- Haskell-Cafe - Haskell.org

Compiling Haskell targetting different OS/arch
by Thiago Negri 27 Aug '12

27 Aug '12

Is it possible to compile Haskell code targetting a OS/arch that differs from the one where the compiler (GHC) is running? Thanks.

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Cabal Test-suites + custom preprocessors
by Iain Nicol 26 Aug '12

26 Aug '12

Hi café, I am successfully using a custom preprocessor to build my main executable. In particular, I am using UUAGC to preprocess .ag files into .hs files. My Setup.hs file uses 'uuagcLibUserHook' from the package uuagc-cabal to do this, which in particular overrides Cabal's 'buildHook' for me. Now I am trying to add a test-suite (of type exitcode-stdio-1.0). I want the test suite to be able to import the modules which implement the main program. Unfortunately, building the test suite fails, because the .ag files don't seem to be built into .hs files. Does anybody know if Cabal actually supports using custom preprocessors for building a test-suite? If so, is 'buildHook' still the right hook to override? Thanks, -- Iain

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Munich Haskell Meeting
by Heinrich Hördegen 26 Aug '12

26 Aug '12

Dear all, once again, it's time for our monthly get-together. Functional programmers from Munich and other cities will meet on Wed, 29th of August, at 19h30 at Max-Emanuel-Brauerei in Munich. If you plan to join, please go to http://www.haskell-munich.de/dates and click the button! With hope to meet many of you there, Heinrich

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ANN: Monad.Reader Issue 20
by Edward Z. Yang 26 Aug '12

26 Aug '12

*It’s not dead, it’s resting!* I am pleased to announce that Issue 20 of the Monad Reader is now available. http://themonadreader.files.wordpress.com/2012/08/issue20.pdf Issue 20 consists of the following three articles: - "Enumeration of Tuples with Hyperplanes" by Tillmann Vogt - "Understanding Basic Haskell Error Messages" by Jan Stolarek - "The MapReduce type of a Monad" by Julian Porter Feel free to browse the source files. You can check out the entire repository using Git: git clone https://github.com/ezyang/tmr-issue20.git If you’d like to write something for Issue 21, please get in touch! Cheers, Edward

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formal semantics
by Ramana Kumar 25 Aug '12

25 Aug '12

Dear Haskell Cafe I'm looking for information on past and current attempts to write semantics for Haskell. Features I'm particularly interested in are: - formal - mechanised - maintainable - up to date Of course, if nothing like that exists then partial attempts towards it could still be useful. My ultimate aims include: 1. Make it viable to define Haskell formally (i.e. so mechanised semantics can take over the normative role of the Haskell reports). 2. Write a verified (or verify an existing) Haskell compiler (where verified means semantics preserving). Cheers, Ramana

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Dealing poker hands, redux
by Matthew 25 Aug '12

25 Aug '12

So I've finally got some code which shuffles a deck of cards and deals out an arbitrary number of hands. https://gist.github.com/19916435df2b116e0edc type DealerState = State [Card] [[Card]] deck :: [Card] deck = [ (s, r) | s <- suits, r <- ranks ] shuffleDeck :: Int -> RVar [Card] shuffleDeck n = shuffle $ concat $ replicate n deck deal :: Int -> ([[Card]] -> DealerState) deal n = \xs -> state $ \s -> (xs ++ [take n s], drop n s) -- |Deal a number of hands a number of cards each. dealHands :: Int -> Int -> ([[Card]] -> DealerState) dealHands hs cs = foldr1 (<=<) $ replicate hs (deal cs) First of all, I have no idea if this is any good. The way I end up calling dealHands and getting a "real" result is `runState (dealHands 3 7 []) deck`. And I see that I've got nested lambdas all in `deal`. But hey it took me forever to figure it out and it "works." I'm using `shuffle` from Data.Random.Extras, which results in an RVar, and thus the beginning of my perplexity. I'm happy to end up with RVar inside a State monad, but I'm not sure where to start. To be honest, I'm only barely understanding what I'm doing with the State monad as it is. :) Happy for any help whatsoever! - Matthew

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haskell cryptogaphic libraries
by marcmo 25 Aug '12

25 Aug '12

In my day job I currently need to deal with a lot of cryptographic processing. For the tests I need to perform various cryptographic functions in haskell. these include: * AES Encryption/Decryption (CBC-Mode) * RSA Public Key Encryption/Decryption * Certificate Handling I'm having some difficulties finding the libraries to use and using them correctly. Performance is *not* my primary concern...rather *correctness* and* ease of use*. Haskell has some great cryptographic libs but for me it seems hard to judge what to use. The ones that seem appropriate are: * The AES package (for symmetric encryption) * The RSA package (for public key cryptography) certificate handling is s.th. I haven't found out about * how to deal with Certificates (e.g. extract the public key from X.509 certificates) is there a recommended package (packages) that suite my purpose? thnx

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Combining State and List Monads
by Henry Laxen 25 Aug '12

25 Aug '12

Dear Cafe, It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following: countCalls = do a <- [1..2] b <- [1..2] modify (+1) return (a,b) where with some combination of ListT, StateT, List, State, or who knows what would result in: ([(1,1),(1,2),(2,1),(2,2)],4) assuming we initialize the state to 0 Is there any way to make this happen? Thanks in advance. Henry Laxen

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How to implement instance of MonadBaseControl IO
by yi huang 25 Aug '12

25 Aug '12

I have a `newtype Yun a = Yun { unYun :: ReaderT YunEnv (ResourceT IO) a }` , and i need to define an instance of `MonadBaseControl IO` for it. Newtype instance deriving don't work here. I guess the answer is simple, i just can't figure it out, hope anybody can lightening me. Best regards. Yihuang.

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ANNOUNCE: readable-0.1
by MightyByte 24 Aug '12

24 Aug '12

I'm happy to announce a new package called readable: http://hackage.haskell.org/package/readable This package provides simple reading of data types from ByteString and Text using MonadPlus to handle failure. It doesn't seem like there's a standard way to do simple reading from ByteString and Text, so hopefully this package will fill the gap. Suggestions/improvements are most welcome.

