December 2011 - Haskell-Cafe

Conduits: Is Source a valid instance of Monad?
by Aristid Breitkreuz 04 Apr '12

04 Apr '12

Hi all, As you may have noticed, Michael Snoyman has been working on an alternative approach to I/O, called conduits. You can find it here: https://github.com/snoyberg/conduit When looking at the Source type (explained here: http://www.yesodweb.com/blog/2011/12/conduits) I noticed that they seem to behave "like lists", and naturally wondered if I could write a Monad instance for them. But first, let's have a brief look at the definition of Source: data SourceResult a = Open a | Closed data PreparedSource m a = PreparedSource { sourcePull :: ResourceT m (SourceResult a) , sourceClose :: ResourceT m () } newtype Source m a = Source { prepareSource :: ResourceT m (PreparedSource m a) } ResourceT deals with resource acquisition and releasing (making sure that all resources are released), and provides a an abstraction over IORef/STRef. For our purposes here, ResourceT is probably close enough to IO. So now the question again is, can we write a Monad instance for this? I have been able to write join (concatenate) and return (a source with a single non-repeated) element. https://gist.github.com/1525471 I _think_ it behaves properly like a Monad, but I'm not quite sure, and neither was Michael. Greg Weber then suggested bringing the question to this forum. What made the question difficult for me is that this would be a stateful Monad transformer, so I'm not quite sure how to test the Monad laws properly. There's a second part to this question: If Source turns out not to be a Monad, is it possibly a ZipList-like Applicative? And either way, which is more useful: The list-like or the ziplist-like instances (of Applicative/Monad)? Thank you, Aristid

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blog software in Haskell?
by Ketil Malde 30 Mar '12

30 Mar '12

Hi, I just upgraded my server, and set up everything again. Except wordpress, as 1) I'm not too fond of its user interface, and 2) it's a big pile of PHP, difficult to keep updated, and basically a disaster waiting to happen (and in fact, it was hacked at one point). Before I enable it again, is there any alternatives I should consider? Preferably written in Haskell, of course, but other suggestions welcome as well. -k -- If I haven't seen further, it is by standing in the footprints of giants

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If you'd design a Haskell-like language, what would you do different?
by Robert Clausecker 09 Mar '12

09 Mar '12

Image you would create your own language with a paradigm similar to Haskell or have to chance to change Haskell without the need to keep any compatibility. What stuff would you add to your language, what stuff would you remove and what problems would you solve completely different? Thanks in advance for all answers, yours Robert Clausecker

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ANNOUNCE: HaskellDB 2.0: Scrap your SQL strings
by Justin Bailey 22 Feb '12

