- Haskell-Cafe - Haskell.org

[ANN] thirdake-0.1 the Makefile-like DSL
by Sergey Mironov 29 Jul '13

29 Jul '13

Hi, cafe. Neil Mitchell mentioned more than ten build systems written in Haskell in his paper [1]. The list includes shake (of cause) Abba, Coadjuke, hake, hmk, and even two cakes. I'm glad to announce the thirdcake - a DSL for make language. Thirdcake allows user to produce clean and safe (I hope) Makefiles for the good old GNU make. The package is in it's early stages and is not on Hackage yet. Consider visiting https://github.com/grwlf/cake3 or just git clone https://github.com/grwlf/cake3 ./Example/Foo contains working toy-project demonstrating the approach. Regards, Sergey [1] - http://community.haskell.org/~ndm/downloads/paper-shake_before_building-10_… PS My English is still buggy, feel free to patch the grammar as well as the rest

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using network+conduit+tls for a client application?
by Petr Pudlák 29 Jul '13

29 Jul '13

Dear Haskellers, I wanted to write a small TLS application (connecting to IMAP over TLS) and it seemed natural to use conduit for that. I found the network-conduit-tls package, but then I realized it's meant only for server applications. Is there something similar for client applications? Thank you, Petr Pudlak

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Is withAsync absolutely safe?
by Roman Cheplyaka 29 Jul '13

29 Jul '13

Can withAsync guarantee that its child will be terminated if the thread executing withAsync gets an exception? To remind, here's an implementation of withAsync: withAsyncUsing :: (IO () -> IO ThreadId) -> IO a -> (Async a -> IO b) -> IO b -- The bracket version works, but is slow. We can do better by -- hand-coding it: withAsyncUsing doFork = \action inner -> do var <- newEmptyTMVarIO mask $ \restore -> do t <- doFork $ try (restore action) >>= atomically . putTMVar var let a = Async t (readTMVar var) r <- restore (inner a) `catchAll` \e -> do cancel a; throwIO e cancel a return r I am interested in the case when an exception arrives which transfers control to 'cancel', and then another exception arrives to the same thread. Even though 'catchAll' (which is a type-restricted synonym for catch) masks the exception handler, 'throwTo' inside 'cancel' is interruptible (as stated by the documentation). Will this scenario lead to a thread leakage? Roman

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Munich Haskell Meeting
by Heinrich Hördegen 29 Jul '13

29 Jul '13

Dear all, our monthly Haskell Meeting will take place tomorrow, Tuesday the 30th of July, at 19h30 in Munich. Please note that we will meet at Max-Emanuel-Brauerei. If you plan to join, please hit the button at http://www.haskell-munich.de/dates Have a nice week, Heinrich

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haskell-gtk entry question
by briand＠aracnet.com 28 Jul '13

28 Jul '13

Hello all, This should be simple, and I thought it had it working, but I've broken it and can't figure out why. What I want is to invoke the callback whenever the user activates and entry in a dialogbox, so I did both this : Gtk.on entry Gtk.entryActivate (boxHandler entry) (I believe this supercedes the previous method which was onEntryActivate) and this Gtk.on entry Gtk.entryPreeditChanged (boxHandler entry) however neither method will invoke the callback. The program compiles and works just fine, it's just that the callback never runs. Thank you, Brian

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casting a into Maybe a
by Joerg Fritsch 27 Jul '13

27 Jul '13

If I have the following type signature transMit :: Serialize a => Socket -> POSIXTime -> KEY -> Maybe a -> TPSQ -> TMap a -> IO () And the function is called with transMit s now key newmsgs q m where newmsgs is whatever type a I get but _not_ a Maybe a then I get the error Could not deduce (a ~ Maybe a) from the context (Serialize a) Can I somehow when I call transmit cast newmsgs into a Maybe newmsgs or so so that the function call fits the type signature? --Joerg

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scrz - container management tool written (partly) in haskell
by Tomas Carnecky 27 Jul '13

27 Jul '13

Scrz (http://scrz.io) is a container (LXC) management tool. It is heavily inspired by docker and heroku, but also provides features which those two don't have. It comes in two parts, one is the supervisor which manages the containers, the other is a server which stores the configuration. The supervisor fetches its configuration from the server. Only the supervisor is written in Haskell, the configuration server is written in Ruby on Rails. I use scrz on my personal server, and it's also used to automate deployments of http://encounter.io (another side project that I have). I just recently updated the homepage and redesigned the configuration server UI (screenshot link at the bottom). It's more difficult to set up than docker, and not that well integrated into the system. This is partly due to lack of good low-level sysadmin libraries for Haskell (eg. to list/create/inspect network interfaces) and partly due to dependencies on new versions of software that are not even available in the latest stable ubuntu (lxc-tools version 0.9.x). The project is maintained on github: https://github.com/scrz. http://cl.ly/image/262v1x1r2T12

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Are type families really this slow, or is this a GHC bug?
by Mike Izbicki 27 Jul '13

27 Jul '13

I'm using the TypeFamilies extension to generate types that are quite large. GHC can handle these large types fine when they are created manually, but when type families get involved, GHC's performance dies. It's doing in quadratic time what looks to me like it should be linear time. I don't know if this is expected behavior, if I'm doing something wrong, or if this is a GHC bug. I've attached a code sample below that demonstrates the problem. Types.hs generates other haskell files. The first parameter is the size of the type (which is type list of that length), and the second specifies which test to run. All tests generate the same type in the end, but some use type families and some don't. Here's an example of running it: These tests show quadratic time when using the type family. I have to increase the context stack size to be greater than the recursion depth of the type family. I don't know if this is a bad sign or to be expected. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.3 $ ghc Types $ ./Types 200 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=250 real 0m2.973s $ ./Types 300 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=350 real 0m6.018s $ ./Types 400 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=450 real 0m9.995s $ ./Types 500 a > test.hs && time ghc test.hs > /dev/null -fcontext-stack=550 real 0m15.645s Without the type family, I get MUCH better performance: $ ./Types 10000 d > test.hs && time ghc test.hs > /dev/null real 0m2.271s ------------------------ -- Types.hs below ------------------------ import System.Environment code :: Int -> String -> String code i test = concat $ map (++"\n") $ [ "{-# LANGUAGE TypeOperators,DataKinds, KindSignatures,TypeFamilies,PolyKinds #-}" , "import GHC.TypeLits" , "data Nat1 = Zero | Succ Nat1" , "type family Replicate1 (n :: Nat1) (x::a) :: [a]" , "type instance Replicate1 Zero x = '[]" , "type instance Replicate1 (Succ n) x = x ': (Replicate1 n x)" , "class Class a where" , " f :: a -> a" , "data Data (xs::a) = X | Y" , " deriving (Read,Show)" , "main = print test1" ] ++ case head test of 'a' -> [ "instance (xs ~ Replicate1 ("++mkNat1 i++") ()) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( Replicate1 ("++mkNat1 i++") () ))" ] 'b' -> [ "instance (xs ~ ("++mkList i++") ) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( Replicate1 ("++mkNat1 i++") () ))" ] 'c' -> [ "instance (xs ~ Replicate1 ("++mkNat1 i++") ()) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( ("++mkList i++") ))" ] otherwise -> [ "instance (xs ~ ("++mkList i++") ) => Class (Data xs) where" , " f X = Y" , " f Y = X" , "test1 = f (X :: Data ( ("++mkList i++") ))" ] mkList :: Int -> String mkList 0 = " '[] " mkList i = " () ': " ++ mkList (i-1) mkNat1 :: Int -> String mkNat1 0 = " Zero " mkNat1 i = " Succ ( " ++ mkNat1 (i-1) ++ ")" main = do numstr : test : xs <- getArgs let num = read numstr :: Int putStrLn $ code num test

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Sneaky method for var-arg fns?
by Micah Cowan 27 Jul '13

27 Jul '13

So, just for fun, I came up with a way to abuse the language in order to define a "function" whose argument is optional: > -- dirty-trick.hs - A sneaky way to do var args fns in Haskell > > {-# LANGUAGE FlexibleInstances #-} > > class Hello a where > hello :: a > > instance Hello (String -> String) where > hello = (\str -> "Hello " ++ str) > > instance Hello String where > hello = hello "World" In ghci, putStrLn $ hello gives "Hello World", while putStrLn $ hello "There" gives "Hello". . I was wondering if there was a way to do it in "pure" Haskell (i.e., no GHC pragmas required), and also the specific reason for why the above example doesn't work without the pragma (I think it's just that in general a -> b is not syntactically allowed for type specifiers within instance declarations)? I'm also interested in alternative approaches to creating variable-argument functions, if there are any. . If anyone's curious, this was prompted by a discussion with a friend (copied), about Haskell and Clojure. He mentioned that Clojure can accept variable arguments, though AFAICT all Clojure functions basically act like they take a list (that supports variable types), so accepting an empty argument list is a bit analogous to Haskell accepting an empty list, rather than no arguments. Part of the reason Haskell can't really take "variable arguments" is that all Haskell functions really just take one argument. But I figured you could use the contextually expected type to decide whether to return a simple value (not /technically/ a function in that case), or a function expecting further arguments, which could then be extended to define a function taking any arbitrary number of arguments. Cheers! -mjc

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Proposal: Non-recursive let
by oleg＠okmij.org 26 Jul '13

