> > To: Michael Turner <michael.eugene.turner(a)gmail.com> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Subject: Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 217, Issue 17 > Message-ID: > <CAKFCL4WyXKKfZ32kdkBJPYDs8k-TJ=4NVNS0L-g5oDy0BjYhbw(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > On Thu, Sep 16, 2021 at 11:57 PM Michael Turner < > michael.eugene.turner(a)gmail.com> wrote: > > > Wow, someone on this thread said of me, from what I wrote on the > > beginner's list, "He's trying to translate Haskell to C/C++." > > > > Actually no, they were talking about someone on StackOverflow. > > -- > brandon s allbery kf8nh > allbery.b(a)gmail.com >

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Re: [Haskell-cafe] Haskell's "historical futurism" needs better writing, not better tools
by Simon Michael 17 Sep '21

17 Sep '21

Hi Michael, I thoroughly enjoyed and appreciate this thread. You're right, our writing and communication often and in many places does fall short of what's needed (sucks). I think your core point, that writing and communicating better could achieve more than tool-building, is very good. I saw some of the books and docs did not work for you. If you have the time to try another, I'd love to know how Mark Watson's "Haskell Tutorial and Cookbook" strikes you. It's the one I (and only I, for some reason) often recommend to practical minded haskellers. (Pardon the unthreaded reply)

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 217, Issue 16
by Anthony Clayden 17 Sep '21

17 Sep '21

>> "The seeds of your confusion are very evident from your message". > The seeds of your unsubstantiated assumptions about me are very evident from yours. OK Michael, I can see I'm not helping. I'll stop. On NLP (a topic on which I have some expertise), I'll save you time in your search: Haskell is not suitable. Use a NLP tool. Programming languages and their semantics are so unlike natural languages as to be useless. I found your "primer for the lexical semanticist" to be full of strained and unhelpful analogies. I was surprised it omitted mention of Montague Grammar -- which draws from Lambda calculus/Combinators, and would be the obvious go-to _if_ Haskell were to be applicable. On unsubstantiated assumptions: > I started programming at 15. Me too. (Dartmouth BASIC on a teletype over a dial-up line to a timeshare service.) > It's been a very rare year, even after leaving the software profession, when I didn't write at least some code, learning new things in the process. Me ditto. > Which means I've been looking at programming languages and how to implement things at the most appropriate level of abstraction possible for just a few months shy of fifty years. For me, a few months over fifty years. (BTW nothing you've said persuades me away from recommending the best way to learn Haskell is via Lambda-calculus.) AntC

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Re: [Haskell-cafe] Haskell reference documentation, laws first or laws last?
by Anthony Clayden 17 Sep '21

17 Sep '21

Hi David, The Foldable Laws are here https://dnssec-stats.ant.isi.edu/~viktor/haskell/docs/libraries/base/Data-F… In the Data-Foldable doco. (There's a menu entry on the sidebar, or text search.) The Data-Traversable doco has only the Traversable Laws. > I'm not talking about whether they're first or last. They're currently not part of the class documentation *at all*. They're only in the Data.Traversable documentation.

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Re: [Haskell-cafe] Haskell reference documentation, laws first or laws last?
by Anthony Clayden 17 Sep '21

17 Sep '21

Hi Viktor, really this is a quite different q than the thread talking about a learner's experience. You're writing deeply-technical detail not at all aimed at beginners. There's a very broad range of reasons and prior knowledge for readers of Library docos. But I disagree with this: > The beginner to intermediate users will be using the library and existing instances for some time before they start to write their own instances. I'll want to write an instance of Foldable as soon as I've declared my datatype, won't I? Or do you expect me to be folding only over Lists? Speaking for myself, if I want to understand a function, I look first at its type then its source definition. The Laws for the particular Foldable example I don't understand at all. I'd say: hide those Laws as far away as possible. In fact, since the blasted FTP re-organisation of that library, I've more or less given up. Too hard/too abstract/too much Category Theory. What's an `Endo`? Thank you for your efforts to document. I expect there's someone you're helping. Not me. I don't think it's that the style "sucks". This style and content is enough for me: https://www.haskell.org/hugs/pages/libraries/base/Data-Foldable.html > Of course the conjectured users who might most benefit from not being intimidated by being exposed to laws before they're ready to understand them might not be present on this forum... I've gone well beyond the point of being intimidated by Category Theorists. I find they're doing a splendid job of putting people off getting to love Haskell as I used to. If you want to tell me off for speaking out of turn, go ahead. (But this forum is the cafe, perhaps this particular discussion should be on glasgow-haskell-users?) AntC On Thu, Sep 16, 2021 at 06:51:42PM -0400, David Feuer wrote: >* The last time I went to look at the laws it took me a couple minutes to *>* find them. I use them to write instances. Pretty important, IMO. * I agree the laws are important to document, I just don't think they belong at the top of the module. The beginner to intermediate users will be using the library and existing instances for some time before they start to write their own instances. If more modules adopt something like the style of the new Data.Foldable, experienced users will know to look for the laws at the end, if not still present at the top of the module. Of course perhaps the community would prefer the original Laws first format, I'm fine with that emerging as the consensus. Perhaps worthy of a separate thread (made it so). Of course the conjectured users who might most benefit from not being intimidated by being exposed to laws before they're ready to understand them might not be present on this forum... --

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Bundle patterns with type aliases
by Keith 16 Sep '21

16 Sep '21

Is there currently a way to 'bundle' a pattern with a type alias? And if not, could that capability be added to the PatternSynonyms GHC extension? (Is this the right place to ask, or should I be asking a GHC list?) --Keith Sent from my phone with K-9 Mail.

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Mixed Fraction data type
by Galaxy Being 15 Sep '21

15 Sep '21

I'm looking at Sandy Maguire's _Thinking With Types_ and he's talking about the cardinality of types. He introduces the product type data MixedFraction a = Fraction { mixedBit :: Word8 , numerator :: a , denominator :: a } How is this a type for holding mixed fractions such as 5-1/2? ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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