22 Feb '12

What is it? ======== The HaskellDB library lets you generate SQL queries without writing any actual SQL. Unlike other query generating libraries, you choose the abstraction level. Queries can be built out of independent fragments, just like your programs. Leave hand-written, string-based, SQL libraries behind and check out HaskellDB. Where to get it? ============ You can get it from Hackage (http://hackage.haskell.org/package/haskelldb) or by using cabal (cabal install haskelldb). You also need drivers for your specific database. HaskellDB currently supports the HDBC family of drivers. First, download haskelldb-hdbc (http://hackage.haskell.org/package/haskelldb-hdbc) then get the appropriate package for your database: * ODBC: haskelldb-hdbc-odbc (http://hackage.haskell.org/package/haskelldb-hdbc-odbc) * Postgres: haskelldb-hdbc-postgresql (http://hackage.haskell.org/package/haskelldb-hdbc-postgresql) * Sqllite: haskelldb-hdbc-sqlite3 (http://hackage.haskell.org/package/haskelldb-hdbc-sqlite3) Documentation, Mailing List, Source, etc ======================= Go to http://trac.haskell.org/haskelldb for detailed information. What's New? ========= * New query capabilities: stored procedures, CASTs, query parameters, constants, and more. * Postgres SQL improvements. * Bug fixes around AND and OR associativity. * Query optimization improvements around aggregate expressions. * More tests. Details ===== Back in June 2009, Bjorn Bringert passed maintainership of HaskellDB to me. I've been working this summer to get a release out, which included: * Adding an examples/ directory to the source distribution. * Archiving and consolidating information to http://trac.haskell.org/haskelldb. * Accepting patches for various issues. I have used a lot of ORM & SQL-wrapper libraries during my career and HaskellDB is the first that I've found which does not require you to eventually break the abstraction. Learning to use the the library was a fun challenge but not one that many people are willing to take, when they can easily write SQL by hand. If the library was easier to get started with, I think it would be much more widely used. Help me get HaskellDB better documentation! If you are using HaskellDB and want to help: go to http://trac.haskell.org/haskelldb and write an article, write a blog post, get in touch with me, etc. When I saw that Rails 3.0 uses the "ARel" gem (based on the relational algebra) I groaned - Haskell has had that forever! We need to get the word out! Why 2.0? -------------- Last year Brian Bloniarz contributed an awesome patch which replaced HaskellDB's home-grown record system with the HList library by Oleg Kiselyov (and colleagues). I intended to work with the library for awhile and get a 1.0 release out. Unfortunately, HList proved too daunting for GHC and for me. Compile times went way up, and I could not isolate it enough to give GHC HQ a repro case. Even worse, HList required too many Oleg units for me to comprehend. I could usually fix compile errors, but found myself struggling to extend HaskellDB's functionality. In the end I felt I needed to back the patch out. I really hated to lose some of the functionality but in the end maintainability won out. At that point I had pushed a few patches with a v1.0 cabal file, so I thought it best to make this release 2.0. Who am I? --------------- I started using HaskellDB in early 2009. I use it more as a query-generating library than for executing SQL statements. I have written a largish system for generating a data-access layer in PHP from query descriptions in HaskellDB. Yes, I'm using Haskell to write PHP ... dirty! Unmaintained & Unrelated Packages ----------------------------------------------------- A couple of haskelldb related packages on Hackage are not maintained by me: * haskelldb-hdbc-mysql - This project was created and uploaded independent of me. Contact the author for specifics. However, the haskelldb-hdbc-odbc library is maintained by me and can be used to talk to MySQL. * HSQL (haskelldb-hsql, etc) - I cannot get this backend to compile on my platform. HSQL seems unmaintained so I have let the drivers decay. You'll notice I have not uploaded new packages or changed their version dependencies. * haskelldb-flat and haskelldb-dynamic - Both of these packages have suffered serious bitrot and are not maintained any longer.

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Cannot understand liftM2
by Nicola Paolucci 13 Feb '12

13 Feb '12

Hi All, I'm loving learning Haskell quite a bit. It is stretching my brain but in a delightfull way. I've googled, I've hoogled but I haven't found a clear explanation for what exactly liftM2 does in the context below. Using the cool lambdabot "pointless" utility I found out that: > \x -> snd(x) - fst(x) is the same as: > liftM2 (-) snd fst I like the elegance of this but I cannot reconcile it with its type. I can't understand it. I check the signature of liftM2 and I get: Prelude> :t liftM2 Prelude> liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r Can someone help me understand what's happening here ? What does a Monad have to do with a simple subtraction ? What is actually the "m" of my example ? I am sure if I get this I'll be another step closer to illumination ... Thanks, Nick

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Monad-control rant
by Mikhail Vorozhtsov 31 Jan '12