26 Jul '13

Jon Fairbairn wrote: > It just changes forgetting to use different variable names because of > recursion (which is currently uniform throughout the language) to > forgetting to use non recursive let instead of let. Let me bring to the record the message I just wrote on Haskell-cafe http://www.haskell.org/pipermail/haskell-cafe/2013-July/109116.html and repeat the example: In OCaml, I can (and often do) write let (x,s) = foo 1 [] in let (y,s) = bar x s in let (z,s) = baz x y s in ... In Haskell I'll have to uniquely number the s's: let (x,s1) = foo 1 [] in let (y,s2) = bar x s1 in let (z,s3) = baz x y s2 in ... and re-number them if I insert a new statement. I once wrote about 50-100 lines of code with the fragment like the above and the only problem was my messing up the numbering (at one place I used s2 where I should've used s3). In the chain of lets, it becomes quite a chore to use different variable names -- especially as one edits the code and adds new let statements. I have also had problems with non-termination, unintended recursion. The problem is not caught statically and leads to looping, which may be quite difficult to debug. Andreas should tell his story. In my OCaml experience, I don't ever remember writing let rec by mistake. Occasionally I write let where let rec is meant, and the type checker very quickly points out a problem (an unbound identifier). No need to debug anything. Incidentally, time and again people ask on the Caml list why 'let' in OCaml is by default non-recursive. The common answer is that the practitioners find in their experience the non-recursive let to be a better default. Recursion should be intended and explicit -- more errors are caught that way. Let me finally disagree with the uniformity principle. It may be uniform to have equi-recursive types. OCaml has equi-recursive types; internally the type checker treats _all_ types as (potentially) equi-recursive. At one point OCaml allowed equi-recursive types in user programs as well. They were introduced for the sake of objects; so the designers felt uniformly warrants to offer them in all circumstances. The users vocally disagreed. Equi-recursive types mask many common type errors, making them much more difficult to find. As the result, OCaml developers broke the uniformity. Now, equi-recursive types may only appear in surface programs in very specific circumstances (where objects or their duals are involved). Basically, the programmer must really intend to use them. Here is an example from the natural language, English. Some verbs go from regular (uniform conjugation) to irregular: http://en.wiktionary.org/wiki/dive

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Fwd: News: FP Complete Offering Free Academic Accounts to FP Haskell Center
by Natalia Muska 26 Jul '13

26 Jul '13

FP Complete Offering Free Academic Accounts to FP Haskell Center San Diego, C.A. (July 25, 2013) FP Complete, the leading developer of commercial Haskell software tools and services, announced today that professors and students will be offered free accounts to FP Haskell Center, the world’s first commercial Haskell IDE and deployment platform. FP Haskell Center is an integrated development and deployment environment written in Haskell, the most popular and purest functional programming language. It consists of two components, the FP Development Environment and the FP Application Server. The FP Development Environment includes a Haskell compiler and a continually updated set of vetted, tested and supported libraries and code templates. There is no need to run Cabal or other installers. The FP Application Server is used to deploy and run Haskell applications directly in the cloud. With many tutorials and sample code included in the product, FP Haskell Center provides both teachers and students with an accessible cloud-based platform to teach/learn, perform Haskell research, and develop real-world solutions. “Haskell was created at some leading universities in the world over 20 years ago and its advancement since then has been significantly driven by the global academic community,” said Aaron Contorer, FP Complete founder and CEO. “As Haskell becomes more mainstream, we want to provide the institutions championing Haskell the most the opportunity to keep it growing at no cost to them” he adds. “Great things are constantly coming out of the university setting and we want to support their research and projects as much as we can.” Eligible teachers and students can get a free account through a simple web sign-up form in mid-August, right before FP Haskell Center becomes generally available in early September. They can get started now by signing up <https://learning-site-staging.fpcomplete.com/business/designer-ide> for a Beta. Check for updates/details at www.fpcomplete.com. About FP Complete FP Complete is dedicated to bringing the Haskell programming language into the mainstream software market by being the leading developer of commercial Haskell software tools and services. Haskell is the new way to make software that accelerates time to market, boosts programmer productivity, lowers cost, and increases product quality. Haskell enables rapid development of high-performing, complex software with high quality, maintainability and scalability. Haskell has proven to be able to significantly cut the billions wasted in the compile- test-debugging-rework cycles where 50% of the software project costs are today. In short, Haskell delivers the competitive advantage and productivity that enterprises need in a global software-driven economy. Visit www.fpcomplete.com for more information. -- Natalia Muska Marketing Manager FP Complete, Inc. Natalia(a)fpcomplete.com 865-506-6513 skype: natalia.s.muska

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Re: [Haskell-cafe] Proposal: Non-recursive let
by oleg＠okmij.org 25 Jul '13

25 Jul '13

ivan.chollet wrote: > let's consider the following: > > let fd = Unix.open ... > let fd = Unix.open ... > > At this point one file descriptor cannot be closed. Static analysis will > have trouble catching these bugs, so do humans. Both sentences express false propositions. The given code, if Haskell, does not open any file descriptors, so there is nothing to close. In the following OCaml code let fd = open_in "/tmp/a" in let fd = open_in "/tmp/v" in ... the first open channel becomes unreachable. When GC collects it (which will happen fairly soon, on a minor collection, because the channel died young), GC will finalize the channel and close its file descriptor. The corresponding Haskell code do h <- openFile ... h <- openFile ... works similarly to OCaml. Closing file handles upon GC is codified in the Haskell report because Lazy IO crucially depends on such behavior. If one is interested in statically tracking open file descriptors and making sure they are closed promptly, one could read large literature on this topic. Google search for monadic regions should be a good start. Some of the approaches are implemented and used in Haskell. Now about static analysis. Liveness analysis has no problem whatsoever determining that a variable fd in our examples has been shadowed and the corresponding value is dead. We are all familiar with liveness analysis -- it's the one responsible for `unused variable' warnings. The analysis is useful for many other things (e.g., if it determines that a created value dies within the function activation, the value could be allocated on stack rather than on heap.). Here is example from C: #include <stdio.h> void foo(void) { char x[4] = "abc"; /* Intentional copying! */ { char x[4] = "cde"; /* Intentional copying and shadowing */ x[0] = 'x'; printf("result %s\n",x); } } Pretty old GCC (4.2.1) had no trouble detecting the shadowing. With the optimization flag -O4, GCC acted on this knowledge. The generated assembly code reveals no traces of the string "abc", not even in the .rodata section of the code. The compiler determined the string is really unused and did not bother even compiling it in. > Disallowing variable shadowing prevents this. > The two "fd" occur in different contexts and should have different names. > Usage of shadowing is generally bad practice. It is error-prone. Hides > obnoxious bugs like file descriptors leaks. > The correct way is to give different variables that appear in different > contexts a different name, although this is arguably less convenient and > more verbose. CS would be better as science if we refrain from passing our personal opinions and prejudices as ``the correct way''. I can't say better than the user Kranar in a recent discussion on a similar `hot topic': The issue is that much of what we do as developers is simply based on anecdotal evidence, or statements made by so called "evangelicals" who blog about best practices and people believe them because of how articulate they are or the cache and prestige that the person carries. ... It's unfortunate that computer science is still advancing the same way medicine advanced with witch doctors, by simply trusting the wisest and oldest of the witch doctors without any actual empirical data, without any evidence, just based on the reputation and overall charisma or influence of certain bloggers or "big names" in the field. http://www.reddit.com/r/programming/comments/1iyp6v/is_there_a_really_an_em…

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Re: [Haskell-cafe] Parsec question
by Kashyap CK 25 Jul '13

25 Jul '13

There is reference in the paper that empty list indicates failure...so could we just use it like Maybe? I'd like it very much if I could get an example of a missed match by not using the complete match. regards, Kashyap Sent from my Windows Phone From: Roman Cheplyaka Sent: 24/07/2013 8:19 PM To: C K Kashyap Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Parsec question Think about this: if you always take only the first element, why do you need lists at all? Roman * C K Kashyap <ckkashyap(a)gmail.com> [2013-07-24 19:56:29+0530] > Dear Cafe, > > I am trying to implement[1] parsec in go using the "Monadic Parser > Combinators" paper [2] . I've been able to implement "plus" "bind" and > "many" > While doing the implementation - I looked at bind closely > > bind :: Parser a -> (a -> Parser b) -> Parser b > p `bind` f = \inp -> concat [f v inp' | (v,inp') <- p inp] > > I wondered if the result needs the complete list - wouldn't just the first > successful value suffice? > Perhaps - > p `bind` f = \inp -> take 1 $ concat [f v inp' | (v,inp') <- p inp] > > Will this miss out matches? > > > Regards, > Kashyap > > [1] https://github.com/ckkashyap/parsec/blob/master/parsec.go > [2] Monadic Parser Combinators: Graham Hutton, Erik Meijer > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Parsec question
by C K Kashyap 25 Jul '13

25 Jul '13

Dear Cafe, I am trying to implement[1] parsec in go using the "Monadic Parser Combinators" paper [2] . I've been able to implement "plus" "bind" and "many" While doing the implementation - I looked at bind closely bind :: Parser a -> (a -> Parser b) -> Parser b p `bind` f = \inp -> concat [f v inp' | (v,inp') <- p inp] I wondered if the result needs the complete list - wouldn't just the first successful value suffice? Perhaps - p `bind` f = \inp -> take 1 $ concat [f v inp' | (v,inp') <- p inp] Will this miss out matches? Regards, Kashyap [1] https://github.com/ckkashyap/parsec/blob/master/parsec.go [2] Monadic Parser Combinators: Graham Hutton, Erik Meijer

