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The fastest way to generate 2d data stucture

Haskell on Reddit - Fri, 10/23/2015 - 4:03pm

Im solving a problem which requires two dimensional data structure.

Im using Matrix from Data.Matrix and fromLists to generate a structure.

However, I need 1000 x 1000 Matrix and the creation process takes too long.

Is there a better / faster way to do that ? Using Data.Matrix is not necessary

submitted by raw909
[link] [2 comments]
Categories: Incoming News

Records with different fields for its constructors can cause a run-time error.

Haskell on Reddit - Fri, 10/23/2015 - 4:02pm

I was just bitten by this behavior.

Here is an example program:

module Main where data ExecResult = ExecFail { exitcode :: Int , stdout :: String , stderr :: String } | ExecSucc { stdout :: String , stderr :: String } deriving (Eq, Show) main :: IO () main = do let failure = ExecFail 1 "fail stdout" "fail stderr" let success = ExecSucc "succ stdout" "succ stderr" putStrLn "failure:" print $ stdout failure print $ stderr failure print $ exitcode failure putStrLn "success:" print $ stdout success print $ stderr success print $ exitcode success putStrLn "done!"

Here is the result:

$ runhaskell -Wall -Werror main.hs failure: "fail stdout" "fail stderr" 1 success: "succ stdout" "succ stderr" main.hs: No match in record selector exitcode

I was surprised by this. I always assumed that this was the kind of run-time error that Haskell should be able to prevent.

I'm also surprised that there isn't a warning about creating such a data-type, since it has the potential to cause run-time errors.

submitted by joehillen
[link] [47 comments]
Categories: Incoming News

'Associative' order of calling

haskell-cafe - Fri, 10/23/2015 - 3:26pm
Is there a name for a fold that promises to call a function such that only an associative function will always return the same result. Or in other words, it has the property that it promises to call a function "associatively" on a set of data? Thanks! Charlie _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

fixed-length list library that uses Nat?

Haskell on Reddit - Fri, 10/23/2015 - 2:17pm

Is there a library in Stackage for fixed-length lists that uses the Nat kind for lengths, rather than hand-written Z/S types? The list type constructor should kind "Nat -> * -> *".

submitted by AshleyYakeley
[link] [19 comments]
Categories: Incoming News

Using cabal to build in subdirectories?

Haskell on Reddit - Fri, 10/23/2015 - 1:56pm


I use haskell to to Project Euler problems, so my "project" directory looks like p1,p2, p3 ... pn for each different problem that I'm working on. inside each subdirectory there's a Main.hs and any other supporting .hs files.

I'm wondering if there's a way to use cabal with unix wildcards or regexes to build everything without needing an explicit executable section for each problem in the .cabal file.

submitted by cptwolf
[link] [7 comments]
Categories: Incoming News

Meta: How does Reddit know my "Recently Viewed Links"?

Haskell on Reddit - Fri, 10/23/2015 - 5:10am

If I click one of the posted links to an external site (not the comments for that post) it appears on the sidebar under "Recently Viewed Links". How does Reddit know that I've viewed the external site?

submitted by tomejaguar
[link] [7 comments]
Categories: Incoming News

Is it appropriate to ask for help writing an algorithm here in exchange for bounties (money)? If not, is there a better place?

Haskell on Reddit - Thu, 10/22/2015 - 11:04pm

So unless a mod posts that's it's fine, I won't place my specific inquiry here in order to not break the spirit of the post.

Basically I need a script that's beyond my ability to write and I'm willing to pay for it. It's not anywhere near a full on software project. I just need to process a set of values and run a (difficult) calculation on them and output the solution to the terminal or to a file or whatever. Based on my non-professional experience, I'd say it's on the level of an extremely hard homework assignment.

Is there a good place for me to ask for this? I'm asking in this sub, because the problem seems particularly suited to Haskell or other functional languages.

submitted by rorriMnmaD
[link] [10 comments]
Categories: Incoming News

Haskell Computer Vision SIFT binding?

