January 2019 - Haskell-Cafe

Feedback on plan for checking code requirements in a GHC plugin
by Chris Smith 04 Apr '19

04 Apr '19

Hey everyone, I'm converging on a design for a new feature of CodeWorld, the Haskell-based educational programming environment that I teach with. I'm wondering if anyone has done something like this before. The goal is that when I give an assignment in a class (e.g., "modify this starting code to generalize the repeated pattern using a function"), I want the user to see a checklist of assignment requirements when they run the code. A prototype implementation is here: https://code.world/haskell#PpwpTF1wv3qIvwhohCfvSrQ There are all sorts of possible requirements, such as: "No lines longer than 80 characters", or "there must be a function called foo", or "all top-level definitions must have type signatures", or "your code must define at least 10 top-level variables, and use at least 3 where clauses", or "your definition of var should be equivalent to this one" (see Joachim Breitner's inspection testing work), or even "the function f must satisfy this QuickCheck property". It's been suggested that the requirements language also include the ability to match patterns in the AST, which I think is a good idea. The current prototype uses a pre-compile step that parses the code using haskell-src-exts, and doesn't implement dynamic requirements (runtime-evaluated) at all. My ultimate plan, though, is to send these requirements to GHC via a plugin, then have it evaluate the static ones at compile time, and generate code to check the dynamic ones. Finally, the plugin would add new code to the beginning of main that will invoke a configurable function with the results of the requirement check (hard-coding the static ones, and evaluating dynamic ones on the fly). (In the CodeWorld environment, this function would display the checklist in the web UI, for example.) Has anyone done anything like this before? Any wisdom to share, or ideas to contribute? Thanks, Chris

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role constraints
by Ben Franksen 13 Feb '19

13 Feb '19

I have a question about roles for type variables. I have a number of data types that more or less represent partial bijections. They take two type parameters that represent domain and range. The phantom type parameters are extremely useful: code that shuffles these things around will fail to type check if domains and ranges don't match up exactly. However, there are some situations where I know more about the semantics than the type checker and need to be able to coerce the phantom parameters. No problem, that's what coerce is for... Except, most of the code is polymorphic over the data type, requiring only a few class constraints. And this polymorphism forces the type checker to assume representational roles for the type parameters. Which means I can't use coerce. Instead I have to use unsafeCoerce which I would like to avoid. Is there a way to express, as a constraint, that a certain parameter has a phantom role? What I want to say is: you are only allowed to instantiate the type variable 'a :: * -> * -> *' with data type 'A' if 'x' and 'y' in 'A x y' have both role phantom. And what I get is that I can call coerce to modify the parameters 'x' and 'y'.

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Let's plan BayHac 2019!
by Dan Burton 05 Feb '19

05 Feb '19

It's time to start thinking about the next BayHac! I am assembling a group of organizers for next year's event. Anyone who has been involved in organizing past BayHac events will be automatically accepted. (I have BCCed a number of past organizers; please reach out to any you know in case I missed anyone.) Anyone else who is interested in helping to plan BayHac, please contact me. Depending on how many past organizers rejoin, we will discuss the applicants and accept 1 to 3 new organizers. BayHac is a weekend of learning, sharing, and hacking on a variety of projects for the Haskell community. BayHac usually occurs in San Francisco, California, Mountain View, California, or thereabouts. I'd like to aim for April/May 2019, though that is, of course, subject to change. A rough outline, in rough chronological order, of what the BayHac organizers will be doing: 0. Assemble the organizers. 1. Select date and venue for BayHac 2019. 2. Secure corporate sponsors to cover costs (mainly, food). 3. Invite talk proposals and select speakers. 4. Open online registration and publicize the event. 5. Plan details like catering, registration desk, recording, and make it happen! If any of that sounds like fun to you, then I'd love to hear from you! -- Dan Burton

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Patents on Maybe and Tuple
by Henk-Jan van Tuyl 01 Feb '19