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How can I use ghci more wisely?
by yi lu 25 Jul '13

25 Jul '13

I am wondering how can I ask ghci to show an infinite list wisely. When I type *fst ([1..],[1..10])* The result is what as you may guess *1,2,3,4,...*(continues to show, cut now) How could I may ghci show *[1..]* this wise way not the long long long list itself? Yi

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Expression problem in the database?
by Manuel Gómez 24 Jul '13

24 Jul '13

Hi café, I don’t know whether this is a good forum to ask about this —perhaps Stack Overflow is better suited—, but since Haskell and related languages are so finely fit for good solutions to the expression problem, I figure this list may have a few helpful pointers regarding this problem. I’m facing a situation that requires saving boolean expressions of more or less arbitrary structure (encoding some business rules) into a SQL database. The expressions can include terms that refer to some other objects in the database. I realize this is somewhat vague, but that’s intentional: I’m really searching for ideas and experience with similar problems, not for a specific solution to my specific situation. Though I’m uncertain of their relative merits, I’ve had a few ideas myself: * I could sacrifice relational integrity and store the expression serialized, perhaps as an AST represented in JSON or somesuch — although the rest of the data model is a rather traditional, normalized relational schema, so this is undesirable in my situation if only for consistency. * I could directly model some form of the expression’s AST in the relational schema, and then endure the usual pain of representing sum types as relations. I imagine this to be quite unpleasant, although I haven’t tried it. * As these are boolean logic predicates, the expressions could be transformed into some normal form that could be more easily represented with relations. I’ll be mostly (but not exclusively) using this database from an application written in Haskell (Yesod using Esqueleto to talk to PostgreSQL), so some native Haskell solution could work almost as well as an SQL solution — but some legacy systems do need to interact with the database, so I’d prefer a solution in the database as long as it’s not horrible to work with in the application. I’ll be grateful for any ideas or pointers — my Googling for the expression problem has not proved fruitful. Thanks!

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memoization
by Christopher Howard 24 Jul '13

24 Jul '13

When I previously asked about memoization, I got the impression that memoization is not something that just happens magically in Haskell. Yet, on a Haskell wiki page about Memoization <http://www.haskell.org/haskellwiki/Memoization#Memoization_with_recursion>, an example given is memoized_fib :: Int -> Integer memoized_fib = (map fib [0 ..] !!) where fib 0 = 0 fib 1 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1) I guess this works because, for example, I tried "memoized_fib 10000" and the interpreter took three or four seconds to calculate. But every subsequent call to "memoized_fib 10000" returns instantaneously (as does "memoized_fib 10001"). Could someone explain the technical details of why this works? Why is "map fib [0 ..]" not recalculated every time I call memoized_fib?

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Expression problem in the database?
by oleg＠okmij.org 24 Jul '13

24 Jul '13

Here is one possible approach. First, convert the propositional formula into the conjunctive normal form (disjunctive will work just as well). Recall, the conjunctive normal form (CNF) is type CNF = [Clause] type Clause = [Literal] data Literal = Pos PropLetter | Neg PropLetter type PropLetter -- String or other representation for atomic propositions We assume that clauses in CNF are ordered and can be identified by natural indices. A CNF can be stored in the following table: CREATE DOMAIN PropLetter ... CREATE TYPE occurrence AS ( clause_number integer, (* index of a clause *) clause_card integer, (* number of literals in that clause *) positive boolean (* whether a positive or negative occ *) ); CREATE TABLE Formula ( prop_letter PropLetter references (table_of_properties), occurrences occurrence[] ); That is, for each prop letter we indicate which clause it occurs in (as a positive or a negative literal) and how many literals in that clause. The latter number (clause cardinality) can be factored out if we are orthodox. Since a letter may occur in many clauses, we use PostgreSQL arrays (which are now found in many DBMS). The formula can be evaluated incrementally: by fetching the rows one by one, keeping track of not yet decided clauses. We can stop as soon as we found a clause that evaluates to FALSE. BTW, `expression problem' typically refers to something else entirely (how to embed a language and be able to add more syntactic forms to the language and more evaluators without breaking previously written code).

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Re: [Haskell-cafe] memoization
by wren ng thornton 24 Jul '13

24 Jul '13

On 7/22/13 7:41 PM, David Thomas wrote: > On Mon, Jul 22, 2013 at 4:04 PM, wren ng thornton <wren(a)freegeek.org> wrote: >> In principle, we could translate between these two forms (the f2 ==> f1 >> direction requires detecting that y does not depend on x). However, in >> practice, the compiler has no way to decide which one is better since it >> involves a space/time tradeoff which: (a) requires the language to keep >> track of space and time costs, (b) would require whole-program analysis to >> determine the total space/time costs, and (c) requires the user's >> objective function to know how to weight the tradeoff ratio. > > I, for one, would love to have a compiler do (a) based on (b), my > specification of (c), and the ability to pin particular things... Oh, so would I! But, having worked on this problem in a different language, I'm well aware of how difficult it is to actually implement (a) and (b). -- Live well, ~wren

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CmdArgs non-flag arguments with newtypes
by Michael Orlitzky 24 Jul '13

24 Jul '13

I ran into an issue wrapping a [String] in a newtype with CmdArgs. You're supposed to be able to declare that a field contains a list of non-flag arguments... this works fine: data Cfg = Cfg { <whatever flags>, usernames :: [String] } arg_spec = Cfg { <whatever flags>, usernames = def &= args } ... If I now call my program with, ./foo --flag1 --flag2 arg1 arg2 then usernames = [ "arg1", "arg2" ] as desired. However, I need to wrap usernames in a newtype for an unrelated reason: newtype Usernames = Usernames [String] Now, CmdArgs unexpectedly drops the first non-flag argument. So, ./foo --flag1 --flag2 arg1 arg2 gives me usernames = [ "arg2" ]. Obviously not what I want! Has anyone else run into this? Figured out a workaround?

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Re: [Haskell-cafe] memoization
by wren ng thornton 23 Jul '13

23 Jul '13

On 7/22/13 9:06 AM, Tom Ellis wrote: > On Mon, Jul 22, 2013 at 07:52:06PM +1200, Chris Wong wrote: >> A binding is memoized if, ignoring everything after the equals sign, >> it looks like a constant. >> >> In other words, these are memoized: > [...] >> f = \x -> x + 1 > [...] >> and these are not: >> >> f x = x + 1 > > In what sense is the former memoised? I'm not aware of any difference > between these two definitions. Consider rather, f1 = let y = blah blah in \x -> x + y f2 x = let y = blah blah in x + y The former will memoize y and share it across all invocations of f1; whereas f2 will recompute y for each invocation. In principle, we could translate between these two forms (the f2 ==> f1 direction requires detecting that y does not depend on x). However, in practice, the compiler has no way to decide which one is better since it involves a space/time tradeoff which: (a) requires the language to keep track of space and time costs, (b) would require whole-program analysis to determine the total space/time costs, and (c) requires the user's objective function to know how to weight the tradeoff ratio. -- Live well, ~wren

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catching IO errors in a monad transformer stack
by Eric Rasmussen 22 Jul '13

22 Jul '13

Hello, I am writing a small application that uses a monad transformer stack, and I'm looking for advice on the best way to handle IO errors. Ideally I'd like to be able to perform an action (such as readFile "file_that_does_not_exist"), catch the IOError, and then convert it to a string error in MonadError. Here's an example of what I'm doing now: {-# LANGUAGE FlexibleContexts #-} import Control.Monad.Error import Control.Monad.State import System.IO.Error (tryIOError) catcher :: (MonadIO m, MonadError String m) => IO a -> m a catcher action = do result <- liftIO $ tryIOError action case result of Left e -> throwError (show e) Right r -> return r This does work as expected, but I get the nagging feeling that I'm missing an underlying pattern here. I have tried catch, catchError, and several others, but (unless I misused them) they don't actually help here. The tryIOError function from System.IO.Error is the most helpful, but I still have to manually inspect the result to throwError or return to my underlying monad. Since this has come up for me a few times now, I welcome any advice or suggestions on alternative approaches or whether this functionality already exists somewhere. Thanks! Eric

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News about the GSoC Project : Communicating with mobile devices
by Marcos Pividori 22 Jul '13

22 Jul '13

Hi, I am working this summer on the project "Communicating with mobile devices" for the GSoC. After working with GCM, I developed an API for communicating through APNS with iOS mobile devices [1]<https://github.com/MarcosPividori/GSoC-Communicating-with-mobile-devices/tr…> . For this, I designed a way of building the messages, sending them and handling the result. To use this service, Apple requires you to be an enrolled iOS Developer or something similar, so I decided to test the API with a local server. It simulates the APNS servers and different common scenes. Now, that I have a tested version, I am looking for some iOS developers to test in with real apps. So, I wait for feedback! Also, I have a blog to show the progress. I have posted about a simple usage of the API for GCM, I developed a test example of a Yesod server, where devices can register to receive GCM messages and users can send messages through the web service. For more information: [3]<http://gsoc2013cwithmobiledevices.blogspot.com.ar/> Thanks, Marcos The code is available on GitHub: - The APNS Api : [1]<https://github.com/MarcosPividori/GSoC-Communicating-with-mobile-devices/tr…> - Test example for APNS: [2]<https://github.com/MarcosPividori/GSoC-Communicating-with-mobile-devices/tr…> [1] https://github.com/MarcosPividori/GSoC-Communicating-with-mobile-devices/tr… [2] https://github.com/MarcosPividori/GSoC-Communicating-with-mobile-devices/tr… [3] http://gsoc2013cwithmobiledevices.blogspot.com.ar/

1 0

Wrapping all fields of a data type in e.g. Maybe
by Michael Orlitzky 22 Jul '13

22 Jul '13

I have a common pattern in my command-line programs; I start out with a configuration data type, which over-simplified looks like: data Cfg = Cfg { verbose :: Bool } Now, there's usually a default configuration, default :: Cfg default = Cfg False The user can override the defaults one of two ways, either via a config file, or from the command-line. If both are specified, the command-line takes precedence. The way I do this is with, data OptionalCfg = OptionalCfg { verbose :: Maybe Bool } And then I define I Monoid instance for OptionalCfg which lets me merge two ofthem. Once the two OptionalCfgs are merged, I merge *that* with the default Cfg. This all works great, except that when there's 20 or so options, I duplicate a ton of code in the definition of OptionalCfg. Is there some pre-existing solution that will let me take a Cfg and create a new type with Cfg's fields wrapped in Maybe?

