- Haskell-Cafe - Haskell.org

Types of partially instantiated polymorphic helper functions
by Todd Wilson 26 Nov '20

26 Nov '20

Dear Cafe, This has got to be a trivial question, but I don't see a solution, nor could I find anything through searching: If I want to write a type declaration for the helper function g here, what do I write? f :: a -> [b] -> [(a,b)] f a bs = g bs where g :: ???? g [] = [] g (x:xs) = (a,x) : g xs Apologies if I'm wasting bandwidth with my ignorance. Todd Wilson

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Looking for idiomatic example of wrapping a Haskell library
by Stuart Hungerford 25 Nov '20

25 Nov '20

Hi Haskellers, In a Haskell learning project I'm looking at using a couple of Haskell packages (from Hackage) for linear algebra and graphics. I've got a strong feeling my choices of these libraries are going to change as the project progresses and to avoid having to change references to these library functions throughout my modules I'd like to "wrap" them in modules of my own. Can anyone point me to a project where a Haskell library has been idiomatically wrapped this way? I can see newtypes and "smart" constructors of the wrapped types will go some way towards this, but I suspect there's more techniques I haven't yet learned. Thanks in advance, Stu

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Re: [Haskell-cafe] What to call Occult Effects
by Olaf Klinke 25 Nov '20

25 Nov '20

> Dear Olaf and everyone, > > the law do {_ <- b; a} = a is equivalent to do {_ <- b; return ()} = > return (), for, assuming the latter, > > do {_ <- b; a} > = do {_ <- do {_ <- b; return ()}; a} > = do {_ <- return (); a} > = a > > and it does actually have some history. It was dubbed > 'discardability' by Thielecke [1] and explored by > Führmann in [2] together with some other important properties of > effects, such as 'centrality' and > 'commutativity'. Combinations of these properties and examples are > explored in the conventional > Haskell-like style in [3] (where 'discardability' is called 'side- > effect freeness' though). > > Cheers, > Sergey > > [1] H. Thielecke, Categorical structure of continuation passing > style, Ph.D. Thesis, University of > Edinburgh, 1997. > > [2] C. Führmann, Varieties of effects, in: M. Nielsen, U. Engberg > (Eds.), Foundations of Software > Science and Computation Structures, FOSSACS 2002, in: Lect. Notes > Comput. Sci., vol. 2303, Springer, > 2002, pp. 144–158. > > [3] L. Schröder, T. Mossakowski, Monad-independent dynamic logic in > HasCasl, J. Logic Comput. 14 (2004) > 571–619 I've got the impression that all three of the above works look at a problem dual to the OP in a certain sense: At least [1] and [2] ask: For a given monad M, what is the subset of computations b with b >> a = a for all a? In contrast, I understood Kim-Ee's question as: What are the monads M where all computations b belong to this class? Assuming this property, the monoid M () is isomorphic to (). Indeed, the monoid M () inherits the property a <> b = b for all a, b :: M () and the only monoid with this property is the one- element monoid. In category theory a monad M with M 1 being isomorphic to the terminal object 1 is called 'affine'. Bart Jacobs in [4] gives a diagram (10) that amounts to return.const = fmap.const and shows that this is equivalent to being affine. Hence we might also use 'affine' for monads with Kim-Ee's property. Bart Jacobs' proof first reduces the general identity return.const = fmap.const :: a -> m b -> m a to the case b = () just as Sergey did above. Then = fmap (const a) = fmap (const a) . (id :: m () -> m ()) = fmap (const a) . return . const () by assumption = return . const a . const () by naturality of return = return . const a property of const I also learned that the school of 'effects' tries to avoid monads (this going back to John Power), so the terms 'discardable' and 'affine' might just be names for the same thing in different communities. Further there is a categorical construction (see [4]) to compute the greatest affine sub-monad of a monad. Interestingly, the affine part of the state monad is the reader monad, that is, functions s -> (a,s) that may use s but not change it. Olaf [4] https://hal.inria.fr/hal-01446033/file/418352_1_En_5_Chapter.pdf

