February 2011 - Haskell

Haskell Weekly News: Issue 168 - February 09, 2011
by Daniel Santa Cruz 10 Feb '11

10 Feb '11

Welcome to issue 168 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 30 to February 5, 2011. Announcements Maciej Piechotka [2]announced version 0.1.1 of nanoparsec. "Nanoparsec is currently simply a port of attoparsec on ListLike (the abstraction of lists used by iteratee)." Graham Hutton [3]announced the availability of studenships in Functional Programming at the University of Nottingham, UK. Ashley Yakeley [4]made a small update to time (1.2.0.4) that "fixes a bug in parseTime." George Giorgidze [5]announced the release to Hackage of Database Supported Haskell (DSH). Simon Hengel [6]announced the release of HackageOneFive, "a tiny Snap app that provides reverse dependency lookup for all packages on Hackage." Quotes of the Week * alpounet: map succ/pred is a scandinavian name generator or what? ... well, scandinavian and aztec * dons: Think of a monad as a spacesuit full of nuclear waste in the ocean next to a container of apples. Now, you can't put oranges in the space suite or the nuclear waste falls in the ocean, *but* the apples are carried around anyway, and you just take what you need. * Apocalisp: a unit of clarity is a clarinet * roconnor: My coq has failed me * roconnor: I mean, I could, and do, say that a lens is just a monoid natural transformation from the coalgebra functiors from haskell functors to haskell types. * Saizan: "not being ML" is a quite sound principle syntax-wise * lpsmith: well, getting rid of airliners certainly *is* one way of eliminating their potential use as weapons against larger buildings * ddarius: The reason edwardk writes so many Haskell packages is to ward the Perl from penetrating his heart. * monochrom: most people tend to deny the usefulness of useful alternative perspectives --- their entrenched vested interests demand it. as they grow older they also have more influence and power over "the state of the art" to preserve their self-fulfilling prophecy that "the alternative is useless" --- by controlling what practice looks like, they can control what looks useless. * edwardk: i learned to program becaise i'd lied and told a kid that i'd written a disassembler, then had to make good on the claim. Top Reddit Stories * Quine Central ... because everyone needs to be able to make nth order quines Domain: blog.sigfpe.com, Score: 40, Comments: 7 On Reddit: http://www.reddit.com/r/haskell/comments/fc4cs/quine_central_because_everyo… Original: http://blog.sigfpe.com/2011/01/quine-central.html * jhc 0.7.7 is out Domain: haskell.org, Score: 38, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fbqo7/jhc_077_is_out/ Original: http://www.haskell.org/pipermail/jhc/2011-January/000874.html * Hulk: A Haskell IRC server Domain: chrisdone.com, Score: 32, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/fbrie/hulk_a_haskell_irc_server/ Original: http://chrisdone.com/posts/2011-01-30-hulk-haskell-irc-server.html * Status update on {code, trac, projects, planet, community}.haskell.org Domain: haskell.org, Score: 25, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fdnjp/status_update_on_code_trac_p… Original: http://www.haskell.org/pipermail/haskell-cafe/2011-February/088829.html * All about MVars : Inside T5 Domain: blog.ezyang.com, Score: 23, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/fduxb/all_about_mvars_inside_t5/ Original: http://blog.ezyang.com/2011/02/all-about-mvars/ * Text: a visual programming language based on Haskell, demo Domain: vimeo.com, Score: 21, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/fbgxd/text_a_visual_programming_la… Original: http://vimeo.com/19273744 * [SO] Why is GHC so big? Domain: stackoverflow.com, Score: 21, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/fczht/so_why_is_ghc_so_big/ Original: http://stackoverflow.com/questions/4858585/why-is-ghc-so-large-big * Haskell Weekly News: Issue 167 Domain: contemplatecode.blogspot.com, Score: 21, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fe63a/haskell_weekly_news_issue_16… Original: http://contemplatecode.blogspot.com/2011/02/haskell-weekly-news-issue-167.h… * The F# Team are Hiring (functional programming jobs) Domain: haskell.org, Score: 20, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/fcvec/the_f_team_are_hiring_functi… Original: http://www.haskell.org/pipermail/haskell/2011-January/022523.html * Recently Uploaded: DSH-0.4.2 Natural Sql Queries without the need to write Sql strings. Domain: hackage.haskell.org, Score: 20, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/fesjj/recently_uploaded_dsh042_nat… Original: