August 2012 - Haskell-Cafe

happstack simpleHTTP & state monad
by Corentin Dupont 30 Aug '12

30 Aug '12

Hi all, I'm trying to make a web server that manages its own state. The user can issue commands that modifies the state. I did like below but unfortunatly the state is not keep after a command is issued... What is the right way to do it? Is there any example sites with an internal state with happstack? *data Game = (the state of my game) type NomicServer = ServerPartT (StateT Game IO) launchWebServer :: Game -> IO () launchWebServer initialState = do putStrLn "Starting web server...\nTo connect, drive your browser to \" http://localhost:8000/Login\"" d <- getDataDir simpleHTTP' unpackStateT nullConf $ server d server :: FilePath -> ServerPartT (StateT Game IO) Response server d sh = mconcat [fileServe [] d, do decodeBody (defaultBodyPolicy "/tmp/" 4096 4096 4096) html <- implSite "http://localhost:8000/" "" nomicSite return $ toResponse html] unpackStateT:: Game -> UnWebT (StateT Game IO) Response -> UnWebT IO Response unpackStateT g w = evalStateT w g --handler for web routes nomicSite :: Site PlayerCommand (NomicServer Html) nomicSite = setDefault (Noop 0) Site { handleSite = \f url -> unRouteT (routedNomicCommands url) f , formatPathSegments = \u -> (toPathSegments u, []) , parsePathSegments = parseSegments fromPathSegments }* Thanks a lot, Corentin

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paths in .cabal/config
by Semen Trygubenko 30 Aug '12

30 Aug '12

Dear Haskell-Cafe, I am wondering if there is a way to use variables in paths that appear in .cabal/config file? I have written a custom config file that I would like to share. However, my config file contains full paths s.a. world-file: /home/semen/.cabal/world etc. Is there any way I could make it less personal, e.g. world-file: /home/${HOME}/.cabal/world Many thanks, Sem -- Семен Тригубенко

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Problem with searching mailing-lists archives
by damodar kulkarni 30 Aug '12

30 Aug '12

Hi Cafe, I was trying to search for the following Haskell functions in the mailing list archives. Here is a summary of the responses I have had from various servers upon searching various "valid and normally used" Haskell function identifiers. Responses from the http://www.mail-archive.com >>=<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=%3E%3E%3D>, It seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=*+-%3E+*>There are 0 results. and so on... Responses from the http://search.gmane.org >>=<http://search.gmane.org/search.php?group=gmane.comp.lang.haskell.cafe&query…> Here, too, it seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://search.gmane.org/?query=*+-%3E+*&author=&group=gmane.comp.lang.haske…> Here it seems the script searched for the string "&*gt*;" and so on... *Note: google badly fails to search these functions identifiers.* My current solution to this problem is: download compressed text archives and do a simple grep on it on my machine. But the archives up to year 2000 only are found at http://www.haskell.org/pipermail/haskell-cafe/. So, as an aside: Where do I find the earlier messages for downloading? Thanks, Damodar

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Darcs fetches too little files
by Ramana Kumar 30 Aug '12

30 Aug '12

On Aug 29, 2012 10:56 PM, "Henk-Jan van Tuyl" <hjgtuyl(a)chello.nl> wrote: > > On Wed, 29 Aug 2012 23:10:24 +0200, Stefan Monnier < monnier(a)iro.umontreal.ca> wrote: > >>> Albert Einstein said: >>> Insanity: doing the same thing over and over again and expecting >>> different results. >>> I repeated the command today and it worked! >> >> >> So, did you expect the result to be different, or did you re-try just to >> confirm that it doesn't work? > > > The day after I tried the command the first time, I thought the result might be different; I have the experience that my browser crashes, and starting it again results in an immediate crash. Logging off and on again solves the problem. This might be because of a corrupted DLL or a corrupted data area of a DLL. > > The command failure could have also been caused by a problem at the server side. > > In conclusion: repeating the same thing could give different results. This depends on what counts as "the same". (There is a nice book called "The Subtlety of Sameness".) I'm surprised that as users of a language that makes a big deal out of referential transparency none of you have said that neither invoking the same command at a different time nor invoking a browser after changing context (logging in again) is doing the same thing. > > > Regards, > Henk-Jan van Tuyl > > > -- > http://Van.Tuyl.eu/ > http://members.chello.nl/hjgtuyl/tourdemonad.html > Haskell programming > -- > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Haskell Weekly News: Issue 242
by Daniel Santa Cruz 29 Aug '12

