July 2015 - Haskell-Cafe

Announcing libravatar @ hackage
by fr33domlover 12 Jul '15

12 Jul '15

Hello! Libravatar - http://libravatar.org - is a free-as-in-freedom and decentralized avatar (profile image delivery service, a replacement of the centralized and proprietary grava7ar. There has been a Haskell library for grava7ar for some time, but not for Libravatar. I just finished writing one, and did a bit of testing which had good results. More testing and feedback etc. very very welcome! I also enourage you all to *integrate libravatar into your desktop/web applications and websites*! Support openness, decentralization and freedom by using Libravatar. --- fr33domlover <http://www.rel4tion.org/people/fr33domlover> GPG key ID: 63E5E57D (size: 4096) GPG key fingerprint: 6FEE C222 7323 EF85 A49D 5487 5252 C5C8 63E5 E57D

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idea: <package>-latest URLs for Hackage
by Ömer Sinan Ağacan 12 Jul '15

12 Jul '15

It's very convenient to have a persistent URL linked to latest version. GHC user manual has this(https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/) and you can create a bookmark with that URL and it stays up-to-date, it appears in first place in Google queries etc. I'm wondering why not implement this for Hackage too? For example, hackage.haskell.org/package/base-latest, hackage.haskell.org/package/base-latest/docs/<module-name>.html etc. Any ideas?

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Fwd: SQL database schema diff tool
by Sergey Mironov 11 Jul '15

11 Jul '15

---------- Forwarded message ---------- From: Sergey Mironov <grrwlf(a)gmail.com> Date: 2015-07-11 22:17 GMT+03:00 Subject: Re: [Haskell-cafe] SQL database schema diff tool To: David Kraeutmann <kane(a)kane.cx> Thanks for the answer, I've looked at simple-migrations application. Unfortunately, it doesn't look like the solution for me. Term 'migration' in PostgreSQL seems to mean 'movement of data from one server to another using the same schema', but I need to move the data from one schema to another as safely as possible having the definitions of both schemas. Regards, Sergey 2015-07-11 21:05 GMT+03:00 David Kraeutmann <kane(a)kane.cx>: > You could try https://github.com/ameingast/postgresql-simple-migration, but I haven't used it myself. "schema migration" seems to be what you want. > On 7/11/2015 7:54 PM, Sergey Mironov wrote: >> Hi, list! I wish I have a tool which would compare two database >> sachemas, where each schema is represented as a list of 'CREATE smth' >> instructions. The result of such a comparison would be a list of >> 'ALTER smth' instructions. This instruction set would change the >> database from first state to second, if the transition is possible >> without data losses. The 'hard' mode - to prepare the transition for >> any inputs, with minimal data losses. >> >> The database of interest is PostgreSQL. Does such a tool exist? If >> not, which package do I better use to build the AST of initial >> schemas? >> >> The example schema is in attach. >> Thanks in advance, >> Sergey >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > >

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SQL database schema diff tool
by Sergey Mironov 11 Jul '15

11 Jul '15

Hi, list! I wish I have a tool which would compare two database sachemas, where each schema is represented as a list of 'CREATE smth' instructions. The result of such a comparison would be a list of 'ALTER smth' instructions. This instruction set would change the database from first state to second, if the transition is possible without data losses. The 'hard' mode - to prepare the transition for any inputs, with minimal data losses. The database of interest is PostgreSQL. Does such a tool exist? If not, which package do I better use to build the AST of initial schemas? The example schema is in attach. Thanks in advance, Sergey

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First call to putStr takes far more time than subsequent calls
by Ben Gunton 11 Jul '15

11 Jul '15

This is a simplified version of an issue I'm running into when timing some functions. In this example, the first putStr inside the replicateM_ takes far longer than subsequent putStrs. If I add in a putStr at the top (the commented line), each iteration takes the same. (Though I presume I just shifted the long execution to the putStr on top). The same thing happens on the first iterations of a function when I need to evaluate some state, or deepseq some value. putStr was just a simpler version that still showed the same issue. Relevant code: http://lpaste.net/136250 What's going on here? Thanks! import Control.Monadimport Data.Time.Clock main :: IO ()main = do -- putStr "" replicateM_ 5 $ do t1 <- getCurrentTime putStr "" t2 <- getCurrentTime print . round $ 1000 * 1000 * diffUTCTime t2 t1 {- Sample output: 49 1 1 1 2 -}

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As reading, what are your favorite Haskell libraries?
by Jeffrey Brown 11 Jul '15

11 Jul '15

A wider question controls also for a library's applicability and (relative) absence of prerequisites, because the funnest reading is not only intrinsically fun, but also extrinsically rewarding and soon feasible (feasible to comprehend or to use). That is, I like simple, beautiful, powerful libraries that do not require me to first learn too many other things. I might value simplicity over power. I'm interested in answers to either question.