http://hackage.haskell.org/package/DSH * Specifying behaviors in a purely functional language Domain: io7m.com, Score: 16, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fdvux/specifying_behaviors_in_a_pu… Original: http://io7m.com/articles/itefs/ Top StackOverflow Answers * Why is GHC so large/big? votes: 29 Read on SO: http://stackoverflow.com/questions/4858585/why-is-ghc-so-large-big/4867236#… It's a bit silly really. Every library that comes with GHC is provided in no less than 4 flavours: static dynamic profiled GHCi The GHCi version is just the static version linked together in a single .o file. The other three versions all have their own set of interface files (.hi files) too. The profiled versions seem to be about twice the size of the ... * Why `(map digitToInt) . show` is so fast? votes: 24 Read on SO: http://stackoverflow.com/questions/4841078/why-map-digittoint-show-is-so-fa… Seeing as I can't add comments yet, I'll do a little bit more work and just analyze all of them. I'm putting the analysis at the top; however, the relevant data is below. (Note: all of this is done in 6.12.3 as well - no GHC 7 magic yet.) Analysis: Version 1: show is pretty good for ints, especially those as short as we have. Making strings actually ... * What Justification for the type of f x = f x in Haskell is there? votes: 15 Read on SO: http://stackoverflow.com/questions/4847008/what-justification-for-the-type-… Okay, starting from the function definition f x = f x, let's step through and see what we can deduce about the type of f. Start with a completely unspecified type variable, a. Can we deduce more than that? Yes, we observe that f is a function taking one argument, so we can change a into a function between two unknown type variables, which we'll call b -> ... * . versus $ in haskell votes: 14 Read on SO: http://stackoverflow.com/questions/4876828/versus-in-haskell/4876900#4876900 Prelude> :t ($) ($) :: (a -> b) -> a -> b Prelude> :t (.) (.) :: (b -> c) -> (a -> b) -> a -> c $ applies a function to a value. . composes two functions. So I can write f $ g x which is "apply f to (g of x)" or f . g $ x which is "apply the composition of f and g to x". One common style is to pile up dots on the left with a ... * Starting a game development in an exotic language I am just learning (Lisp, Haskell, ...) votes: 12 Read on SO: http://stackoverflow.com/questions/4867656/starting-a-game-development-in-a… If you could write a game in C, you can write one in Haskell. The biggest difficulties many people have with it are: Lack of support for OOP, which is what most people are familiar with these days. Old-fashioned procedural code, on the other hand, is actually very pleasant to write in Haskell; the IO type distinguishes effect-free functions from procedures ... * Haskell: Check if Int is in a list of Int's votes: 11 Read on SO: http://stackoverflow.com/questions/4846974/haskell-check-if-int-is-in-a-lis… First find the type of the function you need. To "Check if" means to return either True or False, a Bool. So the function takes an Int, a list of Int (aka [Int]) and returns Bool: Int -> [Int] -> Bool Now ask hoogle. elem :: Eq a => a -> [a] -> Bool Hoogle is a very useful tool. You can integrate it with ghci. Top StackOverflow Questions * Why is GHC so large/big? votes: 32, answers: 5 Read on SO: http://stackoverflow.com/questions/4858585/why-is-ghc-so-large-big * Starting a game development in an exotic language I am just learning (Lisp, Haskell, …) votes: 16, answers: 10 Read on SO: http://stackoverflow.com/questions/4867656/starting-a-game-development-in-a… * Why `(map digitToInt) . show` is so fast? votes: 8, answers: 2 Read on SO: http://stackoverflow.com/questions/4841078/why-map-digittoint-show-is-so-fa… * Transforming expression given in prefix notation, identifying common subexpressions and dependencies votes: 7, answers: 6 Read on SO: http://stackoverflow.com/questions/4843237/transforming-expression-given-in… * Practical use of curried functions? votes: 7, answers: 6 Read on SO: http://stackoverflow.com/questions/4888480/practical-use-of-curried-functio… About the Haskell Weekly News You can find an html version of this newsletter at: http://contemplatecode.blogspot.com/2011/02/haskell-weekly-news-issue-168.h… To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85781 3. https://groups.google.com/d/topic/fa.haskell/rURgYMjEZhU/discussion 4. https://groups.google.com/d/topic/fa.haskell/l-I05W30DWU/discussion 5. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85961 6. https://groups.google.com/d/topic/fa.haskell/vp2P5w8GbkM/discussion