29 Aug '12

Welcome to issue 242 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 19 to 25, 2012. Quotes of the Week * srhb: I think that's going into space rather than diving into the deep end. * edwardk: normally i don't bother with little things like "documentation", "benchmarks" and "tests", but I recently discovered that when I do, I get these curious things called "users". * shachaf: "Number of comonads released by edwardk will exceed the number of words in the English language by 2015, experts report." Top Reddit Stories * Hackage2 server ready for testing Domain: haskell.org, Score: 75, Comments: 16 On Reddit: [1] http://goo.gl/WTebL Original: [2] http://goo.gl/dHkGo * Monads In Pictures Domain: newartisans.com, Score: 60, Comments: 24 On Reddit: [3] http://goo.gl/QMfGv Original: [4] http://goo.gl/ycmvx * A new fast and easy to use CSV library Domain: blog.johantibell.com, Score: 53, Comments: 26 On Reddit: [5] http://goo.gl/eRKSm Original: [6] http://goo.gl/uRmfz * duckduckgo.com just became my favorite search engine due to haskell support Domain: self.haskell, Score: 50, Comments: 14 On Reddit: [7] http://goo.gl/1tBCg Original: [8] http://goo.gl/1tBCg * An example on how ridiculously powerful haskells type system is Domain: chrisdone.com, Score: 44, Comments: 61 On Reddit: [9] http://goo.gl/iwJT2 Original: [10] http://goo.gl/NovM7 * Galois Internship Available: Come build high-assurance autonomous vehicles! Domain: corp.galois.com, Score: 43, Comments: 13 On Reddit: [11] http://goo.gl/iJEih Original: [12] http://goo.gl/8ckL4 * Issue 20 « The Monad.Reader Domain: themonadreader.wordpress.com, Score: 34, Comments: 8 On Reddit: [13] http://goo.gl/K5Cc7 Original: [14] http://goo.gl/dBSdK * Functional Game Design: Making Pong in Elm Domain: elm-lang.org, Score: 29, Comments: 11 On Reddit: [15] http://goo.gl/sCdfE Original: [16] http://goo.gl/RL2i1 * Making HaskellDB slightly more type-safe Domain: chrisdone.com, Score: 28, Comments: 3 On Reddit: [17] http://goo.gl/b16v5 Original: [18] http://goo.gl/7v38l * Identifying outdated packages in cabal install plans Domain: byorgey.wordpress.com, Score: 24, Comments: 5 On Reddit: [19] http://goo.gl/4yTrR Original: [20] http://goo.gl/Zjw7b * ghcLiVE project escapes! Domain: ghclive.wordpress.com, Score: 23, Comments: 16 On Reddit: [21] http://goo.gl/IiXaA Original: [22] http://goo.gl/QzUHW * My view on why relational algebra matters Domain: users.utu.fi, Score: 22, Comments: 16 On Reddit: [23] http://goo.gl/byYZW Original: [24] http://goo.gl/3NSmI * ANN basic-prelude 0.3 (now comes with a fully-featured Prelude replacement) Domain: haskell.org, Score: 21, Comments: On Reddit: [25] http://goo.gl/klUAO Original: [26] http://goo.gl/4Ccyv * I just realized that running monads is applying a left monad action Domain: self.haskell, Score: 21, Comments: 53 On Reddit: [27] http://goo.gl/Geus4 Original: [28] http://goo.gl/Geus4 * You can soon play in the cabal sandbox Domain: blog.johantibell.com, Score: 20, Comments: 10 On Reddit: [29] http://goo.gl/QuVo0 Original: [30] http://goo.gl/6Q9a9 * Unordered tuples and type algebra Domain: byorgey.wordpress.com, Score: 18, Comments: 3 On Reddit: [31] http://goo.gl/N8iuz Original: [32] http://goo.gl/8l11g * Haskell-Type-Exts Passed the Final Evaluation Domain: cleantypecheck.wordpress.com, Score: 16, Comments: 3 On Reddit: [33] http://goo.gl/ok7CL Original: [34] http://goo.gl/ttsFb * LambdaJam Conference (from the creator of Strange Loop) -- Coming to Chicago in Summer 2013! Domain: lambdajam.com, Score: 15, Comments: On Reddit: [35] http://goo.gl/ytWNE Original: [36] http://goo.gl/ziyYc * Hello world in SKI combinator calculus Domain: self.haskell, Score: 14, Comments: 16 On Reddit: [37] http://goo.gl/qVDMw Original: [38] http://goo.gl/qVDMw * Is this Monoid instance in the standard library somewhere? Domain: self.haskell, Score: 14, Comments: 7 On Reddit: [39] http://goo.gl/cFHG7 Original: [40] http://goo.gl/cFHG7 * Applicative functors: definition and syntax (a reply) Domain: tomasp.net, Score: 13, Comments: 7 On Reddit: [41] http://goo.gl/4VPKR Original: [42] http://goo.gl/saU2j Top StackOverflow Questions * Lazy Evaluation and Time Complexity votes: 40, answers: 7 Read on SO: [43] http://goo.gl/f9QTO * What is the purpose of Rank2Types? votes: 21, answers: 3 Read on SO: [44] http://goo.gl/XrPHP * “Modern” HList? votes: 16, answers: 1 Read on SO: [45] http://goo.gl/X0LJ4 * Overloading of operators votes: 14, answers: 1 Read on SO: [46] http://goo.gl/1rB0O * When is a composition of catamorphisms a catamorphism? votes: 11, answers: 4 Read on SO: [47] http://goo.gl/x7eBk * Tacit function composition in Haskell votes: 10, answers: 2 Read on SO: [48] http://goo.gl/TIvMN * Haskell pattern matching on GADTs with Data Kinds votes: 9, answers: 1 Read on SO: [49] http://goo.gl/5fgAR * Store polymorphic callbacks in Haskell votes: 8, answers: 1 Read on SO: [50] http://goo.gl/QMOlq * Having trouble understanding Haskell's type system votes: 8, answers: 2 Read on SO: [51] http://goo.gl/n8FfC * Function composition hint votes: 7, answers: 2 Read on SO: [52] http://goo.gl/kvgZj * Comparing speed of Haskell and C for the computation of primes votes: 6, answers: 4 Read on SO: [53] http://goo.gl/gPh02 Until next time, Daniel Santa Cruz References 1. http://www.haskell.org/pipermail/cabal-devel/2012-August/008977.html 2. http://www.reddit.com/r/haskell/comments/yr9t1/hackage2_server_ready_for_te… 3. http://newartisans.com/2012/08/monads-in-pictures/ 4. http://www.reddit.com/r/haskell/comments/ykmgg/monads_in_pictures/ 5. http://blog.johantibell.com/2012/08/a-new-fast-and-easy-to-use-csv-library.… 6. http://www.reddit.com/r/haskell/comments/yqh7z/a_new_fast_and_easy_to_use_c… 7. http://www.reddit.com/r/haskell/comments/yjdkh/duckduckgocom_just_became_my… 8. http://www.reddit.com/r/haskell/comments/yjdkh/duckduckgocom_just_became_my… 9. http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.ht… 10. http://www.reddit.com/r/haskell/comments/yosoe/an_example_on_how_ridiculous… 11. http://corp.galois.com/blog/2012/8/21/galois-internship-available.html 12. http://www.reddit.com/r/haskell/comments/yl84x/galois_internship_available_… 13. http://themonadreader.wordpress.com/2012/08/25/issue-20/ 14. http://www.reddit.com/r/haskell/comments/yu9bu/issue_20_the_monadreader/ 15. http://elm-lang.org/blog/games-in-elm/part-0/Making-Pong.html 16. http://www.reddit.com/r/haskell/comments/yp27x/functional_game_design_makin… 17. http://chrisdone.com/posts/2012-08-25-haskelldb-more-type-safe.html 18. http://www.reddit.com/r/haskell/comments/ysuvr/making_haskelldb_slightly_mo… 19. http://byorgey.wordpress.com/2012/08/25/identifying-outdated-packages-in-ca… 20. http://www.reddit.com/r/haskell/comments/ytckc/identifying_outdated_package… 21. http://ghclive.wordpress.com/2012/08/20/ghclive-project-escapes/ 22. http://www.reddit.com/r/haskell/comments/yjtql/ghclive_project_escapes/ 23. http://users.utu.fi/machra/posts/2012-08-23-relalgebra.html 24. http://www.reddit.com/r/haskell/comments/ypdox/my_view_on_why_relational_al… 25. http://www.haskell.org/pipermail/haskell/2012-August/023477.html 26. http://www.reddit.com/r/haskell/comments/yralu/ann_basicprelude_03_now_come… 27. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 28. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 29. http://blog.johantibell.com/2012/08/you-can-soon-play-in-cabal-sandbox.html 30. http://www.reddit.com/r/haskell/comments/ys359/you_can_soon_play_in_the_cab… 31. http://byorgey.wordpress.com/2012/08/24/unordered-tuples-and-type-algebra/ 32. http://www.reddit.com/r/haskell/comments/ysuyr/unordered_tuples_and_type_al… 33. http://cleantypecheck.wordpress.com/2012/08/24/haskell-type-exts-passed-the… 34. http://www.reddit.com/r/haskell/comments/yrzcq/haskelltypeexts_passed_the_f… 35. http://lambdajam.com/ 36. http://www.reddit.com/r/haskell/comments/yprlk/lambdajam_conference_from_th… 37. http://www.reddit.com/r/haskell/comments/ypcu7/hello_world_in_ski_combinato… 38. http://www.reddit.com/r/haskell/comments/ypcu7/hello_world_in_ski_combinato… 39. http://www.reddit.com/r/haskell/comments/yqt89/is_this_monoid_instance_in_t… 40. http://www.reddit.com/r/haskell/comments/yqt89/is_this_monoid_instance_in_t… 41. http://tomasp.net/blog/applicative-functors.aspx 42. http://www.reddit.com/r/haskell/comments/ykto4/applicative_functors_definit… 43. http://stackoverflow.com/questions/12057658/lazy-evaluation-and-time-comple… 44. http://stackoverflow.com/questions/12031878/what-is-the-purpose-of-rank2typ… 45. http://stackoverflow.com/questions/12078792/modern-hlist 46. http://stackoverflow.com/questions/12076475/overloading-of-operators 47. http://stackoverflow.com/questions/12103309/when-is-a-composition-of-catamo… 48. http://stackoverflow.com/questions/12061176/tacit-function-composition-in-h… 49. http://stackoverflow.com/questions/12067503/haskell-pattern-matching-on-gad… 50. http://stackoverflow.com/questions/12044490/store-polymorphic-callbacks-in-… 51. http://stackoverflow.com/questions/12047481/having-trouble-understanding-ha… 52. http://stackoverflow.com/questions/12025230/function-composition-hint 53. http://stackoverflow.com/questions/12026668/comparing-speed-of-haskell-and-…

