February 2017 - Haskell-Cafe

Loading JSON into a set of nested index-able structures
by Carl Petersen 15 Feb '17

15 Feb '17

Hello All, I've used Aeson to load a JSON object with nested lists into a set of data structures, but what I really need is to load it into a set of data structures that are index-able such as a HashMap. I've had success loading the JSON into a structure with the lowest level being a HashMap using !Object, but I'm having difficulty in figuring out how to nest the HashMaps. Any hints or direction would be appreciated. Thanks, Carl

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Extensible records & mandatory/optional record fields
by Harendra Kumar 15 Feb '17

15 Feb '17

In my application, I want some of the fields in a record to have default values while others must always be specified by the user. I can implement this using the rawr records library (http://hackage.haskell.org/package/rawr) like so: def :: ("b" := String) -> R ("a" := Int, "b" := String) def t = (R (#a := 0) :*: R t) -- merge with defaults let r = def (#b := "hello") Here field "a" has a default value (i.e. it is optional when "def" is used) but field "b" does not have a default value and has to be explicitly specified by the user. This is made possible by record merging feature of rawr, making the record extensible. I could not find an equivalent way of achieving this using overloaded records (http://hackage.haskell.org/package/overloaded-records) Is record merging possible or can be made possible using overloaded-records or with the ghc overloaded records poposal? If not, is there a plan to make something like this possible in future? Or will this never be possible with the in built record syntax? Note that, this is handy to simulate mandatory and optional keyword arguments to a function by passing them in the form of a record. Something which is available in other languages like python. -harendra

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Myrtle Software is hiring for Haskell positions in Hardware and Machine Learning!
by Joe Hermaszewski 14 Feb '17

14 Feb '17

Myrtle is looking for multiple skilled Haskell developers to join our growing development team writing a compiler for generating highly efficient hardware designs implementing machine learning solutions. We use Haskell for every aspect of our work and developers will get the chance to be involved in every part of the project. As an example, some things we’ve worked on recently are a tiny CPU to implement a custom network protocol to communicate between a PC and an FPGA and a Haskell implementation of part of the Caffe deep learning framework. Specific tasks you could be working on soon include: implementing fixed point neural network evaluation support in our compiler, supporting AWS F1 instances as a backend, investigating and implementing exciting techniques for neural network evaluation such as the Winograd small convolution algorithm and realising full hardware implementations of existing neural networks. A familiarity and interest with hardware design would be viewed positively but is not essential. Applicants should be familiar with Unix based systems; all our development is done on Ubuntu or NixOS. Applicants should send a resume and covering email, stating your suitability for this role, to jobs(a)myrtlesoftware.com. Please include any links to blog posts, repositories or any other media which demonstrates your skill set. Roles will be based at our offices in Cambridge, UK. Closing date for applications 5pm, 13th March 2017 About Myrtle Myrtle is a software company based in Cambridge UK with a long history of working at the forefront of compiler technology. We are best known for having helped produce computer generated images for over twenty major Hollywood movies including the Transformers franchise. Myrtle’s clients have included NYSE and NASDAQ listed companies in LA, Vancouver and London. -- Joe https://www.myrtlesoftware.com/vacancies/

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Re: [Haskell-cafe] Sublime2/3 hsdev patch: testers needed
by Ara Adkins 14 Feb '17

14 Feb '17

Looks like this has fixed an issue I've been having! Seems to have just been merged into master as of ~4 hours ago at the time of writing. Thanks Scott! - Ara > On 13 Feb 2017, at 06:40, Scott Michel <scooter.phd(a)gmail.com> wrote: > > Was wondering if there are any Sublime Text users out there who also use hsdev as their Haskell IDE backend. I'd recently (within the last 2 weeks) encountered random lockups, error messages about resources no longer available. I was able to track them down to half-closed pipes and ended up revamping subprocess interaction. So far, I have left a SublimeText session open for 3 days, done some fair amount of work and haven't had any problems. > > If you are a SublimeText 2/3 hsdev user and are comfortable with checking out Sublime packages directly from github, I could use some extra help shaking out the code to ensure that it is as robust as I think it is. The URL to my Github fork is: > > https://github.com/bscottm/SublimeHaskell.git > > Once you've cloned my fork, check out the 'stderr_stdout' branch and test away -- I'm looking for unexpected lockups (e.g., when you've saved a file and ST's status line says "... Checking and Linting" forever.) > > Thanks! > > > -scooter

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A question about GHC test coverage
by ok＠cs.otago.ac.nz 14 Feb '17