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ANN: listlike-instances
by John Lato 09 Feb '11

09 Feb '11

Hello, I am pleased to announce the first release of listlike-instances. The listlike-instances package provides orphan ListLike instances for strict and lazy Text[0], and regular, storable, and unboxed Vectors[1]. These class instances greatly expand the available options for ListLike users. John [0] http://hackage.haskell.org/package/text [1] http://hackage.haskell.org/package/vector

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ANN: ListLike-3.0.1
by John Lato 09 Feb '11

09 Feb '11

Hello, I am pleased to announce the release of ListLike-3.0.1. ListLike provides polymorphic functions with a Data.List-style api for many different data types, including lists, arrays, and bytestrings. Changes in ListLike-3.0.1: - The ListLike instance for Data.Map.Map has been removed, as it did not obey several important list properties and could lead to unexpected results. - A new instances for Data.Sequence.Seq has been added - bugfixes in the default instances for groupBy and nubBy - Some default methods have improved performance Many thanks to John Goerzon for his ongoing contributions to the Haskell community. Sincerely, John Lato

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ANN: monad-control-0.2 & a challenge
by Bas van Dijk 09 Feb '11

09 Feb '11

Dear all, I just released control-monad-0.2 a library for lifting control operations, like exception catching, through monad transformers: http://hackage.haskell.org/package/monad-control-0.2 darcs get http://bifunctor.homelinux.net/~bas/monad-control/ To quote the NEWS file: * Use RunInBase in the type of idLiftControl. * Added this NEWS file. * Only parameterize Run with t and use RankNTypes to quantify n and o -liftControl :: (Monad m, Monad n, Monad o) => (Run t n o -> m a) -> t m a +liftControl :: Monad m => (Run t -> m a) -> t m a -type Run t n o = forall b. t n b -> n (t o b) +type Run t = forall n o b. (Monad n, Monad o, Monad (t o)) => t n b -> n (t o b) Bumped version from 0.1 to 0.2 to indicate this breaking change in API. * Added example of a derivation of liftControlIO. This derivation of liftControlIO is really enlightening. It shows the recursive structure of liftControlIO applied to a stack of three monad transformers with IO as the base monad: t1 (t2 (t3 IO)) a: (Note that the derivation in the API documentation also shows the types of the intermediate computations) liftControlIO = liftLiftControlBase $ -- instance MonadControlIO t1 liftLiftControlBase $ -- instance MonadControlIO t2 liftLiftControlBase $ -- instance MonadControlIO t3 idLiftControl -- instance MonadControlIO IO = \f → liftControl $ \run1 → -- Capture state of t1 liftControl $ \run2 → -- Capture state of t2 liftControl $ \run3 → -- Capture state of t3 -- At this point we've captured the state of all transformers -- and have landed in the base (IO) monad. -- So we can start executing f: let run ∷ RunInBase (t1 (t2 (t3 IO))) IO run = -- Restore state liftM (join ∘ lift) ∘ liftM (join ∘ lift) -- Identity conversion ∘ liftM (join ∘ lift) ∘ liftM return -- Run ∘ run3 ∘ run2 ∘ run1 in f run This derivation clearly shows what is happening: first the state of all transformers is captured using the liftControl operations. Each capture supplies us a run function that allows us to run a computation of the respected monad transformer. When we arrive in the base (IO) monad we can start executing f. f expects a run function :: ∀ b. t1 (t2 (t3 IO)) b → IO (t1 (t2 (t3 IO)) b) which allows it to run a t1 computation in IO. The IO computation then returns a new t1 computation that has the final state of the given t1 computation. This can later be used to restore that final state. Although the derivation is correct, I'm not really happy with it from a performance perspective. In the created run function there are two places where it can do better: * The identity conversion: (liftM (join ∘ lift) ∘ liftM return) could be replaced with something more efficient like (liftM (join ∘ lift ∘ return)) or (liftM id) or just id. * The restore conversion: (liftM (join ∘ lift) ∘ liftM (join ∘ lift)) could be replaced with (liftM (join ∘ lift ∘ join ∘ lift)). This transformation is correct according to the Functor law: liftM f ∘ liftM g = liftM (f ∘ g). I haven't figured out yet how to change monad-control in such a way that it generates this more efficient derivation. So I'm posting this as a challenge to you! Good luck, Bas