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Re: [Haskell-cafe] [Haskell] [ANN] Initial release of the threepenny-gui library, version 0.1.0.0
by Sergey Mironov 22 Jul '13

22 Jul '13

Hi, I have a Path problem when installing threepenny-gui from Hackage. Probably somtething trivial. Sergey src/BarTab.hs:9:8: Could not find module `Paths' Use -v to see a list of the files searched for. Failed to install threepenny-gui-0.1.0.0 cabal: Error: some packages failed to install: threepenny-gui-0.1.0.0 failed during the building phase. The exception was: ExitFailure 1 2013/7/21 Heinrich Apfelmus <apfelmus(a)quantentunnel.de>: > Dear Haskellers, > > And the shark, he has teeth, > And he wears them in (his) face. > And Macheath, he has a knife, > (But) yes the knife, no-one sees. > > After an obligatory cryptical quotation from a famous writer, I am pleased > to announce the first public release of *threepenny-gui*, a cheap and simple > library to satisfy your immediate GUI needs in Haskell. > > Want to write a small GUI thing but forgot to sacrifice to the giant rubber > duck in the sky before trying to install wxHaskell or Gtk2Hs? Then this > library is for you! > > Threepenny-gui is easy to install (!!) because it uses the web browser as a > display. Internally, we implement a small web server that communicates with > the browser to display GUI elements. Consequently, you can use HTML and CSS > to design the user interface. You can freely manipulate the HTML DOM and > handle browser events by writing Haskell code. > > hackage - http://hackage.haskell.org/package/threepenny-gui > examples - https://github.com/HeinrichApfelmus/threepenny-gui#examples > source - https://github.com/HeinrichApfelmus/threepenny-gui > > Many thanks to Daniel Austin for collaborating with me on this project and > to Chris Done for implementing the Ji library which is the basis for this > effort. > > > On that note, the threepenny API for creating and manipulating GUI elements > departs from earlier traditions. Do you like the new look and feel of the > API? What do you think could be improved? Try it out and send us your > feedback! > > > Best regards, > Heinrich Apfelmus > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

3 2

Choosing an xml parser
by L Corbijn 21 Jul '13

21 Jul '13

Hello Cafe, I am trying to write a library to parse (and process) the OpenGL xml spec into haskell values. The problem is that I don't know what xml library to choose. So far I can think of the following requirements: - Some error reporting, possibly warning for unparsed elements (as that signals that the parser needs updating). - Not too bulky for the not so simple registry schema [1]. - Preferably some way to preprocess some nodes (this could probably be done by any xml library). - Preferably not too memory hungry (the current spec is about 2MB) Does somebody have a recommendation for a xml library to use for this? Regards, Lars [1]: https://cvs.khronos.org/svn/repos/ogl/trunk/doc/registry/public/api/registr…

2 1

Dynamic and equality
by Jose A. Lopes 21 Jul '13

21 Jul '13

Hello, How to define equality for Data.Dynamic ? Cheers, Jose

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ANN: data-fin
by wren ng thornton 21 Jul '13

21 Jul '13

-------------------------------------------- -- data-fin 0.1.0 -------------------------------------------- The data-fin package offers the family of totally ordered finite sets, implemented as newtypes of Integer, etc. Thus, you get all the joys of: data Nat = Zero | Succ !Nat data Fin :: Nat -> * where FZero :: (n::Nat) -> Fin (Succ n) FSucc :: (n::Nat) -> Fin n -> Fun (Succ n) But with the efficiency of native types instead of unary encodings. -------------------------------------------- -- Notes -------------------------------------------- I wrote this package for a linear algebra system I've been working on, but it should also be useful for folks working on Agda, Idris, etc, who want something more efficient to compile down to in Haskell. The package is still highly experimental, and I welcome any and all feedback. Note that we implement type-level numbers using [1] and [2], which works fairly well, but not as nicely as true dependent types since we can't express certain typeclass entailments. Once the constraint solver for type-level natural numbers becomes available, we'll switch over to using that. [1] Oleg Kiselyov and Chung-chieh Shan. (2007) Lightweight static resources: Sexy types for embedded and systems programming. Proc. Trends in Functional Programming. New York, 2--4 April 2007. http://okmij.org/ftp/Haskell/types.html#binary-arithm [2] Oleg Kiselyov and Chung-chieh Shan. (2004) Implicit configurations: or, type classes reflect the values of types. Proc. ACM SIGPLAN 2004 workshop on Haskell. Snowbird, Utah, USA, 22 September 2004. pp.33--44. http://okmij.org/ftp/Haskell/types.html#Prepose -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/data-fin Darcs: http://community.haskell.org/~wren/data-fin Haddock (Darcs version): http://community.haskell.org/~wren/data-fin/dist/doc/html/data-fin -- Live well, ~wren

1 0

ANN: darcsden 1.1 released, darcs hub news 2013/07
by Simon Michael 21 Jul '13

21 Jul '13

Hi all, here is a combined darcsden and darcs hub update. darcsden 1.1 released ===================== darcsden 1.1 is now available on hackage! This is the updated version of darcsden which runs hub.darcs.net, so these changes are also relevant to that site's users. (More darcs hub news below.) darcsden is a web application for browsing and managing darcs repositories, issues, and users, plus a basic SSH server which lets users push changes without a system login. It is released under the BSD license. You can use it: - to browse and manage your local darcs repos with a more comfortable UI - to make your repos browsable online, optionally with issue tracking - to run a multi-user darcs hosting site, like hub.darcs.net http://hackage.haskell.org/package/darcsden - cabal package \ http://hub.darcs.net/simon/darcsden - source \ http://hub.darcs.net/simon/darcsden/issues - bug tracker Release notes for 1.1 --------------------- Fixed: * 16: Layout of links and navigation places them offscreen * 21: anchors on line numbers exist but line numbers not clickable * 28: forking then deleting a private repo makes repos unviewable * 29: darcs get to an invalid ssh repo url hangs * 46: if user kills a push, the lock file is not removed, preventing subsequent pushes New: * the signup page security question is case-insensitive ("darcs") * login redirects to the "my repos" page * a more responsive layout, with content first, buttons at top/right * many other UI updates; font, headings, borders, whitespace, robustness * more context sensitivity in buttons & links * better next/previous page controls * better support for microsoft windows, runs as a service * builds with GHC 7.6 and latest libraries * easier developer builds Brand new, from the Enhancing Darcsden GSOC (some WIP): * you can sign up, log in, and link existing accounts with your Google or Github id * you can reset your password * you can edit files through the web * you can "pack" your repositories, allowing faster darcs get Detailed change log: http://hub.darcs.net/simon/darcsden/CHANGES.md How to help ----------- darcsden is a small, clean codebase that is fun to hack on. Discussion takes place on the #darcs IRC channel, and useful changes will quickly be deployed at hub.darcs.net, providing a tight dogfooding/feedback loop. Here's how to contribute a patch there: 1. register at hub.darcs.net 2. add your ssh key in settings so you can push 3. fork your own branch: http://hub.darcs.net/simon/darcsden , fork 4. copy to your machine: darcs get http://hub.darcs.net/yourname/darcsden 5. make changes, darcs record 6. push to hub: darcs push yourname@hub.darcs.net:darcsden --set-default 7. your change will appear at http://hub.darcs.net/simon/darcsden/patches 8. discuss on #darcs, or ping me (sm, simon(a)joyful.com) to merge it Credits ------- Alex Suraci created darcsden. Simon Michael led this release, which includes contributions from Alp Mestanogullari, Jeffrey Chu, Ganesh Sittampalam, and BSRK Aditya (sponsored by Google's Summer of Code). And last time I forgot to mention two 1.0 contributors: Bertram Felgenhauer and Alex Suraci. darcsden depends on Darcs, Snap, GHC, and other fine projects from the Haskell ecosystem, as well as Twitter Bootstrap, JQuery, and many more. darcs hub news 2013/07 ====================== http://hub.darcs.net , aka darcs hub, is the darcs repository hosting site I operate. It's like a mini github, but using darcs. You can: - browse users, repos, files and changes - publish darcs repos publicly or privately - get, push and pull repos over ssh - grant push access to other members - fork repos, then view and merge upstream and downstream changes - track issues The site was announced on 2012/9/15 (http://thread.gmane.org/gmane.comp.version-control.darcs.user/26556) Since then: - The site has been deploying new darcsden work promptly; it includes all the 1.1 release improvements described above. - The server's ram has doubled from 1G to 2G (thanks Linode). This means app restarts due to excessive memory use are less frequent. - The front page's user list had become slow and has been optimised, halving the page load time. - BSRK Aditya is doing his Google Summer of Code project on enhancing darcsden and darcs hub (mentored by darcs developer Ganesh Sittampalam). Find out more at http://darcs.net/GSoC/2013-Darcsden . - The site is being used, with many small projects and a few well-known larger ones. Quick stats as of 2013/07/19: user accounts 317 repos 579 disk usage 2.5G uptime last 30 days 99.48% average response time last 30 days 1.6s - The site remains free to use, including private repos. Eventually, some kind of funding will be needed to keep it self-sustaining, and could also enable faster development. Donate button ? Gittip ? Charge for private repos ? Let's discuss. Please try it out, report problems, and contribute patches to make it better. Best, -Simon

