December 2014 - Haskell-Cafe

cabal keeps relinking
by Semen Trygubenko / Семен Тригубен ко 23 Jan '15

23 Jan '15

Dear Haskell-Cafe, cabal seems to be re-linking every time I run cabal build Wait was not an issue for small projects but for big ones it is starting to get in the way… How could I persuade cabal to re-link only if there are changes in the source tree and only these exes that require it? Many thanks, Semen PS I googled around and found this https://www.haskell.org/pipermail/libraries/2005-August/004306.html which seems to imply relink is (was?) conditional on multiple exes being present in the project. However, for me it was always relinking regardless. -- Семен Тригубенко http://trygub.com

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Re: [Haskell-cafe] Typesafe API calls
by Fabian Bergmark 12 Jan '15

12 Jan '15

I have uploaded a new version of the library https://hackage.haskell.org/package/apis-0.0.1 I'have included 15 more APIs from Facebook and Dropbox. I have also uploaded documentation to Hackage. I have been working on creating an autotools installation script that builds a makepkg package with the FFI bindings for Python and Java. I have tried it on Arch and it seems to work. If you want to try it out, check out the github repository. Building on top of this, Im working on a library that uses Pipes to create producers (triggers) and consumers (actions), allowing one to create effects (recipes) of the kind, when I am tagged in a photo on Facebook, notify me via email. This very much resembles IFTTT but with static and typesafe compositions. Im experimenting with using my javascript interpreter to allow for more dynamic compositions. 2014-12-18 21:59 GMT+01:00 Jonathan Paugh <jpaugh(a)gmx.us>: > Hello Fabian, > > I have been dreaming of something like this for Haskell since I wrote a > (non type-safe) python library to access any REST API generically[1]. > This comes from a different (and more ambitious) angle than I had imagined. > > I don't have time to test this just now, but I will keep it bookmarked. > Thanks for sharing your hard work! > > Regards, > Jonathan > > [1]: http://github.com/jpaugh/agithub (unmaintained, but maybe interesting) > > On 12/12/2014 07:01 AM, Fabian Bergmark wrote:> I just released a > library on hackage >> (https://hackage.haskell.org/package/apis-0.0.0) The goal of the >> library is to provide typesafe API calls to a large amount of existing >> APIs. By typesafe I mean that, for each API the library provides two >> types, Input and Output, and a function :: Input -> IO Output. >> Internally the library knows how to serialize Input to make an HTTP >> request and how to deserialize the response into Output. >> >> Whats cool about this library is that it uses Template Haskell to >> generate the API interfaces. A combination of Open Data Tables, an >> internal YQL engine and JSON Schemas, all written in Haskell makes >> this possible. >> >> Right now only two APIs are included (Swedish weather data and Swedish >> public announcement), but more APIs are soon to follow. >> >> I have also implemented FFI exports of all types and functions so >> that, in combination with SWIG, this library can be used in almost any >> language. So far I have tried Java and Python with success. >> >> If anyone would like to try it and respond with feedback it would be >> greatly appreciated! If you want any specific API supported, I can try >> to import that right away >> >> Fabian Bergmark >> >

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Postgresql Intervals with Opaleye
by info＠rotnetix.com 06 Jan '15

06 Jan '15

Another opaleye question: What type should I use for the postgresql interval type? Thanks Riaan

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ECMA262 (Javascript) interpreter
by Fabian Bergmark 04 Jan '15

04 Jan '15

I have been implementing an ECMA-262 (Javascript) interpreter over the last few weeks. I have been following http://www.ecma-international.org/ecma-262/5.1 and tried to keep my implementation as close to the specification as possible. The code can be found here https://github.com/fabianbergmark/ECMA-262 So far I have implemented the core language, but has yet to write all the specification functions (Array.splice, String.substring etc.). Before I do this and release it on hackage, I would appreciate feedback on my implementation. The issues I see at the time are: * The parser is slow for deeply nested code * Strict mode is not implemented * Interpret.hs is ~ 6k lines * SubType.hs uses IncoherentInstances, and a better implementation would be nice. At the moment it works but for one case. To test the code, install the cabal package and run ecma262 on a .js file. One host object, console.log is provided and prints to stdout. Fabian Bergmark

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Handling Postgresql array types
by Riaan 03 Jan '15

03 Jan '15

This is a question about database query libraries. I am trying to do some analytics on a Postgresql database and up to now have been using Database.HDBC for most of my querying. One of the columns on the database is a two dimensional float8 array. I tried using Database.PostgreSQL.Simple but got stuck on this array as I could not figure out how to extend the built in conversion functions to cater for something like this. So went back to HDBC. But now my queries are starting to get fairly long and I have been looking at libraries like Persistent with Esqualeto, HaskellDB and Groundhog to make my queries a little more composable and type safe. However I have not been able to find any examples of any of these libraries using postgres arrays and given my postgresql-simple experience I worried that I might similarly be unable, because of my low level of haskell ability, to extend the conversion functions. So to my question. Does anyone have experience with one of these libraries in dealing with postgresql arrays and could someone perhaps send me a simple example that I could use as a basis. Failing that, can anyone advise me on which of these libraries have the most haskell newbie (LYAH trained, but still struggling with RWH) friendly approach to writing new conversion functions? Thanks Riaan

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Re: [Haskell-cafe] Yet Another Forkable Class
by oleg＠okmij.org 02 Jan '15