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Darcs fetches too little files
by Henk-Jan van Tuyl 29 Aug '12

29 Aug '12

I am trying to fetch wxHaskell with the command darcs get --lazy http://code.haskell.org/wxhaskell/ but there are much too little files downloaded; what could be the problem? I'm working on Windows XP, both in an MS-DOS shell and an MSYS shell. Installed Darcs version: 2.8.1 Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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ANNOUNCE: persistent-vector-0.1.0.1
by Tristan Ravitch 29 Aug '12

29 Aug '12

I uploaded a package implementing persistent vectors using array mapped tries (based on the implementation in clojure). Version 0.1.0.0 was broken, so I am starting off with 0.1.0.1. http://hackage.haskell.org/package/persistent-vector Persistent vectors are a sequence container offering efficient and purely functional append (snoc), indexing, and updates. This is similar to Data.Sequence from containers. The array mapped trie is closely related to the data structure used in the unordered-containers package. Comparison to Sequence: * Faster indexing and append * Slightly slower updates to existing elements * O(1) slicing * Sequence offers efficient prepend and concatenate (persistent-vector does not implement prepend, while concatenate is O(n)). I tried to model the API after Sequence as much as was reasonable, but a few functions are still missing. Some are reasonable to implement and some would be difficult to make efficient. The results from criterion (mostly comparing against Sequence and IntMap) are posted here: http://pages.cs.wisc.edu/~travitch/pvec.html The *Vec* runs are the vectors from this package and IM and Seq are IntMap and Sequence from containers. Hopefully someone will find this useful. Comments and suggestions are definitely welcome for anything: API, implementation, test suite, or benchmarks.