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Rigid skolem type variable escaping scope
by Matthew Steele 24 Aug '12

24 Aug '12

While working on a project I have come across a new-to-me corner case of the type system that I don't think I understand, and I am hoping that someone here can enlighten me. Here's a minimal setup. Let's say I have some existing code like this: {-# LANGUAGE Rank2Types #-} class FooClass a where ... foo :: (forall a. (FooClass a) => a -> Int) -> Bool foo fn = ... Now I want to make a type alias for these (a -> Int) functions, and give myself a new way to call foo. I could do this: type IntFn a = a -> Int bar :: (forall a. (FooClass a) => IntFn a) -> Bool bar fn = foo fn This compiles just fine, as I would expect. But now let's say I want to change it to make IntFn a newtype: newtype IntFn a = IntFn (a -> Int) bar :: (forall a. (FooClass a) => IntFn a) -> Bool bar (IntFn fn) = foo fn I had expected this to compile too. But instead, I get an error like this one (using GHC 7.4.1): Couldn't match type `a0' with `a' because type variable `a' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: FooClass a => a -> Int The following variables have types that mention a0 fn :: a0 -> Int (bound at <the line number that implements bar>) In the first argument of `foo', namely `fn' In the expression: foo fn In an equation for `bar': bar (IntFn fn) = foo fn I don't think I am grasping why adding the layer of newtype wrapping/unwrapping, without otherwise changing the types, introduces this problem. Seems to me like the newtype version would also be type-safe if GHC allowed it (am I wrong?), and I'm failing to understand why GHC can't allow it. Is there a simple explanation, or else some reading that someone could point me to? (-: Cheers, -Matt

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explicit annotations on kind polymorphism for data types
by dude 24 Aug '12

24 Aug '12

Hello All: I'm working through Giving Haskell a Promotion. Section 2.4 presents an explicitly annotated data type declaration similar to the following: data EqRefl (a::X)(b::X) where Refl :: forall X. forall (a::X). EqRefl a a Has this been implemented in GHC 7.4.2? 7.8.3 in the GHC User Guide leads me to believe it has not. -- dude

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Handle close on GC leading to wrong reported exit codes?
by Niklas Hambüchen 24 Aug '12

24 Aug '12

According to System.IO, handles are automatically closed once they are garbage-collected. This served me well for files so far, but for processes, it gives me a weird problem: If I don't prevent my stdErr handle from being garbage-collected, the exit code reported by getProcessExitCode is just wrong (for me, it's always ExitSuccess, no matter what the program does). I posted this into #haskell and some poeple could not reproduce it, so I wanted to ask the Cafe what happens if you run this and if you have an idea why it happens to me. Thank you Niklas The following code gives an example. (http://hpaste.org/73593) You can see the real exit code with "./test; echo $?". module Main where import Control.Concurrent (threadDelay) import System.Process import System.IO import System.Exit run :: FilePath -> [String] -> IO (Handle, Handle, Handle, ProcessHandle) run cmd args = do r@(i, o, e, p) <- runInteractiveProcess cmd args Nothing Nothing getProcessExitCode p >>= \me -> case me of Just (ExitFailure 127) -> error $ "command not found: " ++ show cmd _ -> do mapM_ (flip hSetBuffering LineBuffering) [i, o, e] return r main = do (i, o, e, p) <- run "./test" [] putStrLn "spawned" threadDelay 1100000 putStrLn "waited" print =<< getProcessExitCode p -- If you comment these away, the above always returns Just ExitSuccess -- print =<< hGetContents o -- print =<< hGetContents e {- test.c: #include <stdio.h> #include <unistd.h> int main(int argc, char const *argv[]) { sleep (1); printf ("some stdout\n"); fprintf (stderr, "some stderr\n"); fflush (stdout); fflush (stderr); return 2; } -}

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model theory for type classes
by Patrick Browne 24 Aug '12

24 Aug '12

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Bringing knowledge from Haskell to Java
by Thiago Negri 24 Aug '12

24 Aug '12

Hello everyone. I just posted about a fact that happens to everyone who codes in Java while learning Haskell. You end up trading knowledge from both sides. I'll appreciate if you could read and send me feedbacks: http://me-hunz.blogspot.com.br/2012/08/bringing-knowledge-from-haskell-to-j… Thanks, Thiago.

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Re: [Haskell-cafe] regex-pcre is not working with UTF-8
by José Romildo Malaquias 23 Aug '12

23 Aug '12

On Tue, Aug 21, 2012 at 05:50:44PM -0300, José Romildo Malaquias wrote: > On Tue, Aug 21, 2012 at 04:05:28PM +0100, Chris Kuklewicz wrote: > > I do not have time to test this myself right now. But I will unravel my code a > > bit for you. > > > > > By November 2011 it worked without problems in my application. Now that > > > I have resumed developping the application, I have been faced with this > > > behaviour. As it used to work before, I believe it is a bug in > > > regex-pcre or libpcre. > > > > I believe it may be problem in String <-> ByteString conversion. The "base" > > library may have changed and your LOCALE information may be different or may be > > being used differently by "base". > > > > > The (temporary) workaround I found is to convert the strings to > > > byte-strings before matching, and then convert the results back to > > > strings. With byte-strings it works well. > > > > That is an excellent sign that it is your LOCALE settings being picked up by > > GHC's "base" package, see explanation below. [...] > I have written an application to test those things. There are 2 source > files: test.hs and seestr.c, which are attached. > > The test does the following: > > 1. shows the getForeignEncoding > > 2. uses a C function to show the characters from a String (using > withCString) and from a ByteString (using useAsCString) > > 3. matches a PCRE regular expression using String and ByteString > > The test is run twice, with different LANG settings, and its output > follows. [...] > As can be seen, regular expression matching does not work with > en_US.UTF-8. But it works with en_US.ISO-8859-1. > > The test shows that withCString is working as expected too. This > may suggest the problem is really with regex-pcre. The previous tests were run on an gentoo linux with ghc-7.4.1. I have also run the tests on Fedora 17 with ghc-7.0.4, which does not have the bug. The sources are attached. The tests output follows: $ LANG=en_US.ISO-8859-1 && ./test testing with String code: 70, char: p code: 61, char: a code: ffffffed, char: code: 73, char: s result: 4 testing with ByteString code: 70, char: p code: 61, char: a code: ffffffed, char: code: 73, char: s result: 4 regex : pa�s:(.*) text : pa�s:Brasil String match : [["pa\237s:Brasil","Brasil"]] ByteString match : [["pa\237s:Brasil","Brasil"]] $ LANG=en_US.UTF-8 && ./test testing with String code: 70, char: p code: 61, char: a code: ffffffed, char: code: 73, char: s result: 4 testing with ByteString code: 70, char: p code: 61, char: a code: ffffffed, char: code: 73, char: s result: 4 regex : país:(.*) text : país:Brasil String match : [["pa\237s:Brasil","Brasil"]] ByteString match : [["pa\237s:Brasil","Brasil"]] Clearly witchCString has changed from ghc-7.0.4 to ghc-7.4.1. It seems that With ghc-7.0.4 withCString does not obey the UTF-8 locale and generates a latin1 C string. Regards, Romildo