01 Feb '19

L.S., I just read the article "Apple Patents Swift".[0] It says that Apple has two patents regarding Swift: [1][2]; these patents state, amongst others, that there are new types of variables: "Additionally, an embodiment of the new language introduces advanced types not found in Objective-C. These types include Tuples and Optionals. Tuples enable a developer to create and pass groupings of values. Tuples group multiple values into a single compound value." and "In one embodiment, an optional type is provided that handles the absence of a value. Optionals are a way to say either “there is a value, and it equals x” or “there isn't a value at all.”" I hope we don't get arrested for using the Maybe monad or tuples in Haskell. Regards, Henk-Jan van Tuyl [0] https://www.i-programmer.info/news/98-languages/12495-apple-patents-swift.h… [1] https://patents.google.com/patent/US9952841B2 [2] https://patents.google.com/patent/US9329844B2 -- Message from Stanford University: Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://foldingathome.stanford.edu/ -- http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Library for common AST transformations based on haskell-src-exts
by Sandra Dylus 01 Feb '19

01 Feb '19

Hi, in the last couple of years we had a number of projects that used haskell-src-exts [1]. In most of these cases we wanted to do some common AST transformations, e.g., translating function definitions via several rules into one rule using case expressions. So, after needing these transformations a couple of times, I was wondering if there is a library that works on top of haskell-src-exts and implements common transformations like the one mentioned. I took a look at the reverse dependencies of haskell-src-exts, but only found projects implementing these transformations as part of a bigger project. In a nutshell, I am asking if I did miss such a library or if there does not exist such a library yet. If the latter is the case, we will definitely implement such a library as part of the next student project that builds upon haskell-src-exts. Cheers Sandra [1] http://hackage.haskell.org/package/haskell-src-exts <http://hackage.haskell.org/package/haskell-src-exts>

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Seeking: 12 month internship opportunities for Haskell enthusiasts
by Jacques Carette 30 Jan '19

30 Jan '19

I'm writing on behalf of some (quite good) undergraduate students in CS who have been quite taken by Haskell, whose program includes a 12 month internship at a company. Start time could be anytime between May and September, and duration would be 12 months. [Some students who start in May end up doing 16 months]. If anyone here has leads on such opportunities, please contact me. Location is flexible, as long as the language of work is English. Jacques

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Re: [Haskell-cafe] Montreal Haskell
by Doug McIlroy 28 Jan '19

28 Jan '19

> Morgan-Stanley do their own internal version of Haskell, and they're > strongly oriented towards performance and correctness Interestingly eclectic. They also employ Bjarne Stroustup, the eminence grise of C++. Doug McIlroy

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Montreal Haskell
by Asher Klein 28 Jan '19

28 Jan '19

Hi everyone! New member here! Anyone know of Haskell jobs in Montreal for new grads? Thanks!

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Munich Haskell Meeting, 2019-01-28 @ 19:30
by Leah Neukirchen 27 Jan '19

27 Jan '19

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 28 at Cafe Puck at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jan-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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generics-sop : deep flattening of nested algebraic values ?
by Marco Zocca 27 Jan '19

27 Jan '19

Hi all, I'm a bit at a loss regarding this: say we have a value from a nested algebraic type (definitions at the bottom) testT2 :: T2 testT2 = T2 B (Left 42) a generic encoding computed with `from` only goes one level deep, leaving the sub-terms as they are : λ> from testT2 SOP (Z (I B :* (I (Left 42) :* Nil))) However, is it possible to recursively encode and collect all the sub-terms, until primitive types or enumerations are encountered? The encoded sub-terms from the example above are as follows: λ> from B SOP (S (Z Nil)) λ> from (Left 32 :: Either Int Char) SOP (Z (I 32 :* Nil)) where {-# language DeriveGeneric #-} import qualified GHC.Generics as G import Generics.SOP data T1 = A | B | C deriving (G.Generic) instance Generic T1 data T2 = T2 { t21 :: T1 , t22 :: Either Int Char } deriving (G.Generic) instance Generic T2

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