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HaskellWiki images disappeared
by Henk-Jan van Tuyl 21 Jul '13

21 Jul '13

L.S., It looks like the HaskellWiki images have disappeared; can anybody repair this? (See for example http://www.haskell.org/haskellwiki/Special:MostLinkedFiles ) Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

2 1

List Monads and non-determinism
by Matt Ford 21 Jul '13

21 Jul '13

Hi All, I thought I'd have a go at destructing [1,2] >>= \n -> [3,4] >>= \m -> return (n,m) which results in [(1,3)(1,4),(2,3),(2,4)] I started by putting brackets in ([1,2] >>= \n -> [3,4]) >>= \m -> return (n,m) This immediately fails when evaluated: I expect it's something to do with the n value now not being seen by the final return. It seems to me that the return function is doing something more than it's definition (return x = [x]). If ignore the error introduced by the brackets I have and continue to simplify I get. [3,4,3,4] >>= \m -> return (n,m) Now this obviously won't work as there is no 'n' value. So what's happening here? Return seems to be doing way more work than lifting the result to a list, how does Haskell know to do this? Why's it not in the function definition? Are lists somehow a special case? Any pointers appreciated. Cheers, -- Matt

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Default installation path of haskell platform in windows
by Ivan Perez 20 Jul '13

20 Jul '13

Hi café, I just spent the whole day fighting ghc and cabal to get my program compiled in windows (and I haven't succeeded yet :) ) realgcc is not very happy about paths with spaces in them (I think there's an open bug about the way ghc calls realgcc, which manifests if you use extra-lib-dirs in the cabal file). I'd like to ask you: Might it be wise to change the setup file of the Haskell Platform to default to an installation path with no spaces in it, and to tell users why this is a good idea (possibly during the installation process)? Other projects are doing the same, including msys. Please be aware that I know that this "solution" is far from ideal. Ideally, it should just work (with spaces and without them). This is purely a practical decision, a workaround, aiming at making haskell more accessible to windows users, and making it easier for us to develop software that works in all platforms. Regards Ivan

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Re: [Haskell-cafe] Generating Haskell Code out of Haskell AST (GHC API)
by John Blackbox 19 Jul '13

19 Jul '13

I accidentally didn't send that email to haskell-cafe, so I'm pasting it here also: Alan - I do NOT want to generate Haskell code. I want only to generate AST and compile it. The question about generating the code was only to have a "debugging tool" - to see if the generated AST is good - I wanted to generate the Haskell code only to check if its correct, but normally I would not do it, because it makes no sense to generate AST -> code -> AST (by GHC) again etc :) Additional - I want to connect to GHC's type-checking also and translate the errors to be appropriate to my language syntax - so maybe the pure GHC API is the best way to go? 2013/7/19 John Blackbox <blackbox.dev.ml(a)gmail.com> > Additional - I want to connect to GHC's type-checking also and translate > the errors to be appropriate to my language syntax. > > > 2013/7/19 John Blackbox <blackbox.dev.ml(a)gmail.com> > >> Alan - I do NOT want to generate Haskell code. I want only to generate >> AST and compile it. >> The question about generating the code was only to have a "debugging >> tool" - to see if the generated AST is good - I wanted to generate the >> Haskell code only to check if its correct, but normally I would not do it, >> because it makes no sense to generate AST -> code -> AST (by GHC) again etc >> :) >> >> >> 2013/7/19 AlanKim Zimmerman <alan.zimm(a)gmail.com> >> >>> I have not used haskell-src-exts so I may be talking out of turn, but it >>> seems that if you want to generate an AST which you then turn into source >>> code and compile it makes more sense than using than GHC AST, which has a >>> number of wrinkles, including fields that are only valid at certain phases >>> of the compilation process. >>> >>> For my purposes, in the Haskell Refactorer, I need access to the >>> renaming and type-checking, which to my knowledge is not currently >>> available in haskell-src-exts, although there is work happening to bring it >>> in, e.g. https://github.com/haskell-suite/haskell-names. >>> >>> >>> On Fri, Jul 19, 2013 at 10:09 AM, John Blackbox < >>> blackbox.dev.ml(a)gmail.com> wrote: >>> >>>> Thank you! >>>> So, if I'm writing a compiler of custom language, which I want to >>>> generate Haskell AST and further compile it with GHC, you prefer >>>> something like haskell-src-exts over pure GHC API? >>>> >>>> >>>> 2013/7/19 Antoine Latter <aslatter(a)gmail.com> >>>> >>>>> The package haskell-src-exts is a lot less intimidating if all you are >>>>> trying to do is programmatically generate Haskell source: >>>>> >>>>> http://hackage.haskell.org/package/haskell-src-exts >>>>> >>>>> The base types are here: >>>>> >>>>> http://hackage.haskell.org/packages/archive/haskell-src-exts/1.13.5/doc/htm… >>>>> >>>>> This module has some helper function for generating parts of the AST: >>>>> >>>>> http://hackage.haskell.org/packages/archive/haskell-src-exts/1.13.5/doc/htm… >>>>> >>>>> >>>>> On Thu, Jul 18, 2013 at 1:11 PM, John Blackbox >>>>> <blackbox.dev.ml(a)gmail.com> wrote: >>>>> > Hi! >>>>> > I dont know GHC API very well, but I want to generate AST of a >>>>> program using >>>>> > GHC API. >>>>> > Is there any standard method to generate Haskell code out of it? >>>>> (something >>>>> > like "print_this_for_me_please" function? :D >>>>> > >>>>> > _______________________________________________ >>>>> > Haskell-Cafe mailing list >>>>> > Haskell-Cafe(a)haskell.org >>>>> > http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>> > >>>>> >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> >> >

3 2

Generating Haskell Code out of Haskell AST (GHC API)
by John Blackbox 19 Jul '13

19 Jul '13

Hi! I dont know GHC API very well, but I want to generate AST of a program using GHC API. Is there any standard method to generate Haskell code out of it? (something like "print_this_for_me_please" function? :D

2 2

Monad Transformer Space Leak
by Clark Gaebel 19 Jul '13

19 Jul '13

Hi everyone! For some reason, this leaks thunks: module Main where import Control.Monad import Control.Monad.MC -- from monte-carlo import Control.Monad.ST.Strict go :: Int -> MCT (ST s) () go k = replicateM_ k (return ()) main = print $ runST $ evalMCT (go 100000000) rng where rng = mt19937 0 while this does not: module Main where import Control.Monad import Control.Monad.MC go :: Int -> MC () go k = replicateM_ k (return ()) main = print $ evalMC (go 100000000) rng where rng = mt19937 0 Can anyone help me figure out what's going on here? Thanks, - Clark

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ghc and jump table generation
by Erik Rantapaa 19 Jul '13

19 Jul '13

I've been looking at the assembly code that ghc generates for simple pattern matching for situations like: foo :: Int -> Int foo 1 = 3 foo 2 = 10 foo 3 = 2 foo 4 = 42 -- ... other random assignments and I noticed that it doesn't seem to generate a jump table (or even a table lookup) like, for instance, gcc would for this switch statement: int foo(int x) { switch (c) { case 1: return 3; case 2: return 10; case 3: return 2; case 4: return 42; // ... etc. } } Under -O3 ghc seems to produce a binary-search of the cases. Questions: Would generating a jump/lookup table for pattern matches like this (i.e. with a suitable density of cases) be beneficial? And, if so, would it be difficult to add to ghc? And would it be a good first project for someone who wants to get into the ghc code base (perhaps with some mentoring from the community?)

2 1

Automatic error traces
by Alberto G. Corona 19 Jul '13

19 Jul '13

It is important to have execution traces in case of error. specially in server applications that run 24/7 such are web applications Thanks to the wonderful package monadloc by Pepe Iborra, now MFlow can generate a complete execution trace in case of error. The control-monad-exception uses monadLoc to generate stack traces, but MFlow makes use of his backtracking mechanism to generate a complete execution trace. Here I explain what and how: http://haskell-web.blogspot.com.es/2013/07/automatic-error-trace-generation… The MFlow version that implements this is in gitHub. Not in hackage yet. https://github.com/agocorona/MFlow I´m quite proud of it since it is one of the things closest to magic that I have done. Feedback? I do not want to keep MFlow as a single person development. I think that it has many unique and nice features not available in other languages and frameworks, and it can be raised to a serious exploitation level by the Haskell community. This will attract people working in Industrial Web development to Haskell thanks to the edge in expressiveness and safety necessary for creating industrial solutions that Haskell has over other languages. It uses most other Haskell web developments blaze, wai, hamlet etc and there are many other things to come. monadLoc : http://hackage.haskell.org/package/monadloc -- Alberto.