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Second call for submissions: Monad.Reader special poetry and fiction edition
by Brent Yorgey 08 Feb '11

08 Feb '11

Ten days until the submission deadline. "Nein": what I won't take for an answer. Eighth to eighteenth February, Seven thousand minutes twice, Sick's he who can't write aught. Five submissions now; Four more, at least? Three'd be fair To no- One. http://themonadreader.wordpress.com/2011/01/19/call-for-copy-special-poetry…

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ICFP 2011: Second Call for Papers
by Wouter Swierstra 08 Feb '11

08 Feb '11

===================================================================== Second Call for Papers ICFP 2011: International Conference on Functional Programming Tokyo, Japan, Monday 19 -- Wednesday 21 September 2011 http://www.icfpconference.org/icfp2011 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Titles, abstracts & keywords due: Thursday 17 March 2011 at 14:00 UTC Submissions due: Thursday 24 March 2011 at 14:00 UTC Author response: Tuesday & Wednesday 17-18 May Notification: Monday 30 May 2011 Final copy due: Friday 01 July 2011 Conference: Monday-Wednesday 19-21 September 2011 Scope ~~~~~ ICFP 2011 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Particular topics of interest include * Language Design: type systems; concurrency and distribution; modules; components and composition; metaprogramming; relations to imperative, object-oriented, or logic programming; interoperability * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling * Foundations: formal semantics; lambda calculus; rewriting; type theory; mathematical logic; monads; continuations; delimited continuations; global, delimited, or local effects * Transformation and Analysis: abstract interpretation; partial evaluation; program transformation; program calculation; program proofs; normalization by evaluation * Applications and Domain-Specific Languages: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia programming; scripting; system administration; security; education * Functional Pearls: elegant, instructive, and fun essays on functional programming * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working in a particular application Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By 17 March 2011, 14:00 UTC, submit a title, an abstract of at most 300 words, and keywords. * By 24 March 2011, 14:00 UTC, submit a full paper of at most 12 pages (6 pages for a Functional Pearl and for an Experience Report), including bibliography and figures. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.acm.org/sigplan/republicationpolicy.htm In addition, authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no rewiewer will read the annotated copies of the previous reviews. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions are expected to transfer the copyright to the ACM. Presentations will be videotaped and released online if the presenter consents by signing an additional permission form at the time of the presentation. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. If this requirement is a hardship, make contact with the program chair at least one week before the deadline. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available from SIGPLAN at http://www.acm.org/sigs/sigplan/authorInformation.htm Submission: Submissions will be accepted electronically at a URL to be named later. Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 48-hour period, starting at 14:00 UTC on Tuesday 17 May 2010, to read reviews and respond to them. Special Journal Issue: There will be a special issue of the Journal of Functional Programming with papers from ICFP 2011. The program committee will invite the authors of select accepted papers to submit a journal version to this issue. Conference Chairs: Manuel M T Chakravarty, University of New South Wales, Australia Zhenjiang Hu, National Institute of Informatics (NII), Japan Program Chair: Olivier Danvy, Aarhus University, Denmark Program Committee: Kenichi Asai, Ochanomizu University, Japan Josh Berdine, Microsoft Research, UK Adam Chlipala, Harvard University, USA William Cook, University of Texas at Austin, USA Maribel Fernandez, King's College London, UK Ronald Garcia, Carnegie Mellon University, USA Neal Glew, Intel Labs, USA Jacques Garrigue, Nagoya University, Japan Suresh Jagannathan, Purdue University, USA Sam Lindley, University of Edinburgh, UK Frank Pfenning, Carnegie Mellon University, USA Paola Quaglia, University of Trento, Italy Alexis Saurin, University of Paris VII, France Mike Spivey, Oxford University, UK Kristian Stoevring, University of Copenhagen, Denmark Doaitse Swierstra, Utrecht University, The Netherlands David Van Horn, Northeastern University, USA Rene Vestergaard, JAIST, Japan Edwin Westbrook, Rice University, USA

