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Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 20 Mar '17

20 Mar '17

Just adding to the programming contest anecdotes. Here is what happens if you issue an applied math assignment without specifying the programming language to be used. The first student solves it in bash, to demonstrate how inefficient the solution can be. The second student writes a Turing machine simulator in Maple (I can't recall whether it was the brainf*ck dialect), then programs that Turing machine. The third student writes the solution in postscript and makes the printer solve the differential equation. All were languages available in the setting. I have no idea whether any of the above submissions were inspected by the tutor, but received a mark. Olaf

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Loop with the StateT monad
by p75213＠gmail.com 19 Mar '17

19 Mar '17

This is an exercise to learn the StateT monad. The program implements the game Morra. The two players are the computer and a person. The state accumulates the score of the computer and player. The program works for one iteration of function morra only. However I am at a loss how to loop it. I have tried a few things but nothing seems to work. I have narrowed the problem down to the "p <- liftIO getChar" statement. The second time through the loop it isn't executed. I have bracketed the statement with "liftIO $ putStrLn "before" and liftIO $ putStrLn "after". This is the program: module Morra where import Control.Monad.Trans.State.Lazy import Control.Monad.IO.Class import Data.Char (isDigit, digitToInt) import System.Random (randomRIO) import Control.Monad (when) morra :: StateT (Int, Int) IO () morra = do liftIO $ putStrLn "before" p <- liftIO getChar liftIO $ putStrLn "after" when (isDigit p) $ do let p' = digitToInt p c <- liftIO $ randomRIO (1, 2) liftIO $ putStrLn ("P: " ++ [p]) liftIO $ putStrLn ("C: " ++ show c) (pt, ct) <- get if even (c + p') then do liftIO $ putStrLn "Computer Wins" put (pt, ct + 1) else do liftIO $ putStrLn "Player Wins" put (pt + 1, ct) morra main :: IO () main = do putStrLn "-- p is Player" putStrLn "-- c is Computer" putStrLn "-- Player is odds, Computer is evens." (personS,compS) <- execStateT morra (0,0) putStrLn ("Person Score: " ++ show personS) putStrLn ("Computer Score: " ++ show compS) if personS > compS then putStrLn "Winner is Person" else putStrLn "Winner is Computer" and this is the output: *Morra> main -- p is Player -- c is Computer -- Player is odds, Computer is evens. before 1 after P: 1 C: 2 Player Wins before after Person Score: 1 Computer Score: 0 Winner is Person *Morra>

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Re: [Haskell-cafe] benchmarking a function that returns an unlifted type
by David Feuer 19 Mar '17

19 Mar '17

The critical feature of whnf (as I understand it) is that GHC does not use the static argument transformation. The function passed to "go" therefore doesn't inline into it. So you'll have to write your version in some fashion that similarly prevents undesirable inlining. Could you just copy the code and specialize it to the desired type? On Mar 18, 2017 1:32 PM, "Richard Eisenberg" <rae(a)cs.brynmawr.edu> wrote: Apologies for a newbie question, but I am seeking advice on how to benchmark a pure function that returns an unlifted type. I've heard tell that `criterion` is good for benchmarking, and so I'm using that library (but have no particular allegiance to it, if something else is better). Specifically, I'm comparing a function over unlifted types with the same function written over lifted types, trying to observe how much we pay for the boxing/unboxing. To benchmark the lifted version, I just use criterion's whnf function. That seems to be working splendidly. Of course, whnf doesn't work over unlifted types, so I found its source code and inlined it. This compiles. But it doesn't work! No matter what I do, the unlifted version measures in the nanoseconds. I'm guessing that GHC "cleverly" is avoiding recomputation. How can I stop this? Here is my code: > main :: IO () > main = do > initializeTime > let lifted = whnf fastSumPrimes 100000 > > unlifted = Benchmarkable go > where > go n > | n <= 0 = return () > | otherwise = let x = fastSumPrimes# 100000# in go (n-1) > > benchmark lifted > benchmark unlifted I've tried increasing the argument in the unlifted case, to no avail. I've also tried using `case` instead of `let`. -ddump-simpl suggests that fastSumPrimes# is really being called, but I'm dubious. Can anyone offer advice? Thanks! Richard _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: 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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benchmarking a function that returns an unlifted type
by Richard Eisenberg 19 Mar '17