2 3

Not working examples in GHC API documentation
by John Blackbox 18 Jul '13

18 Jul '13

Hi! Please take a look here: http://www.haskell.org/haskellwiki/GHC/As_a_library The examples are not working. Even the simpelst one: import GHC import GHC.Paths ( libdir ) import DynFlags ( defaultLogAction ) main = defaultErrorHandler defaultLogAction $ do runGhc (Just libdir) $ do dflags <- getSessionDynFlags setSessionDynFlags dflags target <- guessTarget "test_main.hs" Nothing setTargets [target] load LoadAllTargets throws: $> ghc -package ghc Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Main.hs:6:25: Couldn't match type `DynFlags' with `[Char]' Expected type: DynFlags.FatalMessager Actual type: DynFlags.LogAction In the first argument of `defaultErrorHandler', namely `defaultLogAction' In the expression: defaultErrorHandler defaultLogAction In the expression: defaultErrorHandler defaultLogAction $ do { runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; .... } } Main.hs:7:7: Couldn't match expected type `DynFlags.FlushOut' with actual type `IO SuccessFlag' In a stmt of a 'do' block: runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; target <- guessTarget "test_main.hs" Nothing; setTargets [target]; .... } In the second argument of `($)', namely `do { runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; .... } }' In the expression: defaultErrorHandler defaultLogAction $ do { runGhc (Just libdir) $ do { dflags <- getSessionDynFlags; setSessionDynFlags dflags; .... } }

2 2

Haskell Programmers for Distributed Systems
by Amanda Schwoegler 18 Jul '13

18 Jul '13

Parallel Scientific, Inc. is a Boulder, CO based, funded startup company with a presence around the globe, working in the cutting edge area of scalable parallelization. We seek experienced Haskell programmers to build a sophisticated, ultra-scalable high availability framework to be used in conjunction with scientific applications, big data applications and I/O systems. Successful candidates will work in a modern, virtual environment, with a distributed international team. All candidates are welcome to apply no matter their location. However, for this particular position, preference may be given to someone residing in or willing to relocate to the UK. Responsibilities include design, implementation in Haskell and in C, testing, infrastructure development, and optionally formal verification. We provide training in advanced processes for software design and implementation, as well as domain specific knowledge. Required skills: - Very strong background in computer science or mathematics. - Experience with Haskell, or network and cluster programming in other functional languages (e.g. Erlang, OCaml, Clojure). - Knowledge of systems programming and operating systems. Experience in one or more of the following areas is desirable: - Experience in distributed and/or parallel programming. - Knowledge of sophisticated distributed algorithms (fault tolerance, consensus, distributed transactions, etc). - Experience with delivering quality products on time and within budget. To apply, please send a resume to jobs(a)parsci.com. About Parallel Scientific Parallel Scientific was founded by Dr Peter Braam in 2010. Peter formerly taught mathematics and computer science at Oxford and Carnegie Mellon. He then contributed file systems to Linux and invented Lustre (which provides storage to 9 of the top 10 systems in the world). He ran several successful startups, and Parallel Scientific is run by a very experienced management team and board. Our expertise is in the areas of distributed scientific parallel computing, compilers, domain specific languages, scalable computing for cloud and HPC, and the synthesis of hardware appliances.

1 0

what is wrong w my IORef Word32 ?
by Joerg Fritsch 18 Jul '13

18 Jul '13

All, what is wrong w the below code? I get an type error related to the operation shiftL import Data.Bits import Data.Word import Data.IORef data Word32s = Word32s { x :: IORef Word32 } bitfield :: Word32 bitfield = 0 mkbitfield :: Word32 -> IO Word32s mkbitfield i = do the_bf <- newIORef i return (Word32s the_bf) sLbitfield :: Integer -> Word32s -> IO () sLbitfield i (Word32s bf) = do modifyIORef bf (shiftL i) main::IO() main = do oper_bf <- mkbitfield bitfield sLbitfield 2 oper_bf bf_003.hs:15:48: Couldn't match type `Int' with `Word32' Expected type: Word32 -> Word32 Actual type: Int -> Word32 In the return type of a call of `shiftL' In the second argument of `modifyIORef', namely `(shiftL i)' In a stmt of a 'do' block: modifyIORef bf (shiftL i) bf_003.hs:15:55: Couldn't match expected type `Word32' with actual type `Integer' In the first argument of `shiftL', namely `i' In the second argument of `modifyIORef', namely `(shiftL i)' In a stmt of a 'do' block: modifyIORef bf (shiftL i) Thanks, --Joerg

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Re: [Haskell-cafe] Optimization flag changing result of code execution
by Azeem -ul-Hasan 18 Jul '13

18 Jul '13

I am using GHC 7.6.1 mwc-random 0.12.0.1 vector 0.9.1 primitive 0.4.1 Azeem On 16.03.2013 13:31, Azeem -ul-Hasan wrote: > Nope that isn't the case either. Even if I make use of defaultSeed > through create the problem still remains. The problem seems to be in the > generation of a vector of (a,a) i.e in the part > > V.generateM ((round $ p*(fromIntegral $ l*z)) `div` 2) (\i-> R.uniformR > ((0,0) , (l-1,l-1)) gen) > > in line 16. Thanks again. > I've tried to run you program and I've got approximately same results regardless of optimization level. Which versions of GHC, mwc-random, vector and primitive do you use?

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Haskell Weekly News: Issue 273
by Daniel Santa Cruz 18 Jul '13

