- Haskell-Cafe - Haskell.org

Subtyping CoC
by james faure 28 Jul '20

28 Jul '20

Hello, I am working on a subtyping calculus of constructions: https://github.com/jfaure/Nimzo, based on Algebraic Subtyping [1] The goal is to leverage the synthesis of subtyping with the CoC for general purpose programming, both for the usual correctness guarantees, but additionally exceptional type inference and powerful optimizations by using subtyping relations on dependent types. I spent the last year researching the theory and practice for this, and with the compiler at now just over 3000 lines of Haskell, I feel like I am no longer advancing as quickly as I would like, so if anyone is interested in helping create this language of the future, please get in touch ! [1]: https://www.cs.tufts.edu/~nr/cs257/archive/stephen-dolan/thesis.pdf James Faure (Discord: J4#0303)

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Job: Medical Solutions with Haskell in Freiburg (German speaker required)
by David Leuschner 28 Jul '20

28 Jul '20

Dear Haskellers, our team is very happily and successfully using Haskell in production for over 10 years. We're currently hiring german-speaking/learning, full-time, on-site Haskell developers to work on user-friendly web- and mobile applications for doctors, nurses, patients and all other people involved. We have a very experienced team and we're looking for new-comers that are eager to learn as well as we're looking for experienced Haskell developers. We'd love to hear from you! Please look at our full announcement on Reddit: https://www.reddit.com/r/haskell/comments/hy8tuv/medical_solutions_with_has… Kind regards, David -- David Leuschner Entwicklungsleitung Lohmann & Birkner Software Solutions GmbH Alt-Reinickendorf 25 13407 Berlin Email: leuschner(a)cp-med.com Web: http://www.checkpad.de Lohmann und Birkner Software Solutions GmbH Geschaeftsfuehrer: Dr. Ruediger Lohmann Handelsregister: Amtsgericht Berlin-Charlottenburg HRB 130806 B

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How to understand the 320-byte heap footprint of UUID ?
by YueCompl 28 Jul '20

28 Jul '20

Hello Cafe, I'm about to introduce UUID into my code, and see https://github.com/haskell-hvr/uuid/issues/24 <https://github.com/haskell-hvr/uuid/issues/24> stating: > Currently, UUID is represented as data UUID = UUID {-# UNPACK #-} !Word32 {-# UNPACK #-} !Word32 {-# UNPACK #-} !Word32 {-# UNPACK #-} !Word32 > However, this suboptimal for 64bit archs (where GHC currently stores this a 320-byte Heap object); ... According to https://wiki.haskell.org/GHC/Memory_Footprint <https://wiki.haskell.org/GHC/Memory_Footprint> I can understand each evaluated `Word32` on 64-bit hardware can take 2 words - 16 bytes, and given they are unpacked and strict, I think one whole UUID record should just take 64 bytes plus a few words, which is far less than 320 bytes. So how comes the 320 bytes? Thanks with regards, Compl

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how to get live info on thread scheduling?
by Johannes Waldmann 25 Jul '20

25 Jul '20

Dear Cafe, With Control.Concurrent, I would want to know, for a ThreadId, the total (up to now) (wallclock) time it was running/paused by the RTS scheduler. I guess that the scheduler's actions can be written to the eventlog for later analysis. Can I access them from inside a running program? I guess this is https://gitlab.haskell.org/ghc/ghc/-/wikis/event-log/live-monitoring , what is the status? - J.