14 Feb '17

Step 1: write a module defining a data type. Step 2: write a module with lots of QuickCheck tests for it. Step 3: Compile with gcc -fhpc to get coverage data. Step 4: run, find bugs, fix bugs, repeat from step 3. Step 5: view coverage .html files, note functions not covered, add tests to step 2 file, repeat. Step 6: Step back with a sigh of content. Coverage high. Step 7, some time later: encounter a bug in an untested function. Why? Because Step 1 includes instance Ord MyType where compare ... ... = ... and the untested function was max. The "inherited" definition is of course fully consistent with compare, and comparison had been tested because I thought of that, but there was an extra requirement which need not detain us here. My problem was that (a) I was too dumb to think of writing tests for a function I hadn't written. (b) Because I hadn't written max, 'hpc' didn't show it to me as something needing a test. Obviously this was my fault: if I have declared instance <some class> <my type> it's my responsibility to test all of the functions in the type-class. But I might not actually *know* what all the functions in the type-class are; Haskell does change from time to type. And being a bear of very little brain, I could use some reminders "Hey, you've inherited this function but not tested it". Is that possible using ghc -hpc?

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Fully funded PhD position in CS at Heriot-Watt University, Edinburgh, Scotland
by Ekaterina Komendantskaya 13 Feb '17

13 Feb '17

Dependable System group, School of Mathematical and Computer Sciences, Heriot-Watt University, Edinburgh, is looking to fill in one PhD position (a PhD stipend of £14,300 per annum, for 48 months, and a fee waiver are offered, no nationality restrictions apply). We are looking for a talented person, with 1st class degree in Mathematics or Computer Science, with keen interest in Logic and Programming languages and good programming skills. The successful candidate will be working under supervision of Dr Ekaterina Komendantskaya (http://www.macs.hw.ac.uk/~ek19/) The exact topic of the PhD dissertation will be determined in the course of discussions with the successful candidate, and will be related to both the student’s and the supervisor's research interests. To apply please contact Ekaterina directly: ek19(a)hw.ac.uk Heriot-Watt University. With a history dating back to 1821, Heriot-Watt University is a research-led university located in Edinburgh, UK. Currently it is Scotland’s most international university, delivering degree programmes to 31,000 students globally and with campuses in Edinburgh, Scottish Borders, Orkney, Dubai and Malaysia. The university is consistently highly ranked, having been The Sunday Times; Scottish University of Year between 2011 and 2013. The university has a strong track record in attracting funding for research from national Research Councils, UK government departments, charities, the European Commission and industrial sponsors. In the REF 2014 survey of all UK universities, 82% of our research was found to be world-class. Dependable Systems group. The Computer Science department, hosting this PhD position, had 78% of its outputs rated `world-leading' or `internationally excellent' in REF 2014.The proposed work will be carried out within the Dependable Systems Group (DSG). With members of staff working on programming languages and compilers (Fensch, Loidl, Michaelson, Scholz), Automated Reasoning and Formal Methods (Georgieva, Grov, Ireland, Komendantskaya), Type Theory (Gabbay), this group will provide an ideal environment for the PhD research. Additionally, the DSG is an active member of the multi-site Mathematical Reasoning Group (joint with U. Edinburgh); the sites holding weekly research meetings and a joint platform grant. ******************************* Dr Ekaterina Komendantskaya Associate Professor in Computer Science School of Mathematical and Computer Sciences Earl Mountbatten Building [ building 21, office G26 ] Heriot-Watt University Edinburgh, UK, EH14 4AS +44 (0)131 451 8283 http://www.macs.hw.ac.uk/~ek19/

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Sublime2/3 hsdev patch: testers needed
by Scott Michel 13 Feb '17

13 Feb '17

Was wondering if there are any Sublime Text users out there who also use hsdev as their Haskell IDE backend. I'd recently (within the last 2 weeks) encountered random lockups, error messages about resources no longer available. I was able to track them down to half-closed pipes and ended up revamping subprocess interaction. So far, I have left a SublimeText session open for 3 days, done some fair amount of work and haven't had any problems. If you are a SublimeText 2/3 hsdev user and are comfortable with checking out Sublime packages directly from github, I could use some extra help shaking out the code to ensure that it is as robust as I think it is. The URL to my Github fork is: https://github.com/bscottm/SublimeHaskell.git Once you've cloned my fork, check out the 'stderr_stdout' branch and test away -- I'm looking for unexpected lockups (e.g., when you've saved a file and ST's status line says "... Checking and Linting" forever.) Thanks! -scooter

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here's my solution for scripting/interpreter approach
by Dennis Raddle 13 Feb '17

13 Feb '17

My application needed the flexibility of writing test code and running it, or configuring its job (playback of music) through Haskell code and immediately testing the result (i.e. making sound and listening). I thought I would use GHCI, but I ran into some trouble. For one thing, my program forks a thread to do the timed control of the music, while the main thread implements a REPL-like "control loop" -- i.e. let me enter commands to influence the playback or halt it. I think this was crashing GHCI, perhaps when an interpreted script forks a thread which is still alive when control returns to the GHCI prompt. Not sure. Anyway here's what I did. Now the process that does real-time control of the playback is an entirely separate process. The control loop is started by me first, and depending on the command I enter, it builds the playback program with "ghc --make" and then spawns it. I send it signals from the control loop. This turns out to be great. I can make changes anywhere in my playback application, from the deepest utility library to the top-level configuration script, and it builds as needed before running. And, in the case it doesn't need to build anything, "ghc --make" finishes quickly so it doesn't feel inefficient to invoke it after every command I enter in the control loop. D