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Re: [Haskell-cafe] A first glimps on the {-# NOUPDATE #-} pragma
by Carter Schonwald 29 Aug '12

29 Aug '12

You got me there. Excellent point On Tuesday, August 28, 2012, Yves Parès wrote: > Monad? Simple strictness anotation is enough in that case: > > upd_noupd n = > let l = myenum' 0 n > h = head l > in h `seq` last l + h > > Le mardi 28 août 2012 22:39:09 UTC+2, Carter Schonwald a écrit : >> >> Hey Joachim, >> isn't this an example where the exact same issue could be solved via some >> suitable use of a monad for ordering those two computations on l? >> >> cheers >> -Carter >> >> On Tue, Aug 28, 2012 at 12:44 PM, Joachim Breitner <brei...(a)kit.edu>wrote: >> >>> Dear GHC users, >>> >>> I am experimenting with ways to /prevent/ sharing in Haskell, e.g. to >>> avoid space leaks or to speed up evaluation. A first attempt was to >>> duplicate closures on the heap to preserve the original one, see >>> http://arxiv.org/abs/1207.2017 for a detailed description and >>> information on the prototype implementation; no GHC patching required >>> for that. >>> >>> Currently I am trying a different angle: Simply avoid generating the >>> code that will update a closure after its evaluation; hence the closure >>> stays a thunk and will happily re-evaluate the next time it is used. >>> >>> Here is a classical example. Take this function (it is basically [f..t] >>> but with a fixed type and no risk of existing rules firing): >>> >>> myenum :: Int -> Int -> [Int] >>> myenum f t = if f <= t >>> then f : myenum (f + 1) t >>> else [] >>> >>> and this example where sharing hurts performance badly: >>> >>> upd_upd n = >>> let l = myenum 0 n >>> in last l + head l >>> >>> The problem is that during the evaluation of "last l", the list is live >>> and needs to be kept in memory, although in this case, re-evaluating l >>> for "head l" would be cheaper. If n is 50000000, then this takes 3845ms >>> on my machine, measured with criterion, and a considerable amount of >>> memory (3000MB). >>> >>> So here is what you can do now: You can mark the value as >>> non-updateable. We change myenum to >>> >>> myenum' :: Int -> Int -> [Int] >>> myenum' f t = if f <= t then f : ({-# NOUPDATE #-} myenum' (f + >>> 1) t) else [] >>> >>> and use that: >>> >>> upd_noupd n = >>> let l = myenum' 0 n >>> in last l + head l >>> >>> The improvement is considerable: 531ms and not much memory used (18MB) >>> >>> >>> Actually, it should suffice to put the pragma in the definition of l >>> without touching myenum: >>> >>> noupd_noupd n = >>> let l = {-# NOUPDATE #-} myenum 0 n >>> in last l + head l >>> >>> but this does not work with -O due to other optimizations in GHC. (It >>> does work without optimization.) >>> >>> >>> The next step would be to think of conditions under which the compiler >>> could automatically add the pragma, e.g. when it sees that evaluation a >>> thunk is very cheap but will increase memory consumption considerable. >>> >>> >>> Also this does not have to be a pragma; it could just as well be a >>> function "noupdate :: a -> a" that is treated specially by the compiler, >>> similar to the "inline" function. >>> >>> >>> If you want to play around this, feel free to fetch it from the unshare >>> branch of my ghc repository at http://git.nomeata.de/?p=ghc.**git<http://git.nomeata.de/?p=ghc.git>or >>> https://github.com/nomeata/ghc for the GitHub-lovers. Note that the >>> branch is repeatedly rebased against ghc master. >>> >>> >>> Greetings, >>> Joachim >>> >>> >>> -- >>> Dipl.-Math. Dipl.-Inform. Joachim Breitner >>> Wissenschaftlicher Mitarbeiter >>> http://pp.info.uni-karlsruhe.**de/~breitner<http://pp.info.uni-karlsruhe.de/%7Ebreitner> >>> >>> ______________________________**_________________ >>> Haskell-Cafe mailing list >>> Haskel...(a)haskell.org >>> http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >>> >>> >>