19 Mar '17

Apologies for a newbie question, but I am seeking advice on how to benchmark a pure function that returns an unlifted type. I've heard tell that `criterion` is good for benchmarking, and so I'm using that library (but have no particular allegiance to it, if something else is better). Specifically, I'm comparing a function over unlifted types with the same function written over lifted types, trying to observe how much we pay for the boxing/unboxing. To benchmark the lifted version, I just use criterion's whnf function. That seems to be working splendidly. Of course, whnf doesn't work over unlifted types, so I found its source code and inlined it. This compiles. But it doesn't work! No matter what I do, the unlifted version measures in the nanoseconds. I'm guessing that GHC "cleverly" is avoiding recomputation. How can I stop this? Here is my code: > main :: IO () > main = do > initializeTime > let lifted = whnf fastSumPrimes 100000 > > unlifted = Benchmarkable go > where > go n > | n <= 0 = return () > | otherwise = let x = fastSumPrimes# 100000# in go (n-1) > > benchmark lifted > benchmark unlifted I've tried increasing the argument in the unlifted case, to no avail. I've also tried using `case` instead of `let`. -ddump-simpl suggests that fastSumPrimes# is really being called, but I'm dubious. Can anyone offer advice? Thanks! Richard

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Tools to assist with development cycle of hackage packages
by Clinton Mead 18 Mar '17

18 Mar '17

I've only just started uploading packages to hackage (my package listing is here <https://hackage.haskell.org/user/clinton>) but currently there's a lot of repetitive activities for creating and updating packages. For example, here's the steps in creating a package: 1. Initialise a repository on github 2. Initialise git repository locally 3. Set github repository as remote 4. Add a LICENSE file 5. Add a standard ".gitignore file" 6. Create a cabal file with the appropriate files that hackage requires including: 1. Git repository source 2. Issues page 3. Licence 4. Licence file 7. Run multi-ghc-travis <https://github.com/hvr/multi-ghc-travis> to create a ".travis.yml" file 8. Make an initial commit and push 9. Refresh travis-ci.org's repository list so it detects the new repository There's also the stack stuff, but the GUI I'm using, IntelliJ with a Haskell plugin, handles most of that. It also creates a cabal file, but it's missing a number of key fields as mentioned above. When I actually want to upload the package I go though these steps: 1. Push to github 2. Wait for Travis-CI to compile the package (this is a test to ensure it builds in a clean remote environment). 3. Run "cabal sdist 2>&1" 4. Parse the output of sdist to see where the dist file is. 5. Run "hup <https://hackage.haskell.org/package/hup> packup fileFromSDist" to upload the package, putting in my hackage user/pass 6. Run "hup docboth", to both build and upload the documentation. 7. Tag the commit as a release Currently, I've got two scripts with help with a lot of this, but it's a bit adhoc, and it's not fully automated (for example, I still have to manually ensure all the correct fields are in the cabal file, usually by copy/pasting from another package and modifying). Are there any tools that I haven't found that make this process a bit more painless? I'm a bit new to this area, and I've only started using git recently as a prelude to uploading my Haskell packages, so admittedly I may have missed something obvious or perhaps I'm just doing it all wrong. But if other people do find it painful like me, perhaps I'll put some effort into rewriting my perl scripts into nice haskell packages and executables for others to use.