31 Jan '12

On 11/12/2011 07:34 AM, Bas van Dijk wrote: >> Are you going to release a new version of monad-control right away > > Not just yet. I've split `monad-control` into two packages: > > * `monad-control`: just exports `Control.Monad.Trans.Control`. This part is finished. > * `lifted-base`: wraps all modules of the `base` package which export `IO` computations and provides > lifted version instead. For example we have `Control.Exception.Lifted`, `Control.Concurrent.Lifted`, etc. > > As you can imagine the latter is a lot of boring work. Fortunately it's easy to do so will probably > not take a lot of time. BTW if by any chance you want to help out, that will be much appreciated! > > The repos can be found [here](https://github.com/basvandijk/lifted-base) Maybe I should elaborate on why I stopped using monad-control and rolled out my own version of lifted Control.Exception in monad-abort-fd package. I'm CC-ing the Cafe just in case someone else might be interested in the matter of IO lifting. Imagine we have a monad for multiprogramming with shared state: -- ContT with a little twist. Both the constructor and runAIO -- are NOT exported. newtype AIO s α = AIO { runAIO ∷ ∀ β . (α → IO (Trace s β)) → IO (Trace s β) } runAIOs ∷ MonadBase IO μ ⇒ s -- The initial state → [AIO s α] -- The batch of programs to run. -- If one program exits normally (without using -- aioExit) or throws an exception, the whole batch -- is terminated. → μ (s, Maybe (Attempt α)) -- The final state and the result. -- "Nothing" means deadlock or that -- all the programs exited with -- aioExit. runAIOs = liftBase $ mask_ $ ... bloody evaluation ... data Trace s α where -- Finish the program (without finishing the batch). TraceExit ∷ Trace s α -- Lift a pure value. TraceDone ∷ α → Trace s α -- A primitive to execute and the continuation. TraceCont ∷ Prim s α → (α → IO (Trace s β)) → Trace s β -- Primitive operations data Prim s α where -- State retrieval/modification PrimGet ∷ Prim s s PrimSet ∷ s → Prim s () -- Scheduling point. The program is suspended until -- the specified event occurs. PrimEv ∷ Event e ⇒ e → Prim s (EventResult e) -- Scheduling point. The program is suspended until the state -- satisfies the predicate. PrimCond ∷ (s → Bool) → Prim s () -- Run computation guarded with finalizer. PrimFin ∷ IO (Trace s α) → (Maybe α → AIO s β) → Prim s (α, β) -- Run computation guarded with exception handler. PrimHand ∷ IO (Trace s α) → (SomeException → AIO s α) → Prim s α aioExit ∷ AIO s α aioExit = AIO $ const $ return TraceExit aioAfter ∷ (s → Bool) → AIO s () aioAfter cond = AIO $ return . TraceCont (PrimCond cond) aioAwait ∷ Event e ⇒ e → AIO s (EventResult e) aioAwait e = AIO $ return . TraceCont (PrimEv e) runAIOs slices the programs at scheduling points and enqueues the individual pieces for execution, taking care of saving/restoring the context (finalizers and exception handlers). The Functor/Applicative/Monad/MonadBase/etc instances are pretty trivial: instance Functor (AIO s) where fmap f (AIO g) = AIO $ \c → g (c . f) instance Applicative (AIO s) where pure a = AIO ($ a) (<*>) = ap instance Monad (AIO s) where return = pure AIO g >>= f = AIO $ \c → g (\a → runAIO (f a) c) instance MonadBase IO (AIO s) where liftBase io = AIO (io >>=) instance MonadState s (AIO s) where get = AIO $ return . TraceCont PrimGet put s = AIO $ return . TraceCont (PrimSet s) instance MonadAbort SomeException (AIO s) where abort = liftBase . throwIO trace ∷ AIO s α → IO (Trace s α) trace (AIO g) = g (return . TraceDone) instance MonadRecover SomeException (AIO s) where recover m h = AIO $ return . TraceCont (PrimHand (trace m) h) instance MonadFinally (AIO s) where finally' m f = AIO $ return . TraceCont (PrimFin (trace m) f) -- finally m = fmap fst . finally' m . const -- No async exceptions in AIO instance MonadMask () (AIO s) where getMaskingState = return () setMaskingState = const id Now we have a problem: we can throw/catch exceptions and install finalizers in AIO, but we can't use Control.Exception.Lifted because we can't declare a MonadBaseControl instance for our "ContT with limited interface". So now, instead of trying to wrap IO control operations uniformly, I just reimplement them in terms of the classes I mentioned above (MonadAbort+MonadRecover+MonadFinally+MonadMask), for example: bracket ∷ (MonadFinally μ, MonadMask m μ) ⇒ μ α → (α → μ β) → (α → μ γ) → μ γ bracket acq release m = mask $ \restore → do a ← acq finally (restore $ m a) (release a) It requires more typing (more classes => more instances), but it works for a broader class of monads and I get proper side affects in finalizers for a bonus. The code is on Hackage (monad-abort-fd) and on the GitHub (https://github.com/mvv/monad-abort-fd/blob/master/src/Control/Monad/Excepti…)

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Parsing workflow
by Nils Schweinsberg 21 Jan '12