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Virtual Munich Haskell Meeting, 2020-11-25 @ 19:30
by Leah Neukirchen 23 Nov '20

23 Nov '20

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, November 25 at 19:30. **Due to meetup limitations in Bavaria, this meeting will take place online!** For details see here: https://muenchen.haskell.bayern/dates.html Everybody is welcome, especially the Haskellers from Bavaria that do not usually come to our Munich meetings due to travel distance! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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xmonad: eyes based navigation between monitors.
by Karel Gardas 21 Nov '20

21 Nov '20

Hello, I'm using xmonad in a setup with 3 monitors and I'm curious if anybody here got kind of eyes-based navigation working between monitors/workspaces. Explanation: I need to type into windows on different monitors but in not specific order. Sometime I got too deep into thinking that I forget to switch workspace and then I type to different window in different workspace than is intended (and which I see with eyes). Basically I'd like some mode where xmonad will switch workspaces based on my eye/head position so only workspace I follow with eyes will be active. I see that such software (following head/eyes) already exist in AI domain (E.g. NV's xavier NX developer kit demo: from 16:00: https://www.youtube.com/watch?v=aUz1bNGm04w) but I'm more interested in amd64/linux setup as this is for my workstation... Any idea highly appreciated! Thanks! Karel

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 207, Issue 16
by Sumit Raja 19 Nov '20

19 Nov '20

> It may be that you have a dependency of this specific test (or the test > itself) that has the following stanza in its Cabal file: > "extra-libraries: gmp" > > Thanks, makes sense. Any shortcuts to try and find which package has gmp enabled? Interestingly this problem doesn't happen with ghc-8.6.5. Does build order decide on if this issue is triggered or not? -Sumit

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ghci module re exporting qualified as
by Immanuel Litzroth 19 Nov '20

19 Nov '20

I'm trying to make a test module to run tests. I want to import the test functionality + imported functionality in the scope of the test module under ghci. I also want to have short names in the Test module for reexported modules. I am using Data.Map and Data.Set ... so lot's of conflicting exports. Is this possible? #+BEGIN_SRC haskell :tangle T1.hs module T1 ( module Data.Map) where import qualified Data.Map #+END_SRC works but no browsing info shows up #+BEGIN_EXAMPLE λ⊢ :show modules T1 .... λ⊢ :show imports import T1 -- added automatically import Prelude -- implicit λ⊢ fromList [] <interactive>:114:1-8: error: Variable not in scope: fromList ∷ [a0] → t λ⊢ Data.Map.fromList [] fromList [] λ⊢ :browse! T1 λ⊢ #+END_EXAMPLE #+BEGIN_SRC haskell :tangle T2.hs module T2 ( module Map) where import qualified Data.Map as Map #+END_SRC #+BEGIN_EXAMPLE λ⊢ :show modules T2 :show imports import T2 -- added automatically import Prelude -- implicit λ⊢ fromList [] <interactive>:119:1-8: error: Variable not in scope: fromList ∷ [a0] → t λ⊢ Map.fromList [] <interactive>:120:1-12: error: Not in scope: ‘Map.fromList’ No module named ‘Map’ is imported. λ⊢ Data.Map.fromList [] fromList [] λ⊢ :browse! T2 λ⊢ #+END_EXAMPLE -- -- Researching the dual problem of finding the function that has a given point as fixpoint.