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ANNOUNCE: intel-aes-0.1.2 (Accelerated AES and RNG)
by Ryan Newton 08 Feb '11

08 Feb '11

Dear Haskellers, I've released a package to address the topic raised by Simon in this thread: http://www.mail-archive.com/haskell-cafe@haskell.org/msg83901.html I am pleased to announce that it generates random numbers as fast as System.Random.StdGen but with better statistical properties when generators are split. When newer hardware supporting AESNI is available, it will run an additional 2X faster. You can find it here: http://hackage.haskell.org/package/intel-aes https://github.com/rrnewton/intel-aes/wiki This release is non-portable (linux 64-bit only). I'm looking for intrepid Windows hackers to help me get it running on Windows. The underlying Intel AESNI Sample library comes with a .bat, but I haven't got it working yet. Likewise someone who knows how to port assembly to Mac OS... Cheers, -Ryan

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ANN: case-insensitive-0.1
by Bas van Dijk 07 Feb '11

07 Feb '11

Dear all, I had this old module laying around that I wrote some time ago. I dusted it off and uploaded it to Hackage: http://hackage.haskell.org/package/case-insensitive-0.1 The package provides the module Data.CaseInsensitive which exports the CI type constructor which can be parameterised by a string-like type like: String, ByteString, Text, etc.. Comparisons of values of the resulting type are then insensitive to cases. It is my hope that one day packages like snap-core[1], http-types[2] or wai[3] all use this common way of making strings case insensitive. Regards, Bas [1] http://hackage.haskell.org/packages/archive/snap-core/latest/doc/html/Data-… [2] http://hackage.haskell.org/packages/archive/http-types/0.3.0/doc/html/Netwo… [3] http://hackage.haskell.org/packages/archive/wai/latest/doc/html/Network-Wai…

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ANNOUNCE: pwstore 1.0 (Secure password storage)
by Peter Scott 07 Feb '11

07 Feb '11

Hi everyone, If you need to store and verify passwords, the usual advice is to use bcrypt. It neatly handles all the security issues, with a simple API. But Haskell doesn't have bcrypt bindings, so people are tempted to roll their own password storage methods, and perhaps get it wrong. I decided to fix that. The pwstore library handles all the details of password storage for you, in a way that should be so easy to use that there's no reason not to use it. WHAT IT DOES You hash and salt passwords with one function, and verify user input against these password hashes with another function. For more information, see the API docs: http://hackage.haskell.org/packages/archive/pwstore-fast/1.0/doc/html/Crypt… HOW IT WORKS The basic algorithm is this: * Combine the user's password with a randomly-generated salt. * Hash this slowly. By iterating SHA-256 a few thousand times, we make brute-force guessing a lot less practical. * Store this has along with the salt. This scheme is essentially an implementation of the PBKDF1 key derivation function (as specified in RFC 2898) with some convenience code around it to make it easy to use and really hard to mess up. WHERE TO GET IT There are two packages on Hackage, which you can get with cabal-install: 1. pwstore-fast is the preferred version. 2. pwstore-purehaskell has the same API, but only pure Haskell dependencies. It's usable, but about 25 times slower than pwstore-fast. http://hackage.haskell.org/package/pwstore-fast http://hackage.haskell.org/package/pwstore-purehaskell The source code is on GitHub: https://github.com/PeterScott/pwstore/ Any comments, questions, or patches are welcome. -Peter

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ANN: monad-control-0.1
by Bas van Dijk 06 Feb '11