21 Jan '12

Hi! I'm having a really hard time to write a correct parser for a small language I've developed. I have been trying to write a parser using parsec, but always get a lot of error messages like "unexpected "\n", expected ..., new-line or..." when trying to run the parser. Then I read about the happy parser and really liked the separation of lexing the text into tokens and parsing the actual logic behind those tokens. Since I couldn't get familiar with the lexer "alex" I gave up on the alex-happy-approach again and went back to parsec. But with that lexer->parser idea on my mind, my parser currently looks a lot like a lexer. So I came up with the idea of using a combination of parsec and happy, where I generate a list of tokens for my text via parsec and analyse it with happy. My questions would be: - Is this a valid approach? - What is your workflow on parsing complex data structures? - What about performance? Since my project is going to be an interpreted language parsing performance might be interesting aswell. I've read that happy is in general faster than parsec, but what if I combine both of them as I said above? I guess that parsing a simple list of tokens without any nested parser structures would be pretty fast? - Do you have any other ideas on how to improve my parser? - What are your general thoughts on happy vs. parsec? Thanks for any replies, Nils

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How to get Cabal to spit out a .a library suitable for linking into C/Objective-C
by David Pollak 20 Jan '12

20 Jan '12

Howdy, I'm trying to figure out how to get Cabal configured to compile and link my Haskell code such that the code can be part of C and/or Objective-C code such that all the Haskell dependencies are rolled into a .a file and can be linked by a normal C linker (e.g., ld). I've been through http://haskell.org/ghc/docs/6.12.2/html/users_guide/ffi-ghc.html#using-own-… the associated linked, but I'm unable to find out the Cabal incantation to output a library that's linkable into my other code. Any pointers or examples would be greatly appreciated. Thanks, David -- Visi.Pro, Cloud Computing for the Rest of Us http://visi.pro Lift, the simply functional web framework http://liftweb.net Follow me: http://twitter.com/dpp Blog: http://goodstuff.im

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Re: [Haskell-cafe] How to get Cabal to spit out a .a library suitable for linking into C/Objective-C
by Donn Cave 20 Jan '12

20 Jan '12

Sorry about the belated response, but this shouldn't be a problem since it isn't going to be very helpful anyway! I've managed to follow the process described on this page: http://www.haskell.org/haskellwiki/Using_Haskell_in_an_Xcode_Cocoa_project to link Haskell code to a non-Haskell main program, via Xcode. You've probably already seen this, it's mostly about a few iterations of trial and error linking, to get the list of GHC library dependencies, which is what you need whether you're using Xcode or not. If you really need a single library with your code and all those dependencies, that may be technically feasible, though awfully tedious. The only clear problem I encountered while experimenting with it is that GHC run time options need ghc -rtsopts, so they're unavailable if your program isn't built by ghc. I believe I worked around that with a special rtsmain.o, could probably recover the details if that's of interest. Donn

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strict, lazy, non-strict, eager
by Albert Y. C. Lai 18 Jan '12

18 Jan '12

Most individuals of the Haskell community have long been maintaining a cognitive dissonance; some cases turn into plain hypocrisy. You might excuse it for its ancient and prominent origin: Richard Bird and/or Philip Wadler themselves wrote like "it is too lazy", "make it more strict" 13 years ago and surely more. But perpetuating it is not helping. I have not written this complaint until now because I have been waiting for unmistakable evidence, a smoking gun, a red hand so caught that you cannot explain away, for example you cannot explain that "one sentence is from one person, the other sentence is from a different person". So, on IRC in #haskell, from the same person, speaking on the same topic in the same context, in the same interval of 3 minutes (the first two sentences in the same minute): 1. a function f is strict if f ⊥ = ⊥ 2. ⊥ represents any computation which does not terminate, i.e. an exception or an infinite loop 3. "strict" describes the denotational semantics People, could you please make up your mind already? It has been more than 13 years. Denotational semantics: A. There are no computational steps. There is no termination, and there is no non-termination, since there are no steps to finish, and no steps to keep going. B. ⊥ represents "no information", not "non-termination". There is no "non-termination" to represent. C. fix id = ⊥ because ⊥ is the least fixed point of id, not because fix id non-terminates. There is nothing to terminate or non-terminate. D. You say strict, more strict, less strict; non-strict, more non-strict, less non-strict. You don't say eager, and you don't say lazy. Operational semantics: A. There is no ⊥; it does not appear in any sequence of computational steps, finitely long or infinitely long. B. You say eager, more eager, less eager; lazy, more lazy, less lazy; or speculative, more speculative, less speculative; or any other adjectives for evaluation strategies. You don't say strict, and you don't say non-strict. "Semantics", "semantically speaking": A. Which semantics, which semantically? There are two. B. Actually there are more, but apparently two is already enough to cause all kinds of incoherent statements. If I draw your attention to algebraic semantics, will you start saying "it is too lazy, need to make it more idempotent"?

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