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ANNOUNCE basic-prelude-0.3
by Dan Burton 23 Aug '12

23 Aug '12

We are pleased to announce a new release of basic-prelude. http://hackage.haskell.org/package/basic-prelude-0.3.0.0 We apparently weren't entirely clear about the original purpose of basic-prelude, and we got repeated feedback of the form "where are the list functions?" Somewhat in response to this, we decided to rename the BasicPrelude module to CorePrelude. This module is an *incomplete* prelude, and the goal of this module is to provide enhanced core Prelude functionality *without* making any controversial decisions. CorePrelude serves as the core of Michael's ClassyPrelude, my ModularPrelude, and now the new BasicPrelude module. (If you ever feel the need to create your own Prelude replacement, we encourage you to build it off of CorePrelude. If there is any reason that CorePrelude would not be well-suited to this purpose, then please let us know!) I believe that by using the same Core, it will be easier for these various Prelude replacements to play nice with each other. We created a new module, BasicPrelude (not to be confused with the old BasicPrelude), which *is* a fully-featured Prelude, emphasizing the use of Text and FilePath, while also providing most of the List functions that you know and love, with a few exceptions, as documented. BasicPrelude is among the simplest examples of how we intend CorePrelude to be used. The next time you are working on a hobby project, please try using BasicPrelude instead of regular Prelude, and let us know how it goes! {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE OverloadedStrings #-} import BasicPrelude -- Dan Burton

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Rigid skolem type variable escaping scope
by oleg＠okmij.org 23 Aug '12

23 Aug '12

Matthew Steele asked why foo type-checks and bar doesn't: > class FooClass a where ... > > foo :: (forall a. (FooClass a) => a -> Int) -> Bool > foo fn = ... > > newtype IntFn a = IntFn (a -> Int) > > bar :: (forall a. (FooClass a) => IntFn a) -> Bool > bar (IntFn fn) = foo fn This and further questions become much simpler if we get a bit more explicit. Imagine we cannot write type signatures like those of foo and bar (no higher-ranked type signatures). But we can define higher-rank newtypes. Since we can't give foo the higher-rank signature, we must re-write it, introducing the auxiliary newtype FaI. > {-# LANGUAGE Rank2Types #-} > class FooClass a where m :: a > > instance FooClass Int where m = 10 > > newtype FaI = FaI{unFaI :: forall a. (FooClass a) => a -> Int} > foo :: FaI -> Bool > foo fn = ((unFaI fn)::(Char->Int)) m > 0 We cannot apply fn to a value: we must first remove the wrapper FaI (and instantiate the type using the explicit annotation -- otherwise the type-checker has no information how to select the FooClass instance). Let's try writing bar in this style. The first attempt > newtype IntFn a = IntFn (a -> Int) > newtype FaIntFn = FaIntFn{unFaIntFn:: forall a. FooClass a => IntFn a} > > bar :: FaIntFn -> Bool > bar (IntFn fn) = foo fn does not work: Couldn't match expected type `FaIntFn' with actual type `IntFn t0' In the pattern: IntFn fn Indeed, the argument of bar has the type FaIntFn rather than IntFn, so we cannot pattern-match on IntFn. We must first remove the IntFn wrapper. For example: > bar :: FaIntFn -> Bool > bar x = case unFaIntFn x of > IntFn fn -> foo fn That doesn't work for another reason: Couldn't match expected type `FaI' with actual type `a0 -> Int' In the first argument of `foo', namely `fn' In the expression: foo fn foo requires the argument of the type FaI, but fn is of the type a0->Int. To get the desired FaI, we have to apply the wrapper FaI: > bar :: FaIntFn -> Bool > bar x = case unFaIntFn x of > IntFn fn -> foo (FaI fn) And now we get the desired error message, which should become clear: Couldn't match type `a0' with `a' because type variable `a' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: FooClass a => a -> Int The following variables have types that mention a0 fn :: a0 -> Int (bound at /tmp/h.hs:16:16) In the first argument of `FaI', namely `fn' In the first argument of `foo', namely `(FaI fn)' When we apply FaI to fn, the type-checker must ensure that fn is really polymorphic. That is, free type variable in fn's type do not occur elsewhere type environment: see the generalization rule in the HM type system. When we removed the wrapper unFaIntFn, we instantiated the quantified type variable a with some specific (but not yet determined) type a0. The variable fn receives the type fn:: a0 -> Int. To type-check FaI fn, the type checker should apply this rule G |- fn :: FooClass a0 => a0 -> Int ----------------------------------- G |- FaI fn :: FaI provided a0 does not occur in G. But it does occur: G has the binding for fn, with the type a0 -> Int, with the undesirable occurrence of a0. To solve the problem then, we somehow need to move this problematic binding fn out of G. That binding is introduced by the pattern-match. So, we should move the pattern-match under the application of FaI: > bar x = foo (FaI (case unFaIntFn x of IntFn fn -> fn)) giving us the solution already pointed out. > So my next question is: why does unpacking the newtype via pattern > matching suddenly limit it to a single monomorphic type? Because that's what removing wrappers like FaI does. There is no way around it. That monomorphic type can be later generalized again, if the side-condition for the generalization rule permits it. You might have already noticed that `FaI' is like big Lambda of System F, and unFaI is like System F type application. That's exactly what they are: http://okmij.org/ftp/Haskell/types.html#some-impredicativity My explanation is a restatement of the Simon Peyton-Jones explanation, in more concrete, Haskell terms.