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GHC Memory Management wiki page
by Kyle Miller 10 Jul '15

10 Jul '15

I have been studying the garbage collection algorithms used in GHC, and it appears to me that section two of [1] is misleading, since thunks permit controlled mutation of the heap, and so objects in older generations may point to younger objects. In fact, the RTS maintains "remembered sets" to keep track of these relations. A Haskell could in principle be implemented in the way described (and execute in surprising complexity classes), but this does not seem to match reality. [1] https://wiki.haskell.org/GHC/Memory_Management Should the wiki be modified? Regards, Kyle Miller

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Confusion with GeneralizedNewtypeDeriving + MultiParamTypeClasses
by William Yager 10 Jul '15

10 Jul '15

Hello all, I have run into a bit of a strange case where I can't get `GeneralizedNewtypeDeriving` to work as I'd like. In the `vector` package, under `Data.Vector.Generic`, there is a generic vector typeclass of the form `Vector v a`, where `v` is the vector type and `a` is the type of items in the vector. I have a newtype wrapper of the form `newtype Neuron a = Neuron (Vector a)`. (Note that "Vector" here is a type "Vector", not the two-parameter constraint also called "Vector".) I would like to be able to use everything in `Data.Vector.Generic` on `Neuron`s. Using `GeneralizedNewtypeDeriving`, I immediately run up against the issue that the `Vector` typeclass doesn't have the correct form of `* -> Constraint`. According to section 7.5.5.1 of https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/deriving.html , we should be able to derive instances of multi-parameter type classes, provided the newtype is the last parameter of the typeclass. The problem here is that it's the first parameter. Using `ConstraintKinds` to make a constraint synonym (of the form `FlippedVectorClass a v`) so that I can do something like `newtype Neuron a = Neuron (Vector a) deriving (FlippedVectorClass a)` also doesn't seem to work. Does anyone have any advice for how to make this work? It's quite tempting just to make `type Neuron a = Vector a`, but I'd rather do a proper wrapper type. Cheers, Will

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darcs 2.10.1 release
by Guillaume Hoffmann 09 Jul '15

09 Jul '15

Hi all, The darcs team is pleased to announce the release of darcs 2.10.1 ! # Downloading # The easiest way to install darcs 2.10.1 from source is by first installing the Haskell Platform (http://www.haskell.org/platform) If you have installed the Haskell Platform or cabal-install, you can install this release by doing: $ cabal update $ cabal install darcs-2.10.1 Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.10.1.tar.gz and build it by hand as explained in the README file. The 2.10 branch is also available as a darcs repository from http://darcs.net/releases/branch-2.10 # What's new in 2.10.1 (since 2.10.0) # - generalized doFastZip for darcsden support - support terminfo 0.4, network 2.6, zlib 0.6, quickcheck 2.8 and attoparsec 0.13 - errorDoc now prints a stack trace (if profiling was enabled) (Ben Franksen) - beautified error messages for command line and default files (Ben Franksen) - fixed the following bugs: - issue2449: test harness/shelly: need to handle mis-encoded/binary data (Ganesh Sittampalam) - issue2423: diff only respecting --diff-command when a diff.exe is present (Alain91) - issue2447: get contents of deleted file (Ben Franksen) - issue2307: add information about 'darcs help manpage' and 'darcs help markdown' (Dan Frumin) - issue2461: darcs log --repo=remoterepo creates and populates _darcs (Ben Franksen) - issue2459: cloning remote repo fails to use packs if cache is on a different partition (Ben Franksen) # Feedback # If you have an issue with darcs 2.10.0, you can report it on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net, or come to #darcs on irc.freenode.net. # Known issues # - issue2269: rebase should warn about stolen patches at suspend, not unsuspend - issue2272: darcs rebase unsuspend should automate or semi-automate handling unrecorded changes - issue2276: darcs rebase unsuspend needs UI improvements for "You are not... Amend anyway?" - issue2359: convert --export mishandles Unicode filenames - issue2372: Please remove "HINT: I could not reach..." message - issue2436: rollback --patches takes ages before first prompt - issue2445: internal error if suspended patch is pulled into repository again

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(no subject)
by None 09 Jul '15

09 Jul '15

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