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Summer Plans with Haskell (no internship)

Haskell on Reddit - Sun, 04/26/2015 - 10:13am

Hey guys! I'm new to this subreddit. I have recently gotten into Haskell, and I'm loving it so far. The functional paradigm is a really fun way to reason about programming.

So, like the title says, this summer I do not have an internship lined up, so my plan is to do a lot of personal projects, post a lot on github, contribute to open source projects, and possibly attend tech meetups to network. As such, I should probably learn as much as I can this summer, right?

I really want to get good at Haskell, though, like really really good. And functional programming in general. My question is, if I spend all of my summer doing Haskell, getting good at it and doing a lot of good work with it, would I have a shot at getting interviews for places that use functional programming, even if I don't have too much experience with other languages?

Either way, I will probably dabble in more popular languages like Scala, Erlang, F#, etc, but as of now I'm making haskell a priority. Should I split the time more evenly, or will learning haskell pay more dividends when it comes to being a better programmer, and still not hurt my chances of getting interviews at places whose stack is heavily functional based?

Thanks for reading!

submitted by uncountableB
[link] [4 comments]
Categories: Incoming News

Using protocol buffers in ghc 7.10

Haskell on Reddit - Sun, 04/26/2015 - 9:48am

I'm starting out in Haskell and trying to build something that requires reading protocol buffers in ghc 7.10. I'm using 7.10 because it's the latest and I don't want to learn old stuff that I have to later unlearn and it's what I have on my Linux distro (Arch Linux). Is there a package for reading protocol buffers that I can use?

protocol-buffers: Build errors when I try to cabal install it.

protobuf: I can't get code to work that's modeled after the first example in the hackage docs. I get a parse error on the part where I express the field tag. (

protobuf-native: Seems to require interfacing with C++. I'm not sure whether I'm ready for that.

submitted by Syncopat3d
[link] [9 comments]
Categories: Incoming News

Joachim Breitner: Fifth place in Godingame World Cup

Planet Haskell - Sun, 04/26/2015 - 9:01am

Last evening, Codingame held a “Programming World Cup” titled “There is no Spoon”. The format is that within four hours, you get to write a program that solves a given task. Submissions are first rated by completeness (there are 13 test inputs that you can check your code again, and further hidden tests that will only be checked after submission) and then by time of submission. You can only submit your code once.

What I like about Codingame is that they support a great number of programming languages in their Web-“IDE”, including Haskell. I had nothing better to do yesterday, so I joined. I was aiming for a good position in the Haskell-specific ranking.

After nearly two hours my code completed all the visible test cases and I submitted. I figured that this was a reasonable time to do so, as it was half-time and there are supposed to be two challenges. I turned out that the first, quite small task, which felt like a warm-up or qualification puzzle, was the first of those two, and that therefore I was done, and indeed the 5th fastest to complete a 100% solution! With only less than 5 minutes difference to the 3rd, money-winning place – if I had known I had such a chance, I had started on time...

Having submitted the highest ranked Haskell code, I will get a T-Shirt. I also defended Haskell’s reputation as an efficient programming language, ranked third in the contest, after C++ (rank 1) and Java (rank 2), but before PHP (9), C# (10) and Python (11), listing only those that had a 100% solution.

The task, solving a Bridges puzzle, did not feel like a great fit for Haskell at first. I was juggling Data.Maps around where otherwise I’d simple attach attributes to object, and a recursive function simulated nothing but a plain loop. But it played off the moment I had to implement guessing parts of the solution, trying what happens and backtracking when it did not work: With all state in parameters and pure code it was very simple to get a complete solution.

My code is of course not very polished, and having the main loop live in the IO monad just to be able to print diagnostic commands is a bit ugly.

The next, Lord of the Ring-themed world cup will be on June 27th. Maybe we will see more than 18 Haskell entries then?