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Haskell/F#/Ocaml Developer Opportunity in Singapore
by mani maran 23 Aug '12

23 Aug '12

Hello Everyone, I am currently recruiting a Functional Programming Developer for one of the bank in Singapore. Anyone interested please contact me at +65-83991283 or please drop a note to me at manimaran_elumalai(a)kellyit.com.sgwith your contact details. Below are some skills my client is looking for... * Strong in a functional programming language (Haskell, OCaml, F#) * Additional useful languages: C++ / javascript * Maths skills useful. * Good communication skills. * Willingness to interact with traders & quants * 1st degree in Computer Science (preferably with some maths courses) * Post grad degree in Computer Science (Msc/Phd) * Publicly available code that can be reviewed. * No finance background required. Regards, Manimaran

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Cloud Haskell real usage example
by Thiago Negri 23 Aug '12

23 Aug '12

Hello everyone. I'm taking my first steps in Cloud Haskell and got some unexpected behaviors. I used the code from Raspberry Pi in a Haskell Cloud [1] as a first example. Did try to switch the code to use Template Haskell with no luck, stick with the verbose style. I changed some of the code, from ProcessId-based messaging to typed channel to receive the Pong; using "startSlave" to start the worker nodes; and changed the master node to loop forever sending pings to the worker nodes. The unexpected behaviors: - Dropping a worker node while the master is running makes the master node to crash. - Master node do not see worker nodes started after the master process. In order to fix this, I tried to "findSlaves" at the start of the master process and send ping to only these ones, ignoring the list of NodeId enforced by the type signature of "startMaster". Now the master finds new slaves. The bad thing is that when I close one of the workers, the master process freezes. It simply stop doing anything. No more messages and no more Pings to other slaves. :( My view of Cloud Haskell usage would be something similar to this: a master node sending work to slaves; slave instances getting up or down based on demand. So, the master node should be slave-failure-proof and also find new slaves somehow. Am I misunderstanding the big picture of Cloud Haskell or doing anything wrong in the following code? Code (skipped imports and wiring stuff): -- newtype Ping = Ping (SendPort Pong) deriving (Typeable, Binary, Show) newtype Pong = Pong ProcessId deriving (Typeable, Binary, Show) worker :: Ping -> Process () worker (Ping sPong) = do wId <- getSelfPid say "Got a Ping!" sendChan sPong (Pong wId) master :: Backend -> [NodeId] -> Process () master backend _ = forever $ do workers <- findSlaves backend say $ "Slaves: " ++ show workers (sPong, rPong) <- newChan forM_ workers $ \w -> do say $ "Sending a Ping to " ++ (show w) ++ "..." spawn w (workerClosure (Ping sPong)) say $ "Waiting for reply from " ++ (show (length workers)) ++ " worker(s)" replicateM_ (length workers) $ do (Pong wId) <- receiveChan rPong say $ "Got back a Pong from " ++ (show $ processNodeId wId) ++ "!" (liftIO . threadDelay) 2000000 -- Wait a bit before return main = do prog <- getProgName args <- getArgs case args of ["master", host, port] -> do backend <- initializeBackend host port remoteTable startMaster backend (master backend) ["worker", host, port] -> do backend <- initializeBackend host port remoteTable startSlave backend _ -> putStrLn $ "usage: " ++ prog ++ " (master | worker) host port" -- [1] http://alenribic.com/writings/post/raspberry-pi-in-a-haskell-cloud

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Haskell Weekly News: Issue 241
by Daniel Santa Cruz 23 Aug '12

23 Aug '12

Welcome to issue 241 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 12 to 18, 2012. Quotes of the Week * monochrom: seq is a suggestion. pseq is an order. * johnw: Haskell is like using a finely crafted blade to whittle yourself a canoe, as your friends pass by on their cruise liner and wonder what you're up to Top Reddit Stories * Yesod gets financial backing. Domain: yesodweb.com, Score: 76, Comments: 23 On Reddit: [1] http://goo.gl/isVxZ Original: [2] http://goo.gl/aTtqC * The category design pattern Domain: haskellforall.com, Score: 65, Comments: 32 On Reddit: [3] http://goo.gl/Owrgh Original: [4] http://goo.gl/iDEen * I remember the time before monads | free variable Domain: free-variable.org, Score: 64, Comments: 5 On Reddit: [5] http://goo.gl/41Utn Original: [6] http://goo.gl/febU9 * GHC 7.6.1 Release Candidate 1 Domain: haskell.org, Score: 55, Comments: 33 On Reddit: [7] http://goo.gl/weAei Original: [8] http://goo.gl/QHtd7 * PVP: upper bounds are not our friends Domain: haskell.org, Score: 49, Comments: 94 On Reddit: [9] http://goo.gl/8ihwE Original: [10] http://goo.gl/0YbGl * MongoDB on the hunt for Haskell Evangelist Domain: haskellers.com, Score: 36, Comments: 17 On Reddit: [11] http://goo.gl/X1WLL Original: [12] http://goo.gl/TYDdE * Raspberry Pi in a Haskell Cloud Domain: alenribic.com, Score: 33, Comments: 6 On Reddit: [13] http://goo.gl/esAHP Original: [14] http://goo.gl/me7ob * Practical Foundations for Programming Languages (first impressions) Domain: blog.ezyang.com, Score: 30, Comments: On Reddit: [15] http://goo.gl/FjrY1 Original: [16] http://goo.gl/8AzpC * Scoutess GSoC project finished! Domain: self.haskell, Score: 28, Comments: 4 On Reddit: [17] http://goo.gl/hRzvC Original: [18] http://goo.gl/hRzvC Top StackOverflow Questions * Subtype polymorphism in Haskell votes: 17, answers: 2 Read on SO: [19] http://goo.gl/xERPq * Is it possible to use a bracketing syntactic sugar for an applicative functor? votes: 14, answers: 2 Read on SO: [20] http://goo.gl/3EH63 * Useful operations on free arrows votes: 11, answers: 0 Read on SO: [21] http://goo.gl/BQ6ry * Against cold boot attacks: how to restrain sensitive information in Haskell votes: 9, answers: 1 Read on SO: [22] http://goo.gl/kUOn2 * GHCi and compiled code seem to behave differently votes: 7, answers: 1 Read on SO: [23] http://goo.gl/q8yJi * Implementing Iota in Haskell votes: 7, answers: 1 Read on SO: [24] http://goo.gl/KonKf * Understanding STG votes: 6, answers: 1 Read on SO: [25] http://goo.gl/uQmKn * Quickcheck for non-boolean tests votes: 6, answers: 0 Read on SO: [26] http://goo.gl/on6jt * Screen capture in Haskell? votes: 6, answers: 3 Read on SO: [27] http://goo.gl/7xIX5 * typeclass challenge: having both variadic arguments and results votes: 6, answers: 2 Read on SO: [28] http://goo.gl/8aA6Z Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/08/joining-forces-advance-haskell 2. http://www.reddit.com/r/haskell/comments/y62j3/yesod_gets_financial_backing/ 3. http://www.haskellforall.com/2012/08/the-category-design-pattern.html 4. http://www.reddit.com/r/haskell/comments/yfhem/the_category_design_pattern/ 5. http://www.free-variable.org/2012/08/i-remember-the-time-before-monads/ 6. http://www.reddit.com/r/haskell/comments/y87rh/i_remember_the_time_before_m… 7. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-August/022671.h… 8. http://www.reddit.com/r/haskell/comments/y3vzd/ghc_761_release_candidate_1/ 9. http://www.haskell.org/pipermail/haskell-cafe/2012-August/102885.html 10. http://www.reddit.com/r/haskell/comments/ydkcq/pvp_upper_bounds_are_not_our… 11. http://www.haskellers.com/jobs/18 12. http://www.reddit.com/r/haskell/comments/yc1no/mongodb_on_the_hunt_for_hask… 13. http://alenribic.com/writings/post/raspberry-pi-in-a-haskell-cloud 14. http://www.reddit.com/r/haskell/comments/ydvn2/raspberry_pi_in_a_haskell_cl… 15. http://blog.ezyang.com/2012/08/practical-foundations-for-programming-langua… 16. http://www.reddit.com/r/haskell/comments/yda6c/practical_foundations_for_pr… 17. http://www.reddit.com/r/haskell/comments/y9v83/scoutess_gsoc_project_finish… 18. http://www.reddit.com/r/haskell/comments/y9v83/scoutess_gsoc_project_finish… 19. http://stackoverflow.com/questions/12002979/subtype-polymorphism-in-haskell 20. http://stackoverflow.com/questions/12014524/is-it-possible-to-use-a-bracket… 21. http://stackoverflow.com/questions/12001350/useful-operations-on-free-arrows 22. http://stackoverflow.com/questions/11932246/against-cold-boot-attacks-how-t… 23. http://stackoverflow.com/questions/11926398/ghci-and-compiled-code-seem-to-… 24. http://stackoverflow.com/questions/11960809/implementing-iota-in-haskell 25. http://stackoverflow.com/questions/11921683/understanding-stg 26. http://stackoverflow.com/questions/11942554/quickcheck-for-non-boolean-tests 27. http://stackoverflow.com/questions/11963561/screen-capture-in-haskell 28. http://stackoverflow.com/questions/12014420/typeclass-challenge-having-both…