06 Feb '11

Dear all, Several attempts have been made to lift control operations (functions that use monadic actions as input instead of just output) through monad transformers: MonadCatchIO-transformers[1] provided a type class that allowed to overload some often used control operations (catch, block and unblock). Unfortunately that library was limited to those operations. It was not possible to use, say, alloca in a monad transformer. More importantly however, the library was broken as was explained[2] by Michael Snoyman. In response Michael created the MonadInvertIO type class which solved the problems. Then Anders Kaseorg created the monad-peel library which provided an even nicer implementation. monad-control is a rewrite of monad-peel that uses CPS style operations and exploits the RankNTypes language extension to simplify and speedup most functions. A very preliminary and not yet fully representative, benchmark shows that monad-control is on average about 2.6 times faster than monad-peel: bracket: 2.4 x faster bracket_: 3.1 x faster catch: 1.8 x faster try: 4.0 x faster mask: 2.0 x faster Note that, although the package comes with a test suite that passes, I still consider it highly experimental. API DOCS: http://hackage.haskell.org/package/monad-control INSTALLING: $ cabal update $ cabal install monad-control TESTING: The package contains a copy of the monad-peel test suite written by Anders. You can perform the tests using: $ cabal unpack monad-control $ cd monad-control $ cabal configure -ftest $ cabal test BENCHMARKING: $ darcs get http://bifunctor.homelinux.net/~bas/bench-monad-peel-control/ $ cd bench-monad-peel-control $ cabal configure $ cabal build $ dist/build/bench-monad-peel-control/bench-monad-peel-control DEVELOPING: The darcs repository will be hosted on code.haskell.org ones that server is back online. For the time being you can get the repository from: $ darcs get http://bifunctor.homelinux.net/~bas/monad-control/ TUTORIAL: This short unpolished tutorial will explain how to lift control operations through monad transformers. Our goal is to lift a control operation like: foo ∷ M a → M a where M is some monad, into a transformed monad like 'StateT M': foo' ∷ StateT M a → StateT M a The first thing we need to do is write an instance for the MonadTransControl type class: class MonadTrans t ⇒ MonadTransControl t where liftControl ∷ (Monad m, Monad n, Monad o) ⇒ (Run t n o → m a) → t m a If you ignore the Run argument for now, you'll see that liftControl is identical to the 'lift' method of the MonadTrans type class: class MonadTrans t where lift ∷ Monad m ⇒ m a → t m a So the instance for MonadTransControl will probably look very much like the instance for MonadTrans. Let's see: instance MonadTransControl (StateT s) where liftControl f = StateT $ \s → liftM (\x → (x, s)) (f run) So what is this run function? Let's look at its type: type Run t n o = ∀ b. t n b → n (t o b) The run function executes a transformed monadic action 't n b' in the non-transformed monad 'n'. In our case the 't' will be a StateT computation. The only way to run a StateT computation is to give it some state and the only state we have lying around is the one from the outer computation: 's'. So let's run it on 's': instance MonadTransControl (StateT s) where liftControl f = StateT $ \s → let run t = ... runStateT t s ... in liftM (\x → (x, s)) (f run) Now that we are able to run a transformed monadic action, we're almost done. Look at the type of Run again. The function should leave the result 't o b' in the monad 'n'. This 't o b' computation should contain the final state after running the supplied 't n b' computation. In case of our StateT it should contain the final state s': instance MonadTransControl (StateT s) where liftControl f = StateT $ \s → let run t = liftM (\(x, s') → StateT $ \_ → return (x, s')) (runStateT t s) in liftM (\x → (x, s)) (f run) This final computation, "StateT $ \_ → return (x, s')", can later be used to restore the final state. Now that we have our MonadTransControl instance we can start using it. Recall that our goal was to lift "foo ∷ M a → M a" into our StateT transformer yielding the function "foo' ∷ StateT M a → StateT M a". To define foo', the first thing we need to do is call liftControl: foo' t = liftControl $ \run → ... This captures the current state of the StateT computation and provides us with the run function that allows us to run a StateT computation on this captured state. Now recall the type of liftControl ∷ (Run t n o → m a) → t m a. You can see that in place of the ... we must fill in a value of type 'm a'. In our case this will be a value of type 'M a'. We can construct