- Haskell-Cafe - Haskell.org

using type variables in type declarations inside function
by Dennis Raddle 13 Apr '18

13 Apr '18

Let's say I've written a function on three types. myFunc :: a -> b -> c myFunc x y z = ... where helper :: a -> [b] helper xx = ... Notice that I'm attempting to declare 'helper' using my type variables. I've noticed that this results in an error. Is this actually possible, and how? D

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Unplanned Hackage Downtime
by Gershom B 12 Apr '18

12 Apr '18

I made a terrible administrative snafu, and we need to take some time to restore Hackage. Luckily we have better mirroring in place now, so cabal-install should be able to fall back automatically to mirrors. If this does not work, you can use http://objects-us-west-1.dream.io/hackage-mirror/ or http://hackage.fpcomplete.com/ manually. Best (and sincere apologies), Gershom

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hackage style
by Ben Franksen 12 Apr '18

12 Apr '18

The new hackage style has nice colors but severe problems w.r.t. readability. The right column with the package meta data causes the Readme text to be cut off. This make it almost impossible to read it. I attached a screenshot for illustration. Cheers Ben

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Join a transformer
by Michael Snoyman 11 Apr '18

11 Apr '18

I'm wondering if this pattern exists and has a name. We have the concept of joining a Monad: join :: Monad m => m (m a) -> ma How about joining a monad transformer? joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a I believe implementing this in terms of MonadTransControl[1] might be possible, but I was wondering if there's an already existing idiom for this. Michael [1] http://haddocks.fpcomplete.com/fp/7.4.2/20130301-40/monad-control/Control-M…

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downgrading GHC version on a Mac
by Dennis Raddle 11 Apr '18

11 Apr '18

I'm on a MacBook Pro. I'm not very familiar with software installation. Up until now I've been using Stack for GHC work, and it seems to take care of having the right version of GHC. Now I'm going to use GHCJS, and I need to have a system GHC installed (I believe). At some point in the past I installed the Haskell Platform for Mac, and I have GHC 8.01. But I want to downgrade that to 7.10 so that I don't have to worry about compatibility with GHCJS. How do I go about uninstalling GHC 8 and installing 7.10 in its place? Can I use Homebrew? D

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looking for a particular paper
by Michael Litchard 11 Apr '18

11 Apr '18

Could someone provide a copy of the following paper? Safe Client/Server Web Development with Haskell Mark Mazumder <https://ieeexplore.ieee.org/search/searchresult.jsp?searchWithin=%22Authors…> ; Timothy Braje <https://ieeexplore.ieee.org/search/searchresult.jsp?searchWithin=%22Authors…>

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GHC Core / STG to supercombinators
by Csaba Hruska 10 Apr '18

10 Apr '18

Hello, I wonder what is the easiest way to compile Haskell to supercombinators (top level functions) using GHC as a library. Is it possible to use GHC simplifier to transform the parsed Haskell source to supercombinators? i.e. to do - eta expansion - closure conversion - lambda lifting Or should it be written from scratch? Is Core or STG suited better for this purpose? Thanks, Csaba

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stack and GHCJS
by Dennis Raddle 09 Apr '18

09 Apr '18

Now I'm trying to use stack to install and run GCHJS. Can someone help me figure out this error? I first created project 'proj01' using the ghcjs template via stack new proj01 ghcjs Then I typed stack build It ran for a while, then gave an error messages that starts "While building custom Setup.hs for package semigroupoids-5.0.0.4 using .... <very long description>, Process exited with code ExitFailure 1" But it didn't halt. Instead it started compiling Main.hs, and ended up saying "at least the following dependencies are missing: tagged >= 0.8.5 && <1 && ==0.8.1" I tried adding "tagged" to the extra-deps in the stack.yaml, but it didn't help. While I'm posting, is there a resource that explains background to this? I don't know much about "packages," "snapshots," "dependencies", means of specifying versions, cabal, or YAML files. Everything I find online seems to assume some background knowledge of these things. For instance the stack documentation talks about snapshots, but I can't find an explanation that I understand for what a "snapshot" actually is. Ditto for all these many concepts that go into configuring and building. D

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PatternSignatures (rant)
by Johannes Waldmann 07 Apr '18

07 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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Windows: how to perform cleanup actions when the console window closes
by Peter Dedecker 06 Apr '18

06 Apr '18

So I looked at this some more, and determined a few things: - Ctrl-C did not do anything in the original program because the main thread was blocked on a socket accept() call, which seemingly could not be interrupted for an asynchronous exception. - I fixed this by having the main thread initialize everything, then running the main body of code in an async and waiting on that. Apparently this wait is interruptible, so Ctrl-C works, bracket nicely cleans up, and the application exits. Good! - Then I added a console handler to capture 'close' events. Here is the code for the handler: f mainThreadID event = case event of ControlC -> putStrLn "HANDLER" >> throwTo mainThreadID UserInterrupt Break -> putStrLn "HANDLER" >> throwTo mainThreadID UserInterrupt Close -> putStrLn "HANDLER" >> throwTo mainThreadID ThreadKilled >> threadDelay maxBound _ -> pure () Where the 'threadDelay' call serves simply to give the main thread time to exit. This works fine for ControlC and Break, but fails for Close: the program exists immediately, and no cleanup is run. So I still haven't solved how to clean up when the window close button is hit. Any thoughts? Thanks Peter

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