1 0

regex-pcre is not working with UTF-8
by José Romildo Malaquias 21 Aug '12

21 Aug '12

Hello. It seems that the regex-pcre has a bug dealing with utf-8: Prelude> :m + Text.Regex.PCRE Prelude Text.Regex.PCRE> "país:Brasil" =~ "país:(.*)" :: (String,String,String,[String]) ("","pa\237s:Brasil","",["rasil"]) Notice the missing 'B' in the result of the regex matching. With regex-posix this does not happen: Prelude> :m + Text.Regex.Posix Prelude Text.Regex.Posix> "país:Brasil" =~ "país:(.*)" ::(String,String,String,[String]) ("","pa\237s:Brasil","",["Brasil"]) I hope this bug can be fixed soon. Is there a bug tracker to report the bug? If so, what is it? Romildo

2 2

salvia
by Sergey Mironov 21 Aug '12

21 Aug '12

Hi. Does anybody know anything about Sebastiaan Visser, the maintainer of Salvia-* packages (web server) ? Looks like his email is dead. Sergey

2 1

Galois internships available
by Lee Pike 21 Aug '12

21 Aug '12

**************************************** * ANNOUNCING: * INTERNSHIP AT GALOIS, Inc. **************************************** Galois, Inc. <www.galois.com> has an internship available in Portland, Oregon, USA. PROJECT OVERVIEW: The project is a 4+ year research project on build high-assurance autonomous vehicles. Galois will be working on three aspects of this problem: * Synthesizing software components from Haskell-based embedded DSLs. * Building/porting a hardware platform for testing our prototypes. * Performing static/dynamic analysis on C/C++ code to detect vulnerabilities. We intend to release open-source most (if not all) source code developed on the project and we will be publishing on our research results. Being an intern for repeated terms is a possibility. LOGISTICS: The length and start date of the internship are all negotiable: anytime from this fall through next fall is acceptable. Ideally, you can be at Galois for at least 3 continuous months. The internship is paid competitively, and the intern will be responsible for her own living arrangements (although we can certainly help you find arrangements). Galois is located in the heart of downtown, with multiple public transportation options available and world-class bicycle infrastructure, so living here without an automobile is a viable option. QUALIFICATIONS: * MUST-HAVES: * The ability to be geographically located in Portland during the internship. * Good knowledge of C/C++. * Some experience with low-level/embedded design. * Excellent software engineering ability and aptitude. * NICE-TO-HAVES (not necessary, but let me know if you have these qualifications!): * Experience with Haskell and particularly embedded DSLs. * Experience with microcontrollers (particularly ARM Cortex M). * Experience with control systems. * Interest in/experience with real-time systems and RTOSes. * Interest in/experience with software security. * Interest in/experience with static analysis. * Experience with ArduPilot <http://code.google.com/p/ardupilot-mega/> or other autopilot systems. * Good writing skills/experience in writing technical papers. * DO NOT NEED: * A specific degree (we're interested in hearing from anyone from post-docs to undergrads). ABOUT GALOIS: Galois, Inc. is located in Portland, Oregon with a mission to create trustworthiness in critical systems. We're in the business of taking blue-sky ideas and turning them into real-world technology solutions. We've been developing real-world systems for over 10 years using Haskell. To get a sense of life at Galois, one of our previous interns documented his internship here: <http://blog.ezyang.com/2010/08/day-in-the-life-of-a-galois-intern/>. TO APPLY: In one email, * Please attach a C.V. (PDF, plain text, or markdown only). * In the body of the email, include a brief non-HTML note stating your interest and experience and any other relevant details. Send this to Lee Pike (leepike at galois.com) with the subject line "Internship 2012". If you follow these directions, you'll get a confirmation from me.

1 0

fclabels > 0.5
by Sergey Mironov 21 Aug '12

21 Aug '12

Hi. I'm porting old code, which uses fclabels <0.5. Old fclabels define Iso typeclass as follows: class Iso f where iso :: a :<->: b -> f a -> f b iso (Lens a b) = osi (b <-> a) osi :: a :<->: b -> f b -> f a osi (Lens a b) = iso (b <-> a) Newer one defines iso: class Iso (~>) f where iso :: Bijection (~>) a b -> f a ~> f b instance Arrow (~>) => Iso (~>) (Lens (~>) f) where iso bi = arr ((\a -> lens (fw bi . _get a) (_set a . first (bw bi))) . unLens) instance Arrow (~>) => Iso (~>) (Bijection (~>) a) where iso = arr . (.) but no osi. I'm not a guru in categories, can you help me define osi? Thanks Sergey.