such a value by calling foo. However, foo expects an argument of type 'M a'. Fortunately we can provide one if we convert the supplied 't' computation of type 'StateT M a' to 'M a' using our run function of type ∀ b. StateT M b → M (StateT o b): foo' t = ... liftControl $ \run → foo $ run t However, note that the run function returns the final StateT computation inside M. So the type of the right hand side is now 'StateT M (StateT o b)'. We would like to restore this final state. We can do that using join: foo' t = join $ liftControl $ \run → foo $ run t That's it! Note that because it's so common to join after a liftControl I provide an abstraction for it: control = join ∘ liftControl Allowing you to simplify foo' to: foo' t = control $ \run → foo $ run t Probably the most common control operations that you want to lift through your transformers are IO operations. Think about: bracket, alloca, mask, etc.. For this reason I provide the MonadControlIO type class: class MonadIO m ⇒ MonadControlIO m where liftControlIO ∷ (RunInBase m IO → IO a) → m a Again, if you ignore the RunInBase argument, you will see that liftControlIO is identical to the liftIO method of the MonadIO type class: class Monad m ⇒ MonadIO m where liftIO ∷ IO a → m a Just like Run, RunInBase allows you to run your monadic computation inside your base monad, which in case of liftControlIO is IO: type RunInBase m base = ∀ b. m b → base (m b) The instance for the base monad is trivial: instance MonadControlIO IO where liftControlIO = idLiftControl idLiftControl directly executes f and passes it a run function which executes the given action and lifts the result r into the trivial 'return r' action: idLiftControl ∷ Monad m ⇒ ((∀ b. m b → m (m b)) → m a) → m a idLiftControl f = f $ liftM $ \r -> return r The instances for the transformers are all identical. Let's look at StateT and ReaderT: instance MonadControlIO m ⇒ MonadControlIO (StateT s m) where liftControlIO = liftLiftControlBase liftControlIO instance MonadControlIO m ⇒ MonadControlIO (ReaderT r m) where liftControlIO = liftLiftControlBase liftControlIO The magic function is liftLiftControlBase. This function is used to compose two liftControl operations, the outer provided by a MonadTransControl instance and the inner provided as the argument: liftLiftControlBase ∷ (MonadTransControl t, Monad base, Monad m, Monad (t m)) ⇒ ((RunInBase m base → base a) → m a) → ((RunInBase (t m) base → base a) → t m a) liftLiftControlBase lftCtrlBase = \f → liftControl $ \run → lftCtrlBase $ \runInBase → f $ liftM (join ∘ lift) ∘ runInBase ∘ run Basically it captures the state of the outer monad transformer using liftControl. Then it captures the state of the inner monad using the supplied lftCtrlBase function. If you recall the identical definitions of the liftControlIO methods: 'liftLiftControlBase liftControlIO' you will see that this lftCtrlBase function is the recursive step of liftLiftControlBase. If you use 'liftLiftControlBase liftControlIO' in a stack of monad transformers a chain of liftControl operations is created: liftControl $ \run1 -> liftControl $ \run2 -> liftControl $ \run3 -> ... This will recurse until we hit the base monad. Then liftLiftControlBase will finally run f in the base monad supplying it with a run function that is able to run a 't m a' computation in the base monad. It does this by composing the run and runInBase functions. Note that runInBase is basically the composition: '... ∘ run3 ∘ run2'. However, just composing the run and runInBase functions is not enough. Namely: runInBase ∘ run ∷ ∀ b. t m b → base (m (t m b)) while we need to have ∀ b. t m b → base (t m b). So we need to lift the 'm (t m b)' computation inside t yielding: 't m (t m b)' and then join that to get 't m b'. Now that we have our MonadControlIO instances we can start using them. Let's look at how to lift 'bracket' into a monad supporting MonadControlIO. Before we do that I define a little convenience function similar to 'control': controlIO = join ∘ liftControlIO Bracket just calls controlIO which captures the state of m and provides us with a runInIO function which allows us to run an m computation in IO: bracket ∷ MonadControlIO m ⇒ m a → (a → m b) → (a → m c) → m c bracket before after thing = controlIO $ \runInIO → E.bracket (runInIO before) (\m → runInIO $ m >>= after) (\m → runInIO $ m >>= thing) I welcome any comments, questions or patches. Regards, Bas [1] http://hackage.haskell.org/package/MonadCatchIO-transformers [2] http://docs.yesodweb.com/blog/invertible-monads-exceptions-allocations/ [3] http://hackage.haskell.org/package/monad-peel

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