March 2017 - Haskell-Cafe

Blast: a simple distributed computing library
by jean-christophe mincke 06 Mar '17

06 Mar '17

Hi everyone. Over the past few months, I have written Blast, a distributed computing library, inspired by Apache Spark. It is now open source and can be found on github: https://github.com/jcmincke/Blast Regards J-C

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Mailing list request for DataHaskell
by Marco Zocca 06 Mar '17

06 Mar '17

Dear all, I write on behalf of DataHaskell [0, 1], an open source community devoted to data science, scientific computing and machine learning. We would like haskell.org to host a mailing list for our community, to archive discussions that might otherwise get lost on the Gitter chatroom [2]. With this email I'd also like to extend the invitation to join to all interested people and companies. Thank you and kind regards, Marco Zocca (https://github.com/ocramz) [0] datahaskell.org [1] github.com/DataHaskell [2] https://gitter.im/dataHaskell/Lobby

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Inlinable (>>>)
by David Sorokin 05 Mar '17

05 Mar '17

Hi Cafe, I offer to define the functions of the Control.Category module inlinable: -- | Right-to-left composition (<<<) :: Category cat => cat b c -> cat a b -> cat a c {-# INLINABLE (<<<) #-} (<<<) = (.) -- | Left-to-right composition (>>>) :: Category cat => cat a b -> cat b c -> cat a c {-# INLINABLE (>>>) #-} f >>> g = g . f Perhaps all functions from this module should be marked by this pragma as possible. I suppose that the current definition without the pragma is the cause why the execution slows down in my applications, when using monad parameters in the constraints. I noticed it yet two or three years ago, while playing with the monad transformers, but decided to write now. Best regards, David Sorokin

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Declaring Functors for Type constrained data types
by Guru Devanla 01 Mar '17

01 Mar '17

Hello All, I am working on a program that will define a bunch of tasks. Each task will have to implement certain methods as part of a type class. -- task 1 data UpdateAcctsTask = UpdateAccts -- task 2 data EmailConfig = EmaiConfig {someattrs::String} data SendEmailTask = SendEmailsTask EmailConfig -- task 3 data GeneralWriterTask a = GeneralWriterTask a Each of these tasks implement a class, Taskable. The return values are simplified for this example. class Taskable a where process :: a -> Bool can_run :: a -> Bool This works fine. I can expand on these tasks and execute them. Now, I wanted to be able to defined dependencies between these (Taskable's). I decided I could create a data type for this dependency and may be also get a FreeMonad around this structure for further processing using a graph of Tasks. But, before that I wanted to create an wrapper for these Taskables and create a functor for it as follows The first thing I did was, define a Task, which generalizes over all the above defined (and future Taskables) data Task a where Task :: (Taskable a) => a -> Task a instance Functor Task where fmap:: (Taskable a, Taskable b) -> (a -> b) -> Task a -> Task b --- THIS DOES NOT WORK fmap f (Task a) = Task $ f a But, I realized that I cannot define an fmap over a type constraint. My questions are: 1. Is there any way to do this. I see there is an answer of SO. I wanted to make sure if there were any improvements to this since that answer' was posted. http://stackoverflow.com/questions/17157579/functor-instance-for-a-gadt-wit… 2. Secondly, I would like to know why this is not possible. Is it a current limitation of GHC or if there is some fundamental category theory concepts that dis-allows such declarations that I need to grok! Appreciate any help on this. Thank you!

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Re: [Haskell-cafe] Looking for maintainers or comaintainers on my Haskell projects
by sighingnow＠163.com 01 Mar '17

01 Mar '17

Hi! I’m interested in co-maintaining the Hindent project. I use Hindent extensively and I have submitted several patches to Hindent on github during the last year. I would like to contribute to improving Hindent and making it better. Thanks. Tao He

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block comments in cabal files?
by Johannes Waldmann 01 Mar '17

01 Mar '17

I totally could not understand why "stack install" suddenly failed with "ghc: unrecognised flag: -" Then it dawned on me: you cannot use {- .. -} for block comments in cabal files. (the syntax has no block comments at all?) yes, Wadler's law and all, but still. - J.

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