02 Jan '15

Perhaps effect libraries (there are several to choose from) could be a better answer to Fork effects than monad transformers. One lesson from the recent research in effects is that we should start thinking what effect we want to achieve rather than which monad transformer to use. Using ReaderT or StateT or something else is an implementation detail. Once we know what effect to achieve we can write a handler, or interpreter, to implement the desired operation on the World, obeying the desired equations. And we are done. For example, with ExtEff library with which I'm more familiar, the Fork effect would take as an argument a computation that cannot throw any requests. That means that the parent has to provide interpreters for all child effects. It becomes trivially to implement: > Another example would be a child that should not be able to throw errors as > opposed to the parent thread. It is possible to specify which errors will be allowed for the child thread (the ones that the parent will be willing to reflect and interpret). The rest of errors will be statically prohibited then. > instance (Protocol p) => Forkable (WebSockets p) (ReaderT (Sink p) IO) where > fork (ReaderT f) = liftIO . forkIO . f =<< getSink This is a good illustration of too much implementation detail. Why do we need to know of (Sink p) as a Reader layer? Would it be clearer to define an Effect of sending to the socket? Computation's type will make it patent the computation is sending to the socket. The parent thread, before forking, has to provide a handler for that effect (and the handler will probably need a socket). Defining a new class for each effect is possible but not needed at all. With monad transformers, a class per effect is meant to hide the ordering of transformer layers in a monad transformer stack. Effect libraries abstract over the implementation details out of the box. Crutches -- extra classes -- are unnecessary. We can start by writing handlers on a case-by-case basis. Generalization, if any, we'll be easier to see. From my experience, generalizing from concrete cases is easier than trying to write a (too) general code at the outset. Way too often, as I read and saw, code that is meant to be reusable ends up hardly usable.

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Type-directed functions
by Julian Arni 02 Jan '15

02 Jan '15

Hi all, I've been playing around with what might be described as type-directed functions. One example is a list-like structure of phantom-typed values {-# LANGUAGE TypeOperators #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} import GHC.TypeLits infixr 6 ::: data a ::: b = a ::: b deriving (Show, Eq) data Tag b = Tag String deriving (Show, Eq) ex1 :: Tag 5 ::: Tag 3 ::: Tag 7 ex1 = Tag "Alice" ::: Tag "Bob" ::: Tag "Carol" And then sorting 'ex1' based on the Nats, such that sort ex1 :: Tag 3 ::: Tag 5 ::: Tag 7 sort ex1 = Tag "Bob" ::: Tag "Alice" ::: Tag "Carol" Notice how it's the types, not the values, that determine the result, but that the value-level also changes. I know how to do this using classes, but it's a little excruciating - it's like programming in a verbose and very restricted Prolog. With type families it's much easier to get the result *type* (pattern matching is simple, recursive calls are natural, and it all looks a lot more like Haskell), but I haven't yet seen a way of effectively using type families to direct the value-level component of the calculation. Are there any examples of how this might be done? Or are there other alternatives to using type-classes that I am missing? Or, alternatively, are there libraries to reduce the boilerplate of this type-class code? Thanks, Julian

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GHC(STG,Cmm,asm) illustrated for hardware persons
by Takenobu Tani 02 Jan '15

02 Jan '15

Dear Haskellers, I love haskell. I'm studying and playing around with Haskell, GHC, STG, Cmm, asm... When I started studying them, I wanted some figures. I've not have some mental models because I am a hardware person. So I am drawing few illustrations for hardware persons. Here is: GHC(STG,Cmm,asm) illustrated for hardware persons http://takenobu-hs.github.io/downloads/haskell_ghc_illustrated.pdf https://github.com/takenobu-hs/haskell-ghc-illustrated If there are some misunderstandings or errors, please teach me. I'll correct or remove them. Thank you :-) I wish you a happy new year, Takenobu

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ordNub
by Niklas Hambüchen 01 Jan '15

01 Jan '15

tldr: nub is abnormally slow, we shouldn't use it, but we do. As you might know, Data.List.nub is O(n²). (*) As you might not know, almost *all* practical Haskell projects use it, and that in places where an Ord instance is given, e.g. happy, Xmonad, ghc-mod, Agda, darcs, QuickCheck, yesod, shake, Cabal, haddock, and 600 more (see https://github.com/nh2/haskell-ordnub) I've taken the Ord-based O(n * log n) implementation from yi using a Set: ordNub :: (Ord a) => [a] -> [a] ordNub l = go empty l where go _ [] = [] go s (x:xs) = if x `member` s then go s xs else x : go (insert x s) xs and put benchmarks on http://htmlpreview.github.io/?https://github.com/nh2/haskell-ordnub/blob/1f… (compare `nub` vs `ordNub`). `ordNub` is not only in a different complexity class, but even seems to perform better than nub for very small numbers of actually different list elements (that's the numbers before the benchmark names). (The benchmark also shows some other potential problem: Using a state monad to keep the set instead of a function argument can be up to 20 times slower. Should that happen?) What do you think about ordNub? I've seen a proposal from 5 years ago about adding a *sort*Nub function started by Neil, but it just died. (*) The mentioned complexity is for the (very common) worst case, in which the number of different elements in the list grows with the list (alias you don't have an N element list with always only 5 different things inside).

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HaskellNet: maintainer(s) needed!
by Jonathan Daugherty 31 Dec '14

31 Dec '14

Hi, TLDR: HaskellNet needs a new maintainer! I am the current maintainer of HaskellNet, a library for accessing POP, IMAP, and SMTP services. Because I do not have the time to be the kind of maintainer this library needs, I need help. If you use HaskellNet in production, care about providing this service to the community, and/or feel qualified to provide technical support for this library, please let me know! I am looking for someone interested enough to take ownership of this package. Other than that, some kinds of help desired include: - improve documentation - improve test suite - resolve GitHub support tickets - test on Windows https://github.com/jtdaugherty/HaskellNet http://hackage.haskell.org/package/HaskellNet Thanks, -- Jonathan Daugherty Software Engineer Galois, Inc.

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