- Haskell-Cafe - Haskell.org

How to turn Integer into Proxy (n :: KnownNat) with n equal to the integer?
by David Banas 03 Aug '18

03 Aug '18

I’d like to use Data.Vector.Sized.slice’, but it wants the starting position and length parameters as Proxys, not Integers. How can I convert an Integer into a Proxy (n :: KnownNat) with n equal to the integer?

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[random-fu] sampleRVar with StdGen in RWS?
by Sergiu Ivanov 03 Aug '18

03 Aug '18

Dear Café, I would like to use random-fu to do some pseudo-random simulations for a given StdGen (so that I can run the same simulation multiple times, if needed). The following works: testState :: StdGen -> Int testState = evalState (sampleRVar $ uniform 1 10) The following doesn't: testRWS :: StdGen -> Int testRWS = fst . evalRWS (sampleRVar $ uniform 1 10) () I get <interactive>:2:26: error: • No instance for (MonadRandom (RWST () b0 StdGen Identity)) arising from a use of ‘sampleRVar’ • In the first argument of ‘evalRWS’, namely ‘(sampleRVar $ uniform 1 10)’ In the second argument of ‘(.)’, namely ‘evalRWS (sampleRVar $ uniform 1 10) ()’ In the expression: fst . evalRWS (sampleRVar $ uniform 1 10) () Indeed, I do see a MonadRandom instance for StateT, but none for RWST [0]. Is there a reason to not have a MonadRandom instance for RWST? Am I looking in the wrong place? - Sergiu [0] https://hackage.haskell.org/package/random-source-0.3.0.6/docs/Data-Random-…

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hp2pretty-0.9
by Claude Heiland-Allen 02 Aug '18

02 Aug '18

Hi all, hp2pretty[0] graphs heap profiles as SVG, much like hp2ps graphs as PostScript. I release hp2pretty-0.9[1], with support for detached keys in a separate text file with individual label images, which lets you construct a single key for multiple graphs, and also workaround the issue of truncated names (running off the edge of the image) when using the +RTS -L option. I applied the same principle to the titles, they can now be omitted from the images and added as captions in a surrounding document if desired. The examples/ folder shows how to embed the output in PDF[2] using the LaTeX 'svg' package (which calls out to 'inkscape'). There's also an HTML5[3] example. Because one can now create PDF output from SVG, the vapourware plans for other output formats have been dropped. Get it from Hackage[4], use `cabal unpack hp2pretty` to access examples. Or browse the git repository[5]. Thanks, Claude [0] https://mathr.co.uk/blog/hp2pretty.html [1] https://mathr.co.uk/blog/2018-08-01_hp2pretty-0.9_released.html [2] https://mathr.co.uk/hp2pretty/0.9/report.pdf [3] https://mathr.co.uk/hp2pretty/0.9/report.html [4] https://hackage.haskell.org/package/hp2pretty [5] https://code.mathr.co.uk/hp2pretty -- https://mathr.co.uk

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why different type variables enforce different types?
by Rui Azevedo 02 Aug '18

02 Aug '18

after using haskell for a while i was surprised by this little thing f::a->b f a=a and this is not making sense to me till now, so please can someone enlighten me, why does haskell assume that b can not be the same type of a? after all having two distinct math variables does not mean they can not have the same value. I know i could use f::a->a, but its not the same thing, on this last case I'm requesting them to be the same type, but on the first I'm not forbidding it, is there a math reason behind it? would type inference be rendered impossible if considering the case of them being equal? thanks Rui Azevedo (neu-rah)

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JOB | Permanent Java Developer (Montreal, Canada)
by James Tobin 01 Aug '18

01 Aug '18

Hello, I'm working with an employer that is looking to hire (for their Montreal office) a permanent Java developer that has extensive hands-on Java coding experience. Consequently I had hoped that some members of this mailing list may like to discuss with me further; off-list. I can be reached using "JamesBTobin (at) Gmail (dot) Com". Kind regards, James

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JOB | Permanent Java Development Manager (Montreal, Canada)
by James Tobin 01 Aug '18

01 Aug '18

Hello, I'm working with an employer that is looking to hire (for their Montreal office) a permanent development manager that has extensive hands-on Java coding experience. Consequently I had hoped that some members of this mailing list may like to discuss with me further; off-list. I can be reached using "JamesBTobin (at) Gmail (dot) Com". Kind regards, James

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Missing common function for Maybe types
by Marc Busqué 31 Jul '18

31 Jul '18

Hi! I have two functions: ``` foo :: a -> Maybe b bar :: a -> Maybe c ``` >From which I want to build a higher order function: ``` foobar :: a -> (a -> Maybe b) -> (a -> Maybe c) -> Either b c ``` The implementation I need is: ``` foobar x f g = case (f x) of Nothing -> g x Just y -> Just y ``` I'm a bit surprised that looking at hoogle I don't find a built-in solution for this quite common need for `Maybe` types (or perhaps for any monad). Am I looking in the wrong way? Does it exist a similar abstraction but with a different shape? Thanks in advance, Marc Busqué http://waiting-for-dev.github.io/about/

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Testing a collision detection system
by mpevnev＠gmail.com 30 Jul '18

30 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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