18 Jul '13

Welcome to issue 273 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 30 to July 13, 2013. Quotes of the Week * monochrom: 8-bit word uses less memory, but if it doesn't have to preserve information, I know how to use 0 bits of memory. * byorgey: you know, servers migrate in the summer to their ancestral breeding grounds * emeijer: It must me said on the other hand compiler writers seem to ignore the existence of formal semantics completely, probably because they don't like greek letters. * byorgey: now we have the pig operator <^(++)^> as well as the robot monkey operator (:[]) * chrisdone: i remember when the internet was black and white and you had to upload web pages back onto the internet when you were done with them * raganwald: Immuteability: The property of functional programmers that prevents them from shutting up about pure functional programming. Top Reddit Stories * On taking the last n elements of a list Domain: joachim-breitner.de, Score: 69, Comments: 37 On Reddit: [1] http://goo.gl/E9hRX Original: [2] http://goo.gl/9Uf74 * Comonads are NOT objects Domain: gelisam.blogspot.ca, Score: 62, Comments: 50 On Reddit: [3] http://goo.gl/tgdhh Original: [4] http://goo.gl/i3EDv * ANNOUNCE: Haste, the Haskell to JS compiler, is now on Hackage Domain: hackage.haskell.org, Score: 60, Comments: 27 On Reddit: [5] http://goo.gl/GTpj5 Original: [6] http://goo.gl/Nzpe7 * Postmodern Haskell and OpenGL: Introducing vinyl-gl Domain: arcadianvisions.com, Score: 57, Comments: 6 On Reddit: [7] http://goo.gl/9mVbl Original: [8] http://goo.gl/Zh5jE * ANNOUNCE: haskell-mode 13.07 (& Survey) Domain: projects.haskell.org, Score: 49, Comments: 18 On Reddit: [9] http://goo.gl/gYU24 Original: [10] http://goo.gl/YSmk4 * Why the world needs Haskell Domain: devalot.com, Score: 46, Comments: 22 On Reddit: [11] http://goo.gl/ekD96 Original: [12] http://goo.gl/LBxRP * Cartesian Closed Comic #20: Weird language Domain: ro-che.info, Score: 42, Comments: 10 On Reddit: [13] http://goo.gl/In50H Original: [14] http://goo.gl/tblYa * Do naming practices hurt Haskell? Domain: self.haskell, Score: 38, Comments: 222 On Reddit: [15] http://goo.gl/mnwO9 Original: [16] http://goo.gl/mnwO9 * ANNOUNCE: [17] http://goo.gl/pp0Vf Domain: self.haskell, Score: 37, Comments: 18 On Reddit: [18] http://goo.gl/MRDO7 Original: [19] http://goo.gl/MRDO7 * Skills Matter : In The Brain of Edsko de Vries: Lazy I/O and Alternatives in Haskell Domain: skillsmatter.com, Score: 37, Comments: 14 On Reddit: [20] http://goo.gl/13zlQ Original: [21] http://goo.gl/QSyTu * Attoparsec tutorial - School of Haskell Domain: fpcomplete.com, Score: 35, Comments: 17 On Reddit: [22] http://goo.gl/Riydl Original: [23] http://goo.gl/P2oiy * FP Haskell Center Beta Released and Beta Accounts Activated Domain: fpcomplete.com, Score: 33, Comments: 7 On Reddit: [24] http://goo.gl/Zs3kU Original: [25] http://goo.gl/vgy5m Top StackOverflow Questions * What advantage does Monad give us over an Applicative? votes: 17, answers: 7 Read on SO: [26] http://goo.gl/4L0gj * Encoding “Less Than” with Haskell votes: 11, answers: 1 Read on SO: [27] http://goo.gl/1Qsvn * Why is factorial calculation much faster in Haskell than in Java votes: 11, answers: 5 Read on SO: [28] http://goo.gl/QTbrl * Haskell: Can typeclasses define types (ala type traits) votes: 10, answers: 2 Read on SO: [29] http://goo.gl/yLSqT * C++ and Haskell codes differs in execution time on different machines votes: 10, answers: 1 Read on SO: [30] http://goo.gl/LRs3k * GHC.Generics or Data.Data? votes: 10, answers: 1 Read on SO: [31] http://goo.gl/yU6Ri * Constraint subset higher-order constraint votes: 9, answers: 1 Read on SO: [32] http://goo.gl/MTTws * How to make catamorphisms work with parameterized/indexed types? votes: 9, answers: 2 Read on SO: [33] http://goo.gl/FNSZg * How to “change” single values in large objects in elegant way? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/3KuJj * What programming languages have something like Haskell’s `newtype` votes: 9, answers: 4 Read on SO: [35] http://goo.gl/vzUAI * Implementing a zipper for length-indexed lists votes: 8, answers: 1 Read on SO: [36] http://goo.gl/ja6VS * Does Haskell concatenate String literals at compile time? votes: 8, answers: 4 Read on SO: [37] http://goo.gl/sr61q * Combining lenses votes: 8, answers: 2 Read on SO: [38] http://goo.gl/qEbPd Until next time, [38]+Daniel Santa Cruz References 1. http://www.joachim-breitner.de/blog/archives/600-On-taking-the-last-n-eleme… 2. http://www.reddit.com/r/haskell/comments/1i64mt/on_taking_the_last_n_elemen… 3. http://gelisam.blogspot.ca/2013/07/comonads-are-neighbourhoods-not-objects.… 4. http://www.reddit.com/r/haskell/comments/1hficb/comonads_are_not_objects/ 5. http://hackage.haskell.org/package/haste-compiler 6. http://www.reddit.com/r/haskell/comments/1htqi2/announce_haste_the_haskell_… 7. http://www.arcadianvisions.com/blog/?p=388 8. http://www.reddit.com/r/haskell/comments/1hhrfn/postmodern_haskell_and_open… 9. http://projects.haskell.org/pipermail/haskellmode-emacs/2013-June/000188.ht… 10. http://www.reddit.com/r/haskell/comments/1hd21v/announce_haskellmode_1307_s… 11. http://www.devalot.com/articles/2013/07/why-haskell.html 12. http://www.reddit.com/r/haskell/comments/1hzz2d/why_the_world_needs_haskell/ 13. http://ro-che.info/ccc/20.html 14. http://www.reddit.com/r/haskell/comments/1i8e6v/cartesian_closed_comic_20_w… 15. http://www.reddit.com/r/haskell/comments/1i0uiq/do_naming_practices_hurt_ha… 16. http://www.reddit.com/r/haskell/comments/1i0uiq/do_naming_practices_hurt_ha… 17. http://www.reddit.com/r/haskell/comments/1i2a9l/announce_httphackagehaskell… 18. http://www.reddit.com/r/haskell/comments/1i2a9l/announce_httphackagehaskell… 19. http://skillsmatter.com/podcast/home/lazy-io-and-alternatives-in-haskell/jd… 20. http://www.reddit.com/r/haskell/comments/1i438a/skills_matter_in_the_brain_… 21. https://www.fpcomplete.com/school/attoparsec 22. http://www.reddit.com/r/haskell/comments/1hkg3g/attoparsec_tutorial_school_… 23. https://www.fpcomplete.com/blog/2013/06/fp-haskell-center-beta-announcement 24. http://www.reddit.com/r/haskell/comments/1hfbgs/fp_haskell_center_beta_rele… 25. http://stackoverflow.com/questions/17409260/what-advantage-does-monad-give-… 26. http://stackoverflow.com/questions/17578728/encoding-less-than-with-haskell 27. http://stackoverflow.com/questions/17584630/why-is-factorial-calculation-mu… 28. http://stackoverflow.com/questions/17398784/haskell-can-typeclasses-define-… 29. http://stackoverflow.com/questions/17412348/c-and-haskell-codes-differs-in-… 30. http://stackoverflow.com/questions/17502603/ghc-generics-or-data-data 31. http://stackoverflow.com/questions/17502985/constraint-subset-higher-order-… 32. http://stackoverflow.com/questions/17503131/how-to-make-catamorphisms-work-… 33. http://stackoverflow.com/questions/17518749/how-to-change-single-values-in-… 34. http://stackoverflow.com/questions/17591276/what-programming-languages-have… 35. http://stackoverflow.com/questions/17501777/implementing-a-zipper-for-lengt… 36. http://stackoverflow.com/questions/17513273/does-haskell-concatenate-string… 37. http://stackoverflow.com/questions/17528119/combining-lenses 38. https://plus.google.com/105107667630152149014/about

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ANNOUNCE: Aivika v0.6, Aivika Experiment v0.3, Aivika Experiment Chart v0.3
by David Sorokin 17 Jul '13

17 Jul '13

Hi! I’m glad to announce new versions of the Aivika [1] simulation library and its additional packages Aivika Experiment [2] and Aivika Experiment Chart [3]. The library is divided to decrease the dependency on GTK, although all three were tested on Linux, Windows and OS X. They allow developing the wide range of simulation models (SD, DES, AM), simulating them and saving the results in form of charts, tables and so on, automating many routine procedures. The architecture is quite flexible. This is a result of my 4-years work since 2009. I finally find the library a quite mature. Among many languages I tried, Haskell was the most suitable. Many simulation concepts can be naturally expressed in terms of Haskell. Many technical details are hidden either under the façade of monads, or similar structures. So, the ordinary differential equations can be elegantly defined using the recursive do-notation. But the discontinuous processes are well expressed using the continuations, if more precisely, three continuations in one computation. I prepared a 55-page PDF document An Introduction to Aivika Simulation Library [4]. For demonstration, it contains some charts prepared with help of Aivika Experiment Chart. There are the deviation charts and histograms. Thanks, David [1] http://hackage.haskell.org/package/aivika [2] http://hackage.haskell.org/package/aivika-experiment [3] http://hackage.haskell.org/package/aivika-experiment-chart [4] https://github.com/dsorokin/aivika/blob/master/doc/aivika.pdf

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Quoting with template haskell
by Jose A. Lopes 17 Jul '13

17 Jul '13

Hi everyone, I am quite new to template haskell and I am still trying to get the hang of it. How can I achieve something like the following Common Lisp code ? `(,fn ,arg1 ,arg2) Or is there a more Haskelley way of doing this ? Cheers, Jose

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Call for Papers PEPM 2014
by publicityifl＠gmail.com 17 Jul '13