2 2

Pure and monadic functions using the Repa arrays
by felipe zapata 21 Aug '12

21 Aug '12

Hi Haskellers, I have been playing with the Repa functions and trying the Repa-examples. In order to gain experience with the Repa functions I have written some small linear algebra utilities and import this module to a bigger project. In the beginning of my project I used the mmultP function from the repa-examples to calculate a big matrix, therefore I have and array of type: arr :: Monad m => m (Array U DIM2 Double) Then I carried this array in a lot of functions which become Monadic function and then it is necessary to introduce the monadic machinery for manipulating this functions . The Question is then if there is the possibility to work with a pure function in place of the monadic version? There is something like a runRepa function? runRepa :: Monad m => m (Array U DIM2 Double) - > Array U DIM2 Double or could I used the unsafePerformIO function ? or the evaluation of the parallel arrays must be postponed until the Repa.Array is called in the main function? Thanks in Advance, Felipe.

2 1

phantom types
by TP 21 Aug '12

21 Aug '12

Hi, I am currently reading documentation on Generalized Algebraic Data Types: http://en.wikibooks.org/wiki/Haskell/GADT I have a question concerning this page. Let us consider the following code proposed in the page: ---------------------------------- -- Phantom type variable a (does not appear in any Expr: it is just a -- dummy variable). data Expr a = I Int | B Bool | Add (Expr a) (Expr a) | Mul (Expr a) (Expr a) | Eq (Expr a) (Expr a) deriving (Show) -- Smart constructors add :: Expr Int -> Expr Int -> Expr Int add = Add i :: Int -> Expr Int i = I b :: Bool -> Expr Bool b = B eval :: Expr a -> a eval (I n) = n ---------------------------------- I obtain the following error: Phantom.hs:27:14: Couldn't match type `a' with `Int' `a' is a rigid type variable bound by the type signature for eval :: Expr a -> a at Phantom.hs:27:1 In the expression: n In an equation for `eval': eval (I n) = n The wiki page explains: """ But alas, this does not work: how would the compiler know that encountering the constructor I means that a = Int? """ I don't understand. When we write "eval (I n) = n", as I is a constructor which takes an Int as argument, we are able to deduce that the type of n is Int; so the type of eval should be in this case "Expr Int -> Int". What do I miss? Thanks, TP

3 2

Re: [Haskell-cafe] [Haskell] Compositional Compiler Construction, Oberon0 examples available
by S. Doaitse Swierstra 21 Aug '12

21 Aug '12

On Aug 21, 2012, at 13:46 , Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Doaitse Swierstra wrote: >> Heinrich Apfelmus wrote: >>> I have a small question: Last I remember, you've mainly been using >>> your UUAGC preprocessor to write attribute grammars in Haskell, >>> especially for UHC. Now that you have first-class attribute >>> grammars in Haskell ("achievement unlocked"), what do you intend to >>> do with the preprocessor? How do these two approaches compare at >>> the moment and where would you like to take them? >> On the page http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo there is >> a link (http://www.fing.edu.uy/~mviera/papers/VSM12.pdf) to a paper >> we presented at LDTA (one of the ETAPS events) this spring. It >> explains how UUAGC can be used to generate first class compiler >> modules. >> We have also a facility for grouping attributes, so one can trade >> flexibility for speed. The first class approach stores list of >> attributes as nested cartesian products, access to which a clever >> compiler might be able to optimize. This however would correspond a >> form of specialisation, so you can hardly say that we have really >> independent modules; as always global optimisation is never >> compositional). From the point of view of the first class approach >> such grouped non-termionals are seen as a single composite >> non-terminal. > > Ah, I see. So the custom syntax offered by UUAGC is still appreciated, but you now intend to compile it to first-class attribute grammars instead of "bare metal Haskell". Makes sense. Thanks! It is not much that it is our intention, but it is an easy way to make an existing compiler extensible. The main (fixed) part of the compiler is constructed in the "old" way from an UUAGC description, and those attributes are grouped (and quite a bit more efficient). On top of this you can define extra attributes and computations, which plug in to the old system. Notice that there is a main difference between the two approaches is that the uuagc route gives you fast compilers, because we can analyse the grammar, and generate efficient tree walking evaluators, whereas the first-class approach gives you great flexibility and the possibility to abstract from common patterns for which others prefer to get lost in stacks of monads, or find out that monads do not work at all since they cannot feed back information into a computation easily. Doaitse > > > Best regards, > Heinrich Apfelmus > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

1 0

Lazy decoding of a list with Data.Binary
by Niklas Hambüchen 21 Aug '12

21 Aug '12

I just encountered the following problem: http://stackoverflow.com/questions/11695373/lazy-decoding-of-a-list-with-da… If I get that correctly, there once was a discussion about adding a Data.Binary.Lazy to binary: http://www.haskell.org/pipermail/haskell-cafe/2007-November/034758.html What happened to this? And if binary is strict by default now, shouldn't the binary-strict package be deprecated? Is there an alternative these days how to lazily write/read binary? Thanks Niklas

1 0

Wanted: Haskell binding for libbdd (buddy)
by Johannes Waldmann 21 Aug '12

21 Aug '12

Are there any Haskell bindings for BDD libraries (reduced ordered binary decision diagrams)? E.g., it seems "buddy" is commonly used http://packages.debian.org/squeeze/libbdd-dev and it has an Ocaml binding. Yes, there is http://hackage.haskell.org/package/obdd but I need better performance (with the same API, ideally). Thanks - J.W. PS: I wonder where performance goes out the window ... I suspect "Map (Int,Int) whatever" should really be a hashtable but I don't like it in IO, it should be in ST?

3 3

Data structure containing elements which are instances of the same type class
by oleg＠okmij.org 21 Aug '12

21 Aug '12

> data A = A deriving Show > data B = B deriving Show > data C = C deriving Show > > data Foo = forall a. Show a => MkFoo a (Int -> Bool) > > instance Show Foo where > show (MkFoo a f) = show a I'd like to point out that the only operation we can do on the first argument of MkFoo is to show to it. This is all we can ever do: we have no idea of its type but we know we can show it and get a String. Why not to apply show to start with (it won't be evaluated until required anyway)? Therefore, the data type Foo above is in all respects equivalent to > data Foo = MkFoo String (Int -> Bool) and no existentials are ever needed. The following article explains elimination of existentials in more detail, touching upon the original problem, of bringing different types into union. http://okmij.org/ftp/Computation/Existentials.html

