- Haskell-Cafe - Haskell.org

Testing a collision detection system
by mpevnev＠gmail.com 31 Jul '18

31 Jul '18

As a part of a project, I write a collision detection system. It is in dire need of testing, but designing and writing tests for all possible pairs of types of colliding geometry would be a pretty big effort - not only I would have to calculate the fact of collision manually for 20-something pairs of types of colliding geometry, I would also have to do so multiple times for each pair, since each pair requires several test cases. So the idea is to use an existing collision detection library to generate (a lot of) test cases from random data. I've found two such libraries for Haskell - HODE and Bullet. The problem is, Bullet bindings aren't documented at all, and HODE (which isn't really documented either, but at least lists available functions) is extremely ugly with IO all over the place, and manual tracking of objects' lifetimes (at least that's what I infer from `create :: World -> IO Body` and `destroyBody :: Body -> IO ()`, because again - no documentation). So my question is: does anyone know a library I could use? I'll pretty much settle for whatever. -- Michail.

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using Haskell more like scripting language
by Dennis Raddle 29 Jul '18

29 Jul '18

I'm actually working in Purescript, but the issue is pretty much the same as one would encounter in Haskell. My application (animation of math-related ideas) is changing constantly. It's research, and I don't know where I'll be with it next week, or even tomorrow. For instance, the kinds of "objects" I'm animating (numbers, equations) and the way they move and interact with each other is a research project, and I'm constantly ripping out parts of my application and scavenging them for variations on a theme. My objects need state. At this point I'm thinking of just putting the state into a map with strings as keys and some kind of sum type to represent each data item I might need. So if I want to represent that an object has a particular color and position and velocity, itemState = M.fromList [("color" , DataColor blue ) ,("position", DataPoint 500.0 100.0) ,("velocity", DataVector 50.0 45.0 ) ] Which starts to look more like a dynamically-typed scripting language. But it's not hard to work with. I can uses lenses/prisms to inspect and update state, and it's okay if my program crashes on encountering malformed state. What this lets me do is create new objects that mix and match parts of old objects, and copy and paste a lot of the code. Is there a better way to do this? Is there a compromise I can make that will allow the compiler to help me out more with finding errors at compile time? D

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stack test hangs when using 1.7.1 but completes with 1.4.0
by Olaf Klinke 29 Jul '18

29 Jul '18

Dear cafe, I recently upgraded (with some trouble) to stack-1.7.1 from 1.4.0. I have an older project using lts-9.2 that builds just fine when using the new stack, but it hangs on `stack test`. The difference is shown below. I suspect it is something that needs to be upgraded alongside stack. Maybe Cabal? Upgrading stack was troublesome because of different index formats, which is why using the other stack causes the rebuilds of my extra-deps, I suppose. When using stack --verbosity debug I see messages of the form Error while decoding $HOME/.stack/indices/Hackage/01-index.cache ... (this might not be an error, when switching between stack versions) Any help welcome. Olaf $> /usr/bin/stack-1.7.1 test Populated index cache. $MYPACKAGE: unregistering (missing dependencies: glpk-hs) gasp-1.0.1.0: unregistering (old configure information not found) glpk-hs-0.5: unregistering (missing dependencies: gasp) gasp-1.0.1.0: configure gasp-1.0.1.0: build gasp-1.0.1.0: copy/register glpk-hs-0.5: configure glpk-hs-0.5: build glpk-hs-0.5: copy/register $MYPACKAGE: configure (lib + exe + test) Configuring $MYPACKAGE... $MYPACKAGE: build (lib + exe + test) Preprocessing library $MYPACKAGE... <<< some more output ... >>> Registering $MYPACKAGE... $MYPACKAGE: test (suite: $PACKAGENAME-test) <<< with --verbosity debug >>> [debug] Run process within $PACKAGEDIR/: $PACKAGEDIR/.stack-work/dist/x86_64-linux/Cabal-1.24.2.0/build/$PACKAGENAME-testStub/$PACKAGENAME-testStub @(src/Stack/Build/Execute.hs:1711:27) Progress 1/2: $MYPACKAGE <<< Hangs from here on, does not consume CPU cycles >>> $> /usr/bin/stack-1.4.0 test Populated index cache. $MYPACKAGE: unregistering (missing dependencies: glpk-hs) gasp-1.0.1.0: unregistering (old configure information not found) glpk-hs-0.5: unregistering (missing dependencies: gasp) gasp-1.0.1.0: configure gasp-1.0.1.0: build gasp-1.0.1.0: copy/register glpk-hs-0.5: configure glpk-hs-0.5: build glpk-hs-0.5: copy/register $MYPACKAGE: configure (lib + exe + test) Configuring $MYPACKAGE... $MYPACKAGE: build (lib + exe + test) Preprocessing library $MYPACKAGE... <<< some more output >>> Progress: 3/4+++ OK, passed 100 tests. <<< some more output >>> Completed 4 action(s).