17 Jul '13

Hello, Please, find below the first call for papers for PEPM 2014. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Co-chair of PEPM 2014 ----------------------------- C A L L F O R P A P E R S ----------------------------- ======= PEPM 2014 =========== ACM SIGPLAN 2014 WORKSHOP ON PARTIAL EVALUATION AND PROGRAM MANIPULATION Mon-Tue, January 20-21, 2014 San Diego, California, USA co-located with POPL'14 Sponsored by ACM SIGPLAN http://www.program-transformation.org/PEPM14 SCOPE The PEPM Symposium/Workshop series aims at bringing together researchers and practitioners working in the areas of program manipulation, partial evaluation, and program generation. PEPM focuses on techniques, theory, tools, and applications of analysis and manipulation of programs. The 2014 PEPM workshop will be based on a broad interpretation of semantics-based program manipulation and continue last years' successful effort to expand the scope of PEPM significantly beyond the traditionally covered areas of partial evaluation and specialization and include practical applications of program transformations such as refactoring tools, and practical implementation techniques such as rule-based transformation systems. In addition, the scope of PEPM covers manipulation and transformations of program and system representations such as structural and semantic models that occur in the context of model-driven development. In order to reach out to practitioners, a separate category of tool demonstration papers will be solicited. Topics of interest for PEPM'14 include, but are not limited to: Program and model manipulation techniques such as: supercompilation, partial evaluation, fusion, on-the-fly program adaptation, active libraries, program inversion, slicing, symbolic execution, refactoring, decompilation, and obfuscation. Program analysis techniques that are used to drive program/model manipulation such as: abstract interpretation, termination checking, binding-time analysis, constraint solving, type systems, automated testing and test case generation. Techniques that treat programs/models as data objects including metaprogramming, generative programming, embedded domain-specific languages, program synthesis by sketching and inductive programming, staged computation, and model-driven program generation and transformation. Application of the above techniques including case studies of program manipulation in real-world (industrial, open-source) projects and software development processes, descriptions of robust tools capable of effectively handling realistic applications, benchmarking. Examples of application domains include legacy program understanding and transformation, DSL implementations, visual languages and end-user programming, scientific computing, middleware frameworks and infrastructure needed for distributed and web-based applications, resource-limited computation, and security. To maintain the dynamic and interactive nature of PEPM, we will continue the category of `short papers' for tool demonstrations and for presentations of exciting if not fully polished research, and of interesting academic, industrial and open-source applications that are new or unfamiliar. Student attendants with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses and other support. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC programme, see its web page. All accepted papers, short papers included, will appear in formal proceedings published by ACM Press. In addition to printed proceedings, accepted papers will be included in the ACM Digital Library. A special issue for Science of Computer Programming is planned with recommended papers from PEPM'14. PEPM has also established a Best Paper award. The winner will be announced at the workshop. SUBMISSION CATEGORIES AND GUIDELINES Regular Research Papers must not exceed 12 pages in ACM Proceedings style (including appendix). Tool demonstration papers and short papers must not exceed 6 pages in ACM Proceedings style (including appendix). At least one author of each accepted contribution must attend the workshop and present the work. In the case of tool demonstration papers, a live demonstration of the described tool is expected. Suggested topics, evaluation criteria, and writing guidelines for both research tool demonstration papers will be made available on the PEPM'14 Web-site. Papers should be submitted electronically via the workshop web site. Authors using LaTeX to prepare their submissions should use the new improved SIGPLAN proceedings style (sigplanconf.cls, 9pt template). IMPORTANT DATES Abstract due: Sat, September 28, 2013 Paper submission: Sat, October 5, 2013, 23:59, GMT Author notification: Mon, November 11, 2013 Camera-ready papers due: * to be announced * INVITED SPEAKERS to be announced PROGRAM CHAIRS Wei Ngan Chin (National University of Singapore, Singapore) Jurriaan Hage (Utrecht University, Netherlands) PROGRAM COMMITTEE velyne Contejean (LRI, CNRS, Universit Paris-Sud, France) Cristina David (University of Oxford, UK) Alain Frisch (LexiFi, France) Ronald Garcia (University of British Columbia, Canada) Zhenjiang Hu (National Institute of Informatics, Japan) Paul H J Kelly (Imperial College, UK) Oleg Kiselyov (Monterey, USA) Naoki Kobayashi (University of Tokyo, Japan) Jens Krinke (University College London, UK) Ryan Newton (University of Indiana, USA) Alberto Pardo (Universidad de la Repblica, Uruguay) Sungwoo Park (Pohang University of Science and Technology, South Korea) Tiark Rompf (Oracle Labs & EPFL, Switzerland) Sukyoung Ryu (KAIST, South Korea) Kostis Sagonas (Uppsala University, Sweden) Max Schaefer (Nanyang Technological University, Singapore) Harald S¿ndergaard (The University of Melbourne, Australia) Eijiro Sumii (Tohoku University, Japan) Eric Van Wyk (University of Minnesota, USA) Jeremy Yallop (University of Cambridge, UK)

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ANNOUNCE: module-management-0.9.3 - clean import lists, split and merge modules
by David Fox 17 Jul '13

17 Jul '13

I am pleased to announce the first release of module-management, a package for cleaning import lists, and splitting and merging modules. You can see a description at the top of the documentation for Language.Haskell.Modules (once it appears) here: http://hackage.haskell.org/package/module-management It is worth noting that the split and merge operations cause global changes to the module tree, all the modules that import the split or merged modules get their import lists adjusted. For this reason, before these operations can run the caller needs to specify the "moduVerse", the set of modules to scan for import changes. A number of bugs in GHC's -ddump-minimal-imports feature cropped up while I was writing this, and when the fixes for these issues make it into the compiler things will work a bit more smoothly, particularly if you are using NoImplicitPrelude or type families. http://hackage.haskell.org/trac/ghc/ticket/7963 http://hackage.haskell.org/trac/ghc/ticket/7969 http://hackage.haskell.org/trac/ghc/ticket/8000 http://hackage.haskell.org/trac/ghc/ticket/8011 There are workarounds for 7963 and 7969. Not surprisingly, template haskell can cause problems, but frequently it does not. CPP directives do not work. I hope you find this useful - all feedback is welcome! -david

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FP Complete Accepting Submissions for Haskell Competition
by Natalia Muska 17 Jul '13

17 Jul '13

We are excited to share we are accepting submissions for our *FP Haskell Competition<https://www.fpcomplete.com/business/competition/competition-overview> *. Your code and tutorials can earn you $1,000 in cash prizes each month. We're looking for sample Haskell projects that solve specific real-world engineering and business problems. There is no limit to how many projects you can submit or how many times you can win. - $1,000 cash prize for the winning entry each month - $500 each to the 3 runners-ups - There may be none at all if none meet the winning criteria The contest kicks off August 1, 2013. To participate, you'll need to sign up for our FP Haskell Center Beta<https://www.fpcomplete.com/business/haskell-center/overview>. We look forward to seeing your submissions and highlighting the talent of this great Haskell community. if you have any questions, please contact us at competition(a)fpcomplete.com. -- Natalia Muska Marketing Manager FP Complete, Inc. Natalia(a)fpcomplete.com 865-506-6513 skype: natalia.s.muska

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TH splicing and recompilation checking
by Johannes Waldmann 16 Jul '13

16 Jul '13

Hi. we are using template Haskell to splice in some code that is produced by reading and transforming the contents of another file. now, if this "other file" is touched (by editing), but not the main file, then ghc (and cabal) do not realize that the main file does need to be recompiled. is there a way to tell them about the dependency? (example main file: https://github.com/apunktbau/co4/blob/master/CO4/Test/Queens.hs) - J.W.

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Call for Papers IFL 2013
by publicityifl＠gmail.com 16 Jul '13

16 Jul '13

Hello, Please, find below the third call for papers for IFL 2013. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL CALL FOR PAPERS 25th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2013 RADBOUD UNIVERSITY NIJMEGEN, THE NETHERLANDS ACM In-Cooperation / ACM SIGPLAN AUGUST 28 - 30 2013 "Landgoed Holthurnsche Hof" http://ifl2013.cs.ru.nl We are proud to announce that the 25th edition of the IFL series returns to its roots at the Radboud University Nijmegen in the Netherlands. The symposium is held from 28th to 30th of August 2013. Scope ----- The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2013 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2013 will use a post-symposium review process to produce the formal proceedings which will be published in the ACM Digital Library. All participants of IFL 2013 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings do not count as publication for the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. Invited Speaker --------------- Lennart Augustsson, currently employed by the Standard Chartered Bank, well-known for his work on Haskell, parallel Haskell, Cayenne, and Bluespec, is the invited speaker of IFL 2013. He will be talking about practical applications of functional programming. Submission Details ------------------ Submission deadline draft papers: July 31 Notification of acceptance for presentation: August 2 Early registration deadline: August 7 Late registration deadline: August 14 Submission deadline for pre-symposium proceedings: August 21 25th IFL Symposium: August 28-30 Submission deadline for post-symposium proceedings: November 11 Notification of acceptance for post-symposium proceedings: December 18 Camera-ready version for post-symposium proceedings: February 3 2014 Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must adhere to the standard ACM two columns conference format. For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. A suitable document template for LaTeX can be found at: http://www.acm.org/sigs/sigplan/authorInformation.htm Papers are to be submitted via the conference's EasyChair submission page: https://www.easychair.org/conferences/?conf=ifl2013 Topics ------ IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2013, please contact the PC chair at rinus(a)cs.ru.nl. Topics of interest include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honoured article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee ------------------- - Thomas Arts, Quviq, Gothenburg, Sweden - Andrew Butterfield, Trinity College, Dublin, Ireland - Edwin Brady, University of St. Andrews, UK - Clemens Grelck, University of Amsterdam, Netherlands - Adam Granicz, IntelliFactory, Budapest, Hungary - Jeremy Gibbons, University of Oxford, UK - Fritz Henglein, University of Copenhagen, Denmark - Stephan Herhut, Intel Labs, Santa Clara, US - Ralf Hinze (co-chair), University of Oxford, UK - Zoltan Horvath, Eotvos Lorand University, Budapest, Hungary - Zhenjiang Hu, University of Tokyo, Japan - Mauro Jaskelioff, Universidad Nacional de Rosario, Argentina - Johan Jeuring, University of Utrecht, Netherlands - Rita Loogen, University of Marburg, Germany - Marco T. Morazan, Seton Hall University, New Jersey, US - Dominic Orchard, University of Cambridge, UK - Rinus Plasmeijer (chair), Radboud University Nijmegen, Netherlands - Tim Sheard, Portland State University, US - Sam Tobin-Hochstadt, Northeastern University / Indiana University, US - Peter Thiemann, University of Freiburg, Germany - Simon Thompson, University of Kent, UK Venue ----- The 25th IFL is organized by the Radboud University Nijmegen, Model Based Software Development Department at the Nijmegen Institute for Computing and Information Sciences. The event is held in the Landgoed Holthurnsche Hof, a rural estate in the woodlands surrounding Nijmegen. It can be reached quickly and easily by public transport.

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What does it mean to derive "equations of restricted from" in Haskell?
by Daryoush Mehrtash 16 Jul '13

16 Jul '13

In John Hughes's "The Design of Pretty printing library" paper, he says: "The implementations which we are trying to derive consist of equations of > a restricted form. We will derive implementations by proving their > constituent equations from the specification. By itself this is no > guarantee that the implemented functions satisfy the specification because > we might not have proved enough equation But if we also check that the > derived definitions are terminating and exhaustive then this property is > guaranteed" What does "restricted form" mean? What is the meaning and significance of "definitions are terminating and exhaustive"? -- Daryoush Weblog: http://onfp.blogspot.com/

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