12 24

Flipping type constructors
by Tony Morris 20 Aug '12

20 Aug '12

I have a data-type that is similar to EitherT, however, I have ordered the type variables like so: data EitherT (f :: * -> *) (a :: *) (b :: *) = ... This allows me to declare some desirable instances: instance Functor f => Bifunctor (EitherT f) instance Foldable f => Bifoldable (EitherT f) instance Traversable f => Bitraversable (EitherT f) However, I am unable to declare a MonadTrans instance: instance MonadTrans (EitherT a) -- kind error I looked at Control.Compose.Flip to resolve this, but it does not appear to be kind-polymorphic. http://hackage.haskell.org/packages/archive/TypeCompose/0.9.1/doc/html/src/… I was wondering if there are any well-developed techniques to deal with this? Of course, I could just write my own Flip with the appropriate kinds and be done with it. Maybe there is a more suitable way? -- Tony Morris http://tmorris.net/

3 2

Haskell master thesis project
by Francesco Mazzoli 20 Aug '12

20 Aug '12

Hi list(s), I've been hooked on Haskell for a while now (some of you might know me as bitonic on #haskell), and I find myself to decide on a project for my masters thesis. Inspired by the David Terei's master thesis (he wrote the LLVM backend), I was wondering if there were any projects requiring similar effort that will benefit the Haskell community. -- Francesco * Often in error, never in doubt

2 1

Haskell position in Israel
by Michael Snoyman 20 Aug '12

20 Aug '12

Hi all, Just passing on this job opportunity for another company. SQream is looking for Haskellers located in Israel. They are working on high performance solutions for large databases using Haskell. If you're interested, please contact me off-list, and I'll pass your information along. Thanks, Michael

1 0

Compositional Compiler Construction, Oberon0 examples available
by Doaitse Swierstra 20 Aug '12

20 Aug '12

On Aug 19, 2012, at 10:40 , Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Doaitse Swierstra wrote: >> Over the years we have been constructing a collection of Embedded >> Domain Specific Languages for describing compilers which are >> assembled from fragments which can be compiled individually. In this >> way one can gradually ``grow a language'' in a large number of small >> steps. The technique replaces things like macro extensions or >> Template Haskell; it has become feasible to just extend the language >> at hand by providing extra modules. The nice thing is that existing >> code does not have to be adapted, nor has to be available nor has to >> be recompiled. >> Recently we have been using (and adapting) the frameworks such that >> we could create an entry in the ldta11 (http://ldta.info/tool.html) >> tool challenge, where one has to show how one's tools can be used to >> create a compiler for the Oberon0 language, which is used a a running >> example in Wirth's compiler construction book. >> We have uploaded our implementation to hackage at: >> http://hackage.haskell.org/package/oberon0. >> More information can be found at the wiki: >> http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo >> You may take a look at the various Gram modules to see how syntax is >> being defined, and at the various Sem modules to see how we use our >> first class attribute grammars to implement the static semantics >> associated with the various tasks of the challenge. >> We hope you like it, and comments are welcome, > > Awesome! > > I have a small question: Last I remember, you've mainly been using your UUAGC preprocessor to write attribute grammars in Haskell, especially for UHC. Now that you have first-class attribute grammars in Haskell ("achievement unlocked"), what do you intend to do with the preprocessor? How do these two approaches compare at the moment and where would you like to take them? > > > Best regards, > Heinrich Apfelmus On the page http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo there is a link (http://www.fing.edu.uy/~mviera/papers/VSM12.pdf) to a paper we presented at LDTA (one of the ETAPS events) this spring. It explains how UUAGC can be used to generate first class compiler modules. We have also a facility for grouping attributes, so one can trade flexibility for speed. The first class approach stores list of attributes as nested cartesian products, access to which a clever compiler might be able to optimize. This however would correspond a form of specialisation, so you can hardly say that we have really independent modules; as always global optimisation is never compositional). From the point of view of the first class approach such grouped non-termionals are seen as a single composite non-terminal. Doaitse > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

1 0

Network.Curl cookie jar madness
by Michael Orlitzky 19 Aug '12

19 Aug '12

I'm one bug away from a working program and need some help. I wrote a little utility that logs into LWN.net, retrieves an article, and creates an epub out of it. Full code here: git clone http://michael.orlitzky.com/git/lwn-epub.git This is the code that gets the login cookie: cj <- make_cookie_jar li_result <- log_in cj uname pword case li_result of Left err -> do let msg = "Failed to log in. " ++ err hPutStrLn stderr msg Right response_body -> do hPutStrLn stderr response_body return $ cfg { C.cookie_jar = Just cj } Curl is making the request, but if I remove the (hPutStrLn stderr response_body), it doesn't work! What's even more insane is, this works: hPutStrLn stderr response_body and this doesn't: hPutStrLn stdout response_body whaaaaaaatttttttt? I really don't want to dump the response body to stderr, but I can't even begin to imagine what's going on here. Has anyone got Network.Curl working with a cookie jar? For anyone with an LWN account: you can set, username = foo password = bar in ~/.lwn-epub/lwn-epub.conf and attempt to run with, lwn-epub -o current.epub current/bigpage

3 7

Adding to / subtracting from a LocalTime?
by Adde Nilsson 19 Aug '12

19 Aug '12

Hi. How do you add to or subtract from a LocalTime? I'm trying to subtract a second from a LocalTime value but the only API's I can find act only on the TimeOfDay part. subSec :: LocalTime -> LocalTime subSec (LocalTime d t) = LocalTime d $ timeToTimeOfDay ((timeOfDayToTime t) - (secondsToDiffTime 1)) I'm obviously on the wrong track as this looks way too complex for what it's trying to achieve. Calling with '2012-08-18 00:00:00' gives '2012-08-18 -01:59:59' which isn't very helpful. How do I get to '2012-08-17 23:59:59'? Thanks, Adde

5 6

Compositional Compiler Construction, Oberon0 examples available
by Doaitse Swierstra 19 Aug '12