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Re: [Haskell-cafe] Lenses and records and ecosystem
by Anthony Clayden 12 Feb '17

12 Feb '17

> ON 10/02/17 07:10, Tony Morris wrote: >> On 10/02/17 15:35, Anthony Clayden wrote: >> I see Simon PJ's comment here >> https://github.com/ghc-proposals/ghc-proposals/pull/6#issuecomment-277971802 >> and the lengthy discussions from different advocates. >> >> How much is it true there's a fractured "Haskell ecosystem" >> with 2 (3?) incompatible approaches: >> * H98 style labelled records with various extensions >> to ease the pain. >> * newtype-wrapped Lenses >> * 'raw' (unwrapped) Lenses > Yearss ago, many of us spent a long time > on different designs of lenses, ... > ... it would be foolish to concede that ground > for no significant benefit. Thanks Tony, but I don't understand: concede what ground? You're perhaps saying that one style of lenses is the 'right' one(?) I am asking if they can co-exist. > > Can you mix those three styles inside one program? > How much pain does it cause? > > Can you import libraries with a mix of styles? How does a > program cope? Why would you? For what practical purpose? A lens/a structure/a record is a means to an end. Applications that are putting/getting stuff are doing it for some purpose. Not merely because lenses are a nice application of category theory. So say you want to use the functionality from 3 different libraries, but each is written using a different style. >> >> I see the plethora of operators Lenses come with >> (mostly to keep the types coherent?). >> Except that Lens composition is plain function composition >> (.). >> >> I remember something from SPJ's video/lecture on Lenses: >> that having Lens composition turn out to be function >> composition is "cute". > The use of (.) here was deliberately designed that way to > assist in working against existing code. Is "against" some clever pun here? Like Concerto for Piano against Orchestra? Certainly (.) for lenses works 'backwards' (i.e. suffix style) compared to function-prefix style. (As any Lens tutorial will say.) >> That suggests to me it's more of a happy accident than a >> necessary feature. > Not necessary, just useful. I am less excited about this > feature than many others, but it is useful nonetheless, > and was specifically designed that way. Whether or not that's justified [**] it's counter-intuitive w.r.t. Haskell, so I don't see a need to use the same symbol. Hence my "wondering" below. >> >> I also see in the ORF proposal, OverloadedLabels part, >> that composing with Control.Category.(.) can lead to type >> ambiguity. >> >> I wonder: >> Could there be a dedicated operator for composing Lenses, >> that has the same semantics as (.), >> but a more specialised type? >> To strengthen type inference/reduce ambiguity. >> [**] the justification for postfix style seems to be so it's more like OO. Seems strange: FP just isn't very like OO. And in particular, functional data structures, just aren't like OO/procedural data structures. Anthony

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Re: [Haskell-cafe] Lenses and records and ecosystem
by Anthony Clayden 11 Feb '17

11 Feb '17

> 2017-02-11 09:30 UTC Fumiaki Kinoshita: > Are there any real cases that the polymorphism of > Control.Category.. becomes a real problem? I think > ambiguity rarely occurs on the kind (* -> * -> *). > Hi Fumiaki, the remark is here https://github.com/adamgundry/ghc-proposals/blob/overloaded-record-fields/p… The composition is: fooBar = #foo . #bar ... However, if labels are interpreted using IsLabel, the inferred type of fooBar is (Category cat, IsLabel "foo" (cat s t), IsLabel "bar" (cat r s)) => cat r t There's no FunDep on IsLabel (nor could there be). So `s` is ambiguous. Same as the classical (show . read). The usual type for raw/unwrapped Lenses is: type Lens s t a b = forall f. Functor f => (a -> f b) -> (s -> f t) Which is of kind (* -> * -> *), but you need to squint a bit. > > 2017-02-10 14:35 GMT+09:00 Anthony Clayden: > > > > > > I see the plethora of operators Lenses come with > > (mostly to keep the types coherent?). > > Except that Lens composition is plain function > > composition (.). > > > > I remember something from SPJ's video/lecture on Lenses: > > that having Lens composition turn out to be function > > composition is "cute". > > That suggests to me it's more of a happy accident than a > > necessary feature. > > > > I also see in the ORF proposal, OverloadedLabels part, > > that composing with Control.Category.(.) can lead to > > type ambiguity. > > > > I wonder: > > Could there be a dedicated operator for composing Lenses > > , that has the same semantics as (.), > > but a more specialised type? > > To strengthen type inference/reduce ambiguity. > > AntC

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