Haskell on Reddit - Thu, 10/22/2015 - 10:41pm

Anyone know a way to run a SIFT detector/descriptor from Haskell?

I've been doing my Machine Perception homework in Haskell so far, and don't want to stop here.

submitted by CarbonFire
[link] [13 comments]
Categories: Incoming News

Why don't we hear people complaining about Clean's overloaded `length`?

Haskell on Reddit - Thu, 10/22/2015 - 9:21pm

Wasn't one of Clean's original design goals to be an educational language for strongly-typed, lazy functional programming? So how come we're not hearing educators complaining that Clean's length function is overloaded and the one for [] is just an instance?

In fact, I happened to go to a university where (at least at that time) Clean was used in the introductory functional programming class, and I specifically remember that when length was used in an early example, the lecturer mentioned that it's overloaded, we'll be covering typeclasses in a couple lectures, just pretend that its type is specialized to [], and we swiftly moved on.

Why does the similar situation in Haskell suddenly freak out so many people?

submitted by gergoerdi
[link] [44 comments]
Categories: Incoming News

[ANN] haskell-ide project

haskell-cafe - Thu, 10/22/2015 - 8:06pm
The ongoing effort to save us as a community from separately implementing the same functionality over and over around IDEs[1] has found a home in the haskell-ide project on github [2]. This mail serves as a call to collaborate for all parties interested in this space. It is still at a very early stage, but the intent is to pool resources so that we can have really good support for IDEs that can extend through the entire tool chain from changes to GHC to better support tooling through the build environment and community information management. This should mean that people who are familiar with a particular part of the chain can focus their efforts there, knowing that the other parts will be covered too. This is a deliberately broad and vague scope, the reality is likely to be much more limited initially, but the sooner we can get started the sooner we will have something concrete to criticise and improve. For those wishing to get involved the following resources currently exist * the haskell-ide maili
Categories: Offsite Discussion

Expressiveness of Morte

Haskell on Reddit - Thu, 10/22/2015 - 4:52pm

I’m wondering if it would be possible to translate GHC Core or STG to Morte (calculus of constructions). More generally, is there any information/papers on the difficulties/limitations of Morte/CoC?

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

Breaking Changes and Long Term Support Haskell

libraries list - Wed, 10/21/2015 - 10:33am
There seems to be a fair amount of friction between those who want to introduce new features or fix significant historical warts in the base libraries - even if this requires breaking changes - and those who insist on no significant breaking changes in new releases, regardless of the reason or how much warning was given. The rest of the industry has already solved this with long-term/extended support releases. Some version of the platform is blessed with long-term support, and will continue to receive bug fixes and security patches for a number of years, but no other changes. This solution exists for Haskell as well, in the form of Long-Term Support Haskell ( by FP Complete. Not only GHC but an entire ecosystem of libraries are frozen when a new LTS version is released and only bug/security fixes are allowed going forward. Users requiring long-term stability over new features could use LTS Haskell, while those who accept some fixing up of old code
Categories: Offsite Discussion

3 release policy

libraries list - Tue, 10/20/2015 - 8:55pm
A "3 release policy" has been recently mentioned several times, whereby it should always be possible to write code that compiles with the last three releases of GHC, without generating any -Wall warnings. The no warning requirement seems excessively harsh. Will early warnings of impending breakage really cause so much trouble that accepted proposals have to be dragged out over several years to avoid them? If so, would a flag to suppress the warnings suffice? I should note that GHC has traditionally had no qualms about introducing new warnings, on by default. -- View this message in context: Sent from the Haskell - Libraries mailing list archive at
Categories: Offsite Discussion

Default definition for fromRational

libraries list - Thu, 10/15/2015 - 11:51pm
A suitable default definition for fromRational could be the following: fromRational n = fromInteger (numerator n) / fromInteger (denominator n) Changing the MINIMUM pragma to just {-# MINIMAL recip | (/) #-} - Joe
Categories: Offsite Discussion

New gtk2hs 0.12.4 release

gtk2hs - Wed, 11/21/2012 - 12:56pm

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!


Categories: Incoming News