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A first glimps on the {-# NOUPDATE #-} pragma
by Joachim Breitner 28 Aug '12

28 Aug '12

Dear GHC users, I am experimenting with ways to /prevent/ sharing in Haskell, e.g. to avoid space leaks or to speed up evaluation. A first attempt was to duplicate closures on the heap to preserve the original one, see http://arxiv.org/abs/1207.2017 for a detailed description and information on the prototype implementation; no GHC patching required for that. Currently I am trying a different angle: Simply avoid generating the code that will update a closure after its evaluation; hence the closure stays a thunk and will happily re-evaluate the next time it is used. Here is a classical example. Take this function (it is basically [f..t] but with a fixed type and no risk of existing rules firing): myenum :: Int -> Int -> [Int] myenum f t = if f <= t then f : myenum (f + 1) t else [] and this example where sharing hurts performance badly: upd_upd n = let l = myenum 0 n in last l + head l The problem is that during the evaluation of "last l", the list is live and needs to be kept in memory, although in this case, re-evaluating l for "head l" would be cheaper. If n is 50000000, then this takes 3845ms on my machine, measured with criterion, and a considerable amount of memory (3000MB). So here is what you can do now: You can mark the value as non-updateable. We change myenum to myenum' :: Int -> Int -> [Int] myenum' f t = if f <= t then f : ({-# NOUPDATE #-} myenum' (f + 1) t) else [] and use that: upd_noupd n = let l = myenum' 0 n in last l + head l The improvement is considerable: 531ms and not much memory used (18MB) Actually, it should suffice to put the pragma in the definition of l without touching myenum: noupd_noupd n = let l = {-# NOUPDATE #-} myenum 0 n in last l + head l but this does not work with -O due to other optimizations in GHC. (It does work without optimization.) The next step would be to think of conditions under which the compiler could automatically add the pragma, e.g. when it sees that evaluation a thunk is very cheap but will increase memory consumption considerable. Also this does not have to be a pragma; it could just as well be a function "noupdate :: a -> a" that is treated specially by the compiler, similar to the "inline" function. If you want to play around this, feel free to fetch it from the unshare branch of my ghc repository at http://git.nomeata.de/?p=ghc.git or https://github.com/nomeata/ghc for the GitHub-lovers. Note that the branch is repeatedly rebased against ghc master. Greetings, Joachim -- Dipl.-Math. Dipl.-Inform. Joachim Breitner Wissenschaftlicher Mitarbeiter http://pp.info.uni-karlsruhe.de/~breitner

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Conduit: Where to run monad stacks?
by Niklas Hambüchen 28 Aug '12

28 Aug '12

Today I was surprised that transPipe is called for every chunk of data going through my pipe, rendering the StateT I put in useless, because it was always restarted with the initial value. It would be nice to have some explanation about this, as it makes it easy to write compiling code that has completely unexpected behaviour. I wrote this function (also on http://hpaste.org/73538) conduitWithState :: (MonadIO m) => Conduit Int (StateT Int m) String conduitWithState = do liftIO $ putStrLn $ "Counting Int->String converter ready!" awaitForever $ \x -> do i <- lift get lift $ modify (+1) liftIO $ putStrLn $ "Converting " ++ show x ++ " to a string! " ++ "Processed so far: " ++ show i yield (show x) and ran it like this: countingConverterConduit :: (MonadIO m) => Conduit Int m String countingConverterConduit = transPipe (\stateTint -> evalStateT stateTint 1) conduitWithState main :: IO () main = do stringList <- CL.sourceList [4,1,9,7,3] $= countingConverterConduit $$ CL.consume print stringList However, the output is not what I expected, but only: Processed so far:1 Processed so far:1 ... Dan Burton proposed a fix, making the whole sink-conduit-source construction run on the StateT: main = do stringList <- flip evalStateT 1 $ ... So the question is: What is the rationale for this? I was expecting that if I have an IO pipe in my main conduit, I could easily run stuff on top of that in parts of the pipe. Thanks Niklas

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