Categories: Offsite Blogs

503 on upload to Hackage?

haskell-cafe - Sun, 04/26/2015 - 7:55am
Yesterday and today I made a few releases of a package to Hackage. On each upload I get a 503 error. This is the text: ~~~ Error 503 backend read error backend read error Guru Mediation: Details: cache-fra1229-FRA 1430030130 1668038750 Varnish cache server ~~~ I'm guessing this isn't expected behaviour. Has anyone else seen this? /M
Categories: Offsite Discussion

Why do packages need to be precompiled?

Haskell on Reddit - Sun, 04/26/2015 - 5:03am

It seems to me that many of the problems with cabal are due to precompiling packages, because, as far as I can see, Haskell never guarantees ABI compatibility.

Why, then, are packages precompiled, instead of coherently building all the modules used from source when the program is compiled?

submitted by 1-05457
[link] [8 comments]
Categories: Incoming News

20k /r/haskell subscribers. Nice!

Haskell on Reddit - Sun, 04/26/2015 - 3:42am

Pretty cool that we made it to 20k readers. That makes /r/haskell the 7th largest language community on reddit. Not bad for an obscure academic language ;)

submitted by dons
[link] [50 comments]
Categories: Incoming News

Coplanarity or Colinearity [Was: low-cost matrixrank?]

haskell-cafe - Sat, 04/25/2015 - 2:53pm
Well, none of the suggested solutions for computing the rank of a matrix really suited my needs, as dragging in something like BLAS introduce more cost than just integrating the bed-and-breakfast library into my own library. So let me try a different track. My real problem is that I've got a list of points in R3 and want to decide if they determine a plane, meaning they are coplanar but not colinear. Similarly, given a list of points in R2, I want to verify that they aren't colinear. Both of these can be done by converting the list of points to a matrix and finding the rank of the matrix, but I only use the rank function in the definitions of colinear and coplanar. Maybe there's an easier way to tackle the underlying problems. Anyone got a suggestion for such? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

GHCi shows result of (IO a) only if (a) is in classShow

haskell-cafe - Sat, 04/25/2015 - 1:21pm
I wrote a parser and it took me a while to realize why GHCi suddenly did not show any result nor an error message anymore. My parsing function has type (IO Expression), Expression is in class Show. After changing the parser to (IO Declaration), it did not show anything anymore, because Declaration was not in class Show. When I typed (parseFile "input.txt" >>= print), I got the error message and understood what was going on. But for I while I was really confused what's happening. Just wanted to share this. Best, Daniel
Categories: Offsite Discussion

Haskell as a main language with basic maths skills

Haskell on Reddit - Sat, 04/25/2015 - 9:23am


I learned haskell some years ago, but then I stopped and now I really want to learn it, but what stopped me the first time was that I found haskell like a toy-language or a language for specific application, somewhere I am a amateur programmer and I'd like to use it for web developpement, little interpreter/compilater etc.

So the question : Can a student with basic skills in maths (ie no formation in maths) can become good in Haskell, or I will be blocked by my lack of skills anywhere in my learning?

Too, does Haskell really can be use for web developpement and general purpose, or its kinda tricky to use it like that?

Thanks for your answers and sorry for my english.

submitted by vildric
[link] [26 comments]
Categories: Incoming News

Neil Mitchell: Cleaning stale files with Shake

Planet Haskell - Sat, 04/25/2015 - 8:21am

Summary: Sometimes source files get deleted, and build products become stale. Using Shake, you can automatically delete them.

Imagine you have a build system that compiles Markdown files into HTML files for your blog. Sometimes you rename a Markdown file, which means the corresponding HTML will change name too. Typically, this will result in a stale HTML file being left, one that was previously produced by the build system, but will never be updated again. You can remove that file by cleaning all outputs and running the build again, but with the Shake build system you can do better. You can ask for a list of all live files, and delete the build products not on that list.