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Why is my 'text' missing an instance?
by amindfv＠mailbox.org 18 Nov '20

18 Nov '20

tl;dr: I seem to have two versions of 'text' on my system, both of which should have an instance for (Binary Text). However, one version seems to claim not to have that instance. $ cat test.hs import qualified Data.Text as T import qualified Data.Binary as B main = print $ B.encode $ T.pack "hello" $ /usr/local/bin/ghci --version The Glorious Glasgow Haskell Compilation System, version 8.10.2 $ /usr/local/bin/ghci test.hs GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:4:16: error: • No instance for (B.Binary T.Text) arising from a use of ‘B.encode’ • In the second argument of ‘($)’, namely ‘B.encode $ T.pack "hello"’ In the expression: print $ B.encode $ T.pack "hello" In an equation for ‘main’: main = print $ B.encode $ T.pack "hello" | 4 | main = print $ B.encode $ T.pack "hello" | ^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. $ /usr/local/bin/ghc-pkg list text /usr/local/lib/ghc-8.10.2/package.conf.d text-1.2.3.2 /home/name/.ghc/x86_64-linux-8.10.2/package.conf.d text-1.2.4.1 $ ghc-pkg-8.10.2 list text [same result] $ /usr/local/bin/ghci -package text-1.2.3.2 test.hs GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, one module loaded. $ /usr/local/bin/ghci -package text-1.2.4.1 test.hs GHCi, version 8.10.2: https://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:4:16: error: • No instance for (B.Binary T.Text) arising from a use of ‘B.encode’ • In the second argument of ‘($)’, namely ‘B.encode $ T.pack "hello"’ In the expression: print $ B.encode $ T.pack "hello" In an equation for ‘main’: main = print $ B.encode $ T.pack "hello" | 4 | main = print $ B.encode $ T.pack "hello" | ^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Both text-1.2.3.2 and text-1.2.4.1 have a Binary instance for Text. What's going on? (After writing this out I rebuilt an unmodified clone of text-1.2.4.1 but I see the same results.) Thanks, Tom

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GHC linker error 8.8.4 Windows
by Sumit Raja 17 Nov '20

17 Nov '20

Hi, Suddenly started getting the following linker error after I moved code around in my project. It only seems to occur when I am building one specific cabal test target. When building a dependent build it build successfully but cabal repl test-xxx fails. I have no consistent way to repro but I will keep trying and file a defect.. GHCi, version 8.8.4: https://www.haskell.org/ghc/ :? for help GHC runtime linker: fatal error: I found a duplicate definition for symbol __gmpn_sec_sqr_itch whilst processing object file C:\Users\sumit\Apps\ghc-8.8.4\mingw\lib\libgmp.dll.a The symbol was previously defined in C:\Users\sumit\Apps\ghc-8.8.4\lib\integer-gmp-1.0.2.0\HSinteger-gmp-1.0.2.0.o This could be caused by: * Loading two different object files which export the same symbol * Specifying the same object file twice on the GHCi command line * An incorrect `package.conf' entry, causing some object to be loaded twice. ghc.exe: panic! (the 'impossible' happened) (GHC version 8.8.4 for x86_64-unknown-mingw32): loadArchive "C:\\Users\\sumit\\Apps\\ghc-8.8.4\\mingw\\lib\\libgmp.dll.a": failed Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug CallStack (from HasCallStack): die', called at .\\Distribution\\Client\\ProjectOrchestration.hs:1041:55 in main:Distribution.Client.ProjectOrchestration cabal.exe: repl failed for mkr-txn-0.1.0.0-inplace-test-mkr-txn. Regards Sumit

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Re: [Haskell-cafe] Reading Haddock sources on hackage no longer possible with simple browsers?
by M Douglas McIlroy 16 Nov '20

16 Nov '20

> I am pretty sure nobody writes code like this > I confirmed your example, reading with lynx and w3m. > wouldn’t just directly opening the sources (in a text editor) be simpler? Is there a (simple) way to preserve the experience when browsing using Lynx without also holding back features aimed at a regular browsing experience? I second that. I have always been annoyed by the ransom-note appearance of colorized listings. But that's just my taste. I suspect, though, that lots of people have been frustrated when they discover they can't use the code they see because it's festooned with markup. Golang.org offers similar listings, but also provides a button for selecting the underlying plain text. I would like hackage a lot better if it did so, too. Doug

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