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Monad Reader pubs?
by Donald Sitompul 28 Jul '18

28 Jul '18

I’m looking for past publications of the Monad Reader, where are they now? Donald Sitompul

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Re: [Haskell-cafe] Why does this result in a runtime loop instead of a type error or typechecker loop?
by Anthony Clayden 27 Jul '18

27 Jul '18

On Thurs, Jul 26, 2018 at 05:39:00, Emil Axelsson wrote: >> Den 2018-07-26 kl. 01:02, skrev Anthony Clayden: >> >>> * On Tue, Jul 24, 2018 at 5:52 AM,Ryan Reich wrote: *>*>> This seems like the canonical illustration of the dangers of * >>>* UndecidableInstances.* >> >>* No I don't buy that explanation. The worst UndecidableInstances can do * >>* is send the type checker into a loop.* > I used to have a counter-example to that claim: https://mail.haskell.org/pipermail/haskell-cafe/2010-January/072357.html > That example was causing looping dictionaries at run time on GHC 6.10. > But now, on GHC 8.2, the program terminates. 6.10 was rather a long time ago. The whole type inference/improvement engine has had several overhauls since 2010. To be clear: you can write looping programs without type classes at all -- such as mutually recursive functions. You can write looping programs with type classes -- by turning those functions into methods in classes. You can write looping programs with type classes that need `UndecidableInstances` -- by making those classes mutual superclasses or mutual constraints on instances. That doesn't mean the `UndecidableInstances` are to 'blame' for the looping. I would blame the functions/method calls for not reducing the problem. > But one wonders if there are other ways to create looping dictionaries? With `UndecidableInstances`, yes there's plenty. And I expect that'll make the typechecker loop at compile time. The O.P. here is not getting typechecker looping, so I think the explanation is to do with method calls -- for which we need to see what's going on in the imports. AntC

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Integrate GHC-API with Cabal
by Félix Baylac 26 Jul '18

26 Jul '18

Dear cafe, I am currently working on a project [1] which aims to index the code published on Stackage in order to provide a "code example" database. Basically, for each library, I want to generate a real-world example corpus for each exported symbol. Before generating this database, I need to retrieve the exported symbol of a package. So far, I have been gathering the dependencies and integrating them in the GHC pkg database using an external stack call, I then parse the cabal file and load the exposed modules using the GHC API and gather the exported symbols*. It works pretty reliably on the simple packages, however, I end up with some missing dynamic flags for some more advanced packages (missing c includes, c libraries, ASM flags, etc.). I then started to parse and load these missing attributes to GHC until I stepped back for a moment and realized I was re-implementing cabal build. I then looked for a way to "hook" my GHC API code in cabal build. This would allow cabal to both handle the dependencies gathering as well as setting up the correct GHC dyn-flags. The only resource I found was [2]. It's really clever, however, looks a bit hacky to me. Is there a better way to perform this kind of "hook"? [1] https://git.alternativebit.fr/NinjaTrappeur/Exhs [2] http://blog.ezyang.com/2017/02/how-to-integrate-ghc-api-programs-with-cabal/ * I am aware of the hoogle index generated by haddock. However, this index is missing when the haddock documentation of a package cannot be generated.

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Re: [Haskell-cafe] Why does this result in a runtime loop instead of a type error or typechecker loop?
by Anthony Clayden 26 Jul '18