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Package takeover: dbus
by Andrey Sverdlichenko 18 Mar '17

18 Mar '17

I'd like to take over maintenance of the package dbus. It depends on several other packaged with upper bounds too restrictive for the latest versions. I tried to contact owner a few times weeks ago, but to no avail. My hackage username is blaze.

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Ideas for Haskell Summer of Code
by Niki Vazou 18 Mar '17

18 Mar '17

Hello, As a preparation for the 2017 edition of Haskell Summer of Code, we are crowdsourcing project ideas. Compiler optimizations, new libraries, desired features, or any other ideas you would like to see in the Haskell ecosystem. You can submit tickets to the Haskell Summer of Code Ideas page: https://ghc.haskell.org/trac/summer-of-code/report/1 Alternatively, you can tell us your SoC idea by responding to this thread (and I can generate the ticket). Best, Niki

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How can I use “cabal repl” without base
by Adam Steen 17 Mar '17

17 Mar '17

Hi How can I use cabal repl (ghci) without base? The following contrived example shows the error -- minimal.hsmodule Minimal where myId :: a -> a myId x = x and -- minimal.cabal name: minimal version: 0.0.1 build-type: Simple cabal-version: >= 1.10 library exposed-modules: Minimal --build-depends: base default-language: Haskell2010 default-extensions: NoImplicitPrelude cabal build succeeds $ cabal build Building minimal-0.0.1... Preprocessing library minimal-0.0.1...[1 of 1] Compiling Minimal ( Minimal.hs, dist\build\Minimal.o ) but cabal repl fails $ cabal repl Preprocessing library minimal-0.0.1... GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help <interactive>:1:6: error: Not in scope: `System.IO.hSetBuffering' No module named `System.IO' is imported. <interactive>:1:30: error: Not in scope: `System.IO.stdin' No module named `System.IO' is imported. <interactive>:1:46: error: Not in scope: data constructor `System.IO.NoBuffering' No module named `System.IO' is imported. <interactive>:1:70: error: Not in scope: `System.IO.hSetBuffering' No module named `System.IO' is imported. <interactive>:1:94: error: Not in scope: `System.IO.stdout' No module named `System.IO' is imported. <interactive>:1:111: error: Not in scope: data constructor `System.IO.NoBuffering' No module named `System.IO' is imported. <interactive>:1:135: error: Not in scope: `System.IO.hSetBuffering' No module named `System.IO' is imported. <interactive>:1:159: error: Not in scope: `System.IO.stderr' No module named `System.IO' is imported. <interactive>:1:176: error: Not in scope: data constructor `System.IO.NoBuffering' No module named `System.IO' is imported. if i restore "build-depends: base" if works as expected. $ cabal repl Preprocessing library minimal-0.0.1... GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help[1 of 1] Compiling Minimal ( Minimal.hs, interpreted ) Ok, modules loaded: Minimal.*Minimal> i know just keeping "build-depends: base" will make it work, but thats not what i am asking. Note: this also applies for stack Cheers Adam ps this is cross posted from stackoverflow <http://stackoverflow.com/q/42823547/415681>

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Re: [Haskell-cafe] To [] Or Not To []
by Olaf Klinke 17 Mar '17

17 Mar '17

Johannes, Thanks for writing this. Actually, your negative abstract turns out to be for a quite positive article. List are like iterators, that's the essence, and as long as you are iterating, it's fine to use []. The prominence of singly linked lists in Haskell has tought me to write my data processing programs in a streaming style. To think about what data must be held in memory (use Maps or whatever for this) and read the other data in as a list. I'd be interested whether there is a way to check which of my lists in the source code the compiler managed to "deforest" away. Which intermediate files should I look at? What are the tools to inspect? Olaf

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amortized time and space for set operations
by Michael Litchard 17 Mar '17

17 Mar '17

What's the best way to calculate the amortized time and space complexity for the operations in "Implementing Sets Efficiently in a Functional Language". http://groups.csail.mit.edu/mac/users/adams/BB/92-10.ps

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