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GADT/Typeable/existential behaviour that I don't understand
by Tom Ellis 24 Jul '20

24 Jul '20

I can define the following import Data.Typeable data Foo where Foo :: Typeable x => x -> Foo eq = (\a b -> eqT) :: (Typeable a, Typeable b) => a -> b -> Maybe (a :~: b) and then these expressions work as expected > case Foo "Hello" of Foo x1 -> case eq x1 "" of { Nothing -> "It was not a string"; Just Refl -> "It was a string" } "It was a string" > case Foo 1 of Foo x1 -> case eq x1 "" of { Nothing -> "It was not a string"; Just Refl -> "It was a string" } "It was not a string" But if I omit the 'Nothing' branch (as below) I get "Couldn't match expected type ‘p’ with actual type ‘[Char]’ ‘p’ is untouchable". Can anyone explain why this happens? > case Foo "Hello" of Foo x1 -> case eq x1 "" of { Just Refl -> "It was not a string" } > case Foo 1 of Foo x1 -> case eq x1 "" of { Just Refl -> "It was not a string" }

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Question about zippers on trees
by Dominik Schrempf 24 Jul '20

24 Jul '20

Hello Cafe! I am trying to modify a large 'Data.Tree.Tree'. I managed to modify node labels with specific indices in the form of @[Int]@ as they are defined in, for example, 'Control.Lens.At.Ixed' or 'Lens.Micro.GHC'. However, I also need to 1. modify the node label using information from nearby nodes (e.g., the children); 2. modify the tree structure itself; for example, I may want to change the sub-forest. Basically, I need a lens that focuses not on the node label, but on the node itself. I perceived that this is more difficult. I tried to use 'Control.Zipper'. I can use zippers to achieve point 1, albeit in a complicated way: (1) I need to go downwards to focus the specific node; (2) I need to traverse the children to collect data and save the data somewhere (how? in let bindings?); (3) I then go back upwards and change the node label using the collected data. Even so, I do not really manage to change the actual structure of the tree. I also briefly had a look at plates, but do not manage to use them in a proper way, maybe because the depth of my structures may be several hundred levels. Did you encounter similar problems in the past or could you point me to resources discussing these issues? Thank you! Dominik

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Re: [Haskell-cafe] Question about zippers on trees
by Olaf Klinke 22 Jul '20

22 Jul '20

> A direct implementation without using a library is interesting, thank > you. I > refrained from doing that, because I thought that Control.Zipper > would actually > do this for me. Actually, I was pretty successful with using > Control.Zipper to > change node labels, but failed doing more complicated stuff. > Isn't that a strong indicator that zippers are an improper abstraction for your purpose? Perhaps after rolling your own implementation you can more easily discover how to represent the algrorithm as a zipper. Olaf

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ghc profiling
by Mika Pöhls 22 Jul '20

22 Jul '20

Hello everyone, I have a question regarding ghc profiling. The .hp file, created during profiling, displays the memory usage until a certain point, which seems to be the end of the execution of the program (based on the exectuion time the program usually takes), as exprected. But after that an extra sample is created, that contains no data (no cost centers, it is just begin sample followed by end sample). This sample is also created out of the 0.1 second gaps used before. For example the last "correct" sample is at 17s and the empty sample is at 40s. When plotting the data, this results in an monotonous increasing graph (for all centers), that has its maximum at the last correct sample (17s) followed by a straight line to zero (40s). Does anyone know, what leads to this behavior and how it can be prevented? Thanks in advance Mika

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Fwd: Judge for programming language contest?
by Michal J Gajda 22 Jul '20

22 Jul '20

Hi, I just noticed that in the age of cambrian explosion among new programming languages we also have programming language jam organized this August: https://blog.repl.it/langjam Given that there are many prolific programming language designers and implementors here -- I wonder if we get a Haskell judge so that advanced type systems can get a fair ruling? (In case you got another round of reviews from traditional PL conferences that described your paper as "badly typeset" and "wrong formatting was intentional", "repetitive" and "hard to understand", or even rude "expect a boring talk".) -- Cheers Michał

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Haskell Love Conference (the 31st of July & 1st of August, 2020)
by Ida Bzowska 20 Jul '20

20 Jul '20

Hi There! We are organizing the online event on Haskell language https://haskell.love, free to attend. The CFP for the event has started (ends on the 1st of July, 2020), please consider joining https://konfy.care/cfp Tickets (free) will be available soon, follow us on twitter https://twitter.com/_haskellove for the updates and feel free to hop on our slack http://tinyurl.com/haskelloveslack λCheers Ida Bzowska

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