A basic Markdown to HTML converter

Let's start with a simple website generator. For each Markdown file, with the extension .md, we generate an HTML file. We can write that as:

import Development.Shake
import Development.Shake.FilePath

main :: IO ()
main = shakeArgs shakeOptions $ do
action $ do
mds <- getDirectoryFiles "." ["//*.md"]
need ["output" </> x -<.> "html" | x <- mds]

"output//*.html" %> \out -> do
let src = dropDirectory1 out -<.> "md"
need [src]
cmd "pandoc -s -o" [out, src]

phony "clean" $ do
removeFilesAfter "output" ["//*.html"]

Nothing too interesting here. There are three parts:

  • Search for all .md files, and for each file foo/ require output/foo/bar.html.
  • To generate an .html file, depend on the source file then run pandoc.
  • To clean everything, delete all .html files in output.

Using a new feature in Shake 0.15, we can name save this script as Shakefile.hs and then:

  • shake will build all the HTML files.
  • shake -j0 will build all the files, using one thread for each processor on our system.
  • shake output/foo.html will build just that one HTML file.
  • shake clean will delete all the HTML files.

Removing stale files

Now let's imagine we've added a blog post Before publishing we decide to rename our post to If we've already run shake then there will be a stale file output/using-pipes.html. Since there is no source .md file, Shake will not attempt to rebuild the file, and it won't be automatically deleted. We can do shake clean to get rid of it, but that will also wipe all the other HTML files.

We can run shake --live=live.txt to produce a file live.txt listing all the live files - those that Shake knows about, and has built. If we run that after deleting it will tell us that and output/ are both "live". If we delete all files in output that are not mentioned as being live, that will clean away all our stale files.

Using Shake 0.15.1 (released in the last hour) you can write:

import Development.Shake
import Development.Shake.FilePath
import Development.Shake.Util
import System.Directory.Extra
import Data.List
import System.IO

pruner :: [FilePath] -> IO ()
pruner live = do
present <- listFilesRecursive "output"
mapM_ removeFile $ map toStandard present \\ map toStandard live

main :: IO ()
main = shakeArgsPrune shakeOptions pruner $ do
... as before ...

Now when running shake --prune it will build all files, then delete all stale files, such as output/using-pipes.html. We are using the shakePrune function (just sugar over --live) which lets us pass a pruner function. This function gets called after the build completes with a list of all the live files. We use listFilesRecursive from the extra package to get a list of all files in output, then do list difference (\\) to delete all the files which are present but not live. To deal with the / vs \ path separator issue on Windows, we apply toStandard to all files to ensure they match.

A few words of warning:

  • If you run shake output/foo.html --prune then it will only pass output/foo.html and as live files, since they are the only ones that are live as you have asked for a subset of the files to be built. Generally, you want to enable all sensible targets (typically no file arguments) when passing --prune.
  • Sometimes a rule will generate something you care about, and a few files you don't really bother tracking. As an example, building a GHC DLL on Windows generates a .dll and a .dll.a file. While the .dll.a file may not be known to Shake, it probably doesn't want to get pruned. The pruning function may need a few special cases, like not deleting the .dll.a file if the .dll is live.
Categories: Offsite Blogs

Finding subset summing to zero.

Haskell on Reddit - Sat, 04/25/2015 - 3:38am

Often, I when reconciliating accounting stuff, I need to decompose a sum of numbers as a sum of other numbers (all numbers are given). For example I have an invoice of 10, two payments of 3 and 5 and credit note of 2. I would need something which given [2,3,5,10] returns [-10, 2,3,5] ("3+5 - 10 +2 == 0" ) as well all other possibilties.

I'm sure I kind do it myself, but I don't like to reinvent the wheel, so does anybody knows if such a package exists or one which could facilitate this problem ?

submitted by maxigit
[link] [18 comments]
Categories: Incoming News

in this Functor typeclass implementation, what is 'f'?

Haskell on Reddit - Sat, 04/25/2015 - 12:57am

class Functor f where
fmap :: (a -> b) -> f a -> f b

I guess f is not a type , but rather a type constuctor?

submitted by tmtwd
[link] [14 comments]
Categories: Incoming News