26 Jul '18

On Tue, Jul 24, 2018 at 5:52 AM,Ryan Reich wrote: > This seems like the canonical illustration of the dangers of UndecidableInstances. No I don't buy that explanation. The worst UndecidableInstances can do is send the type checker into a loop. For mutually recursive functions to cause looping, you don't need classes at all: foo :: Bool -> () foo x = bar x bar :: () -> Bool bar x = foo x No typechecking will detect that loop. This `HasBool` example is just an obfuscated way to write that loop(?) There's some odd thigs in the OP: Why use OVERLAPPABLE? Neither of those instances need it. Or are there more instances in the imports? (There must at least be the definition for class `Has`.) If so, what are their constraints? And what are their implementations? Perhaps that's causing the looping? Will `runRIO` or `view` use one of those other instances? What does this mean: "I can compile an equivalent program; it loops on execution."? There doesn't need to be a `Bool` in the "environment": the `HasBool a` instance introduces `Bool` in its constraint. I suggest boiling this down to a stand-alone program not needing imports. Then tell the whole story. AntC On Mon, Jul 23, 2018 at 2:10 PM, Theodore Lief Gannon < tanuki at gmail.com <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>> wrote: >* λ. :set -XFlexibleContexts -XFlexibleInstances -XMultiParamTypeClasses *>* -XUndecidableInstances *>* λ. import RIO *>* λ. import Data.Has *>* λ. class HasBool a where boolL :: Lens' a Bool *>* λ. instance {-#OVERLAPPABLE#-} HasBool a => Has Bool a where hasLens = *>* boolL *>* λ. instance {-#OVERLAPPABLE#-} Has Bool a => HasBool a where boolL = *>* hasLens *>* λ. runRIO () $ view boolL *>* ^CInterrupted. *>>* The RIO environment () doesn't contain a Bool. I can see how the *>* typechecker might get lost in this, but I can compile an equivalent *>* program; it loops on execution. *>>* _______________________________________________ *>* Haskell-Cafe mailing list *>* To (un)subscribe, modify options or view archives go to: *>* http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe> *>* Only members subscribed via the mailman list are allowed to post. *>

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Re: [Haskell-cafe] Integrate GHC-API with Cabal
by Daniel Gröber 26 Jul '18

26 Jul '18

Hi, On Wed, Jul 25, 2018 at 08:30:48PM +0200, Félix Baylac wrote: > Thanks a lot for this really helpful message. > > > I think you might be interrested in one of my packages, > > [cabal-helper](https://hackage.haskell.org/package/cabal-helper). It's > > geard more towards editor tooling but I think it should work for your > > use case too. > > Indeed, looks like what I'm looking for. > > > Basically you'd have to do `cabal install --only-dependencies && cabal > > configure && cabal build` for each package you want to inspect and > > then cabal-helpers API boils down to giving you a list of GHC > > flags/module names you need to pass into the GHC API to make it work. > > > > (The `cabal build` step is needed to run preprocessors like hsc2hs and > > compile C dependencies, but once cabal has taken care of that you can > > usually just load the package without problems.) > > I had to make some modifications and jump in some nix hoops to use > cabal-install instead of stack to build the packages, but indeed, it does work > like a charm. Ah, I forgot you said you were using Stack. You can use it too if you prefer. You just have to use `stack path --dist-dir` (run in the project directory) to get the path to the dist-dir after running `stack build` instead of using the hardcoded `$PWD/dist` value. There have been some reports of issues with stack using weird combinations of Cabal-library versions though which so it might need some tweaking. > This is perfect, I just tried your library following this example on the text > package and it seems to do the job. Thanks a lot, fixing this problem without > re-inventing the wheel is a huge relief. > > Do you mind if I write a small article on this integration. No not at all, send me a link when you're done. > It'll probably prevent some other people from using dodgy hacks. Well I'm not sure I would call cabal-helper not a dodgy hack but at least it's a (sort of) well maintained hack :) --Daniel PS: Looks like I messed up the list-reply on my original message so I resent it and I'm CC'ing haskell-cafe on this one. Sorry about that.

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Haskell Opportunity - New York - Blockchain
by Alex Mesropians 25 Jul '18

25 Jul '18

Hey All, All Haskell engineers in the US we have a new and exciting role with a blockchain firm who are rapidly expanding! Check them out - Haskell Role <http://t.sidekickopen13.com/e1t/c/5/f18dQhb0S7lC8dDMPbW2n0x6l2B9nMJW7t5XZs5…> Thanks, Alex

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how to terminate a (Posix) process and all its subprocesses?
by Johannes Waldmann 25 Jul '18

25 Jul '18

Dear Cafe, I am using https://hackage.haskell.org/package/process-1.6.4.0/docs/System-Process.htm… to start an external command, and wait for its completion. Now I would like to time-out this after a while. I can use https://hackage.haskell.org/package/base-4.11.1.0/docs/System-Timeout.html and this works in simple cases - but not in case the external command has spawned child processes. (cleanupProcess sends SIGTERM but only to the process at the top of the tree ?) I guess I need to use https://hackage.haskell.org/package/unix-2.7.2.2/docs/System-Posix-Process.… but that seems rather low-level - and I don't see how I would get the ProcessID of the process started by readProcess, so I'd also have to re-do that. Is there an abstraction/library that would help here? Thanks - J.W.

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