19 Aug '12

On Aug 19, 2012, at 10:40 , Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Doaitse Swierstra wrote: >> Over the years we have been constructing a collection of Embedded >> Domain Specific Languages for describing compilers which are >> assembled from fragments which can be compiled individually. In this >> way one can gradually ``grow a language'' in a large number of small >> steps. The technique replaces things like macro extensions or >> Template Haskell; it has become feasible to just extend the language >> at hand by providing extra modules. The nice thing is that existing >> code does not have to be adapted, nor has to be available nor has to >> be recompiled. >> Recently we have been using (and adapting) the frameworks such that >> we could create an entry in the ldta11 (http://ldta.info/tool.html) >> tool challenge, where one has to show how one's tools can be used to >> create a compiler for the Oberon0 language, which is used a a running >> example in Wirth's compiler construction book. >> We have uploaded our implementation to hackage at: >> http://hackage.haskell.org/package/oberon0. >> More information can be found at the wiki: >> http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo >> You may take a look at the various Gram modules to see how syntax is >> being defined, and at the various Sem modules to see how we use our >> first class attribute grammars to implement the static semantics >> associated with the various tasks of the challenge. >> We hope you like it, and comments are welcome, > > Awesome! > > I have a small question: Last I remember, you've mainly been using your UUAGC preprocessor to write attribute grammars in Haskell, especially for UHC. Now that you have first-class attribute grammars in Haskell ("achievement unlocked"), what do you intend to do with the preprocessor? How do these two approaches compare at the moment and where would you like to take them? > > > Best regards, > Heinrich Apfelmus On the page http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo there is a link (http://www.fing.edu.uy/~mviera/papers/VSM12.pdf) to a paper we presented at LDTA (one of the ETAPS events) this spring. It explains how UUAGC can be used to generate first class compiler modules. We have also a facility for grouping attributes, so one can trade flexibility for speed. The first class approach stores list of attributes as nested cartesian products, access to which a clever compiler might be able to optimize. This however would correspond a form of specialisation, so you can hardly say that we have really independent modules; as always global optimisation is never compositional). From the point of view of the first class approach such grouped non-termionals are seen as a single composite non-terminal. Doaitse > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

1 0

Predictive parser combinators need four values to report errors
by John Lask 19 Aug '12

19 Aug '12

I am unable to find a copy of the following paper: Andrew Partridge, David Wright: Predictive parser combinators need four values to report errors. Journal of Functional Programming 6(2): 355-364, 1996 does any one have a copy of, that they could send me ?

1 0

3 level hierarchy of Haskell objects
by Patrick Browne 18 Aug '12

18 Aug '12

5 12

Does someone have a compiled binary of the LLVM for Windows 7 or must one compile the source?
by KC 18 Aug '12

18 Aug '12

I just want to get started on some matrix operations with REPA. Or is there a library (package?) like REPA without using the LLVM? -- -- Regards, KC

2 2

Regex string replacement
by José Romildo Malaquias 18 Aug '12

18 Aug '12

Hello. I am looking for a substring replacement based on Perl like regular expressions that would let me use part of the original string in the replacement string. Something like: reReplace "name is (.*)\\." "hi \1!" "my name is john." which would result in "hy john!" Any help? Romildo

2 2

Re: [Haskell-cafe] phantom types
by Florian Hartwig 17 Aug '12

17 Aug '12

> Hi, > > I am currently reading documentation on Generalized Algebraic Data Types: > > http://en.wikibooks.org/wiki/Haskell/GADT > > I have a question concerning this page. Let us consider the following code > proposed in the page: > > ---------------------------------- > -- Phantom type variable a (does not appear in any Expr: it is just a > -- dummy variable). > data Expr a = I Int > | B Bool > | Add (Expr a) (Expr a) > | Mul (Expr a) (Expr a) > | Eq (Expr a) (Expr a) > deriving (Show) > > -- Smart constructors > add :: Expr Int -> Expr Int -> Expr Int > add = Add > > i :: Int -> Expr Int > i = I > > b :: Bool -> Expr Bool > b = B > > eval :: Expr a -> a > eval (I n) = n > ---------------------------------- > > I obtain the following error: > > Phantom.hs:27:14: > Couldn't match type `a' with `Int' > `a' is a rigid type variable bound by > the type signature for eval :: Expr a -> a at Phantom.hs:27:1 > In the expression: n > In an equation for `eval': eval (I n) = n > > The wiki page explains: > > """ > But alas, this does not work: how would the compiler know that encountering > the constructor I means that a = Int? > """ > > I don't understand. When we write "eval (I n) = n", as I is a constructor > which takes an Int as argument, we are able to deduce that the type of n is > Int; so the type of eval should be in this case "Expr Int -> Int". > What do I miss? Since the example uses phantom types, the type of n is not actually related to the type of (I n). It is perfectly possible to have, for example, a value of type Expr String created by the I constructor: *Main> let expr = I 5 :: Expr String *Main> expr I 5 *Main> :t expr expr :: Expr String So if we define eval the way it is defined in the example, the compiler cannot infer that the type of (I n) is Expr Int, even though it knows that n's type is Int. We could prevent the creation of values with the wrong types by only exporting the smart constructors, but that is no help for the type system. Declaring eval as Expr Int -> Int would make the code compile, but since eval should presumably include patterns for other types, such as eval (B b) = b which would have to be of type Expr Bool -> Bool, we cannot do that. I hope I did not misunderstand your question and this helps. Cheers, Florian

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Re: [Haskell-cafe] [Announce] Compositional Compiler Construction, Oberon0 examples available
by Facundo Domínguez 17 Aug '12

17 Aug '12

Sorry, fixed the subject. On Fri, Aug 17, 2012 at 11:11 AM, Facundo Domínguez <facundominguez(a)gmail.com> wrote: > I have one unbearable doubt, how do you accent CoCoCo? CóCoCo? CoCóCo? CoCoCó? > > Facundo > >> Date: Thu, 16 Aug 2012 21:40:28 +0200 >> From: Doaitse Swierstra <doaitse(a)swierstra.net> >> Subject: [Haskell-cafe] [Announce] Compositional Compiler >> Construction, Oberon0 examples available >> To: "haskell-cafe(a)haskell.org Cafe" <haskell-cafe(a)haskell.org>, >> Haskell List <haskell(a)haskell.org> >> Message-ID: <3B4B9FFC-B1BB-4156-BE80-5E60632EE5CD(a)swierstra.net> >> Content-Type: text/plain; charset=us-ascii >> >> Over the years we have been constructing a collection of Embedded Domain Specific Languages for describing compilers which are assembled from fragments which can be compiled individuallu. In this way one can gradually ``grow a langauge'' in a large number of small steps. The technique replaces things like macro extensions or Template Haskell; it has become feasable to just extend the language at hand by providing extra modules. The nice thing is that existing code does not have to be adapted, nor has to be available nor has to be recompiled. >> >> Recently we have been using (and adapting) the frameworks such that we could create an entry in the ldta11 (http://ldta.info/tool.html) tool challenge, where one has to show how one's tools can be used to create a compiler for the Oberon0 language, which is used a a running example in Wirth's compiler construction book. >> >> We have uploaded our implementation to hackage at: http://hackage.haskell.org/package/oberon0. >> >> More information can be found at the wiki: http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo >> >> You may take a look at the various Gram modules to see how syntax is being defined, and at the various Sem modules to see how we use our first class attribute grammars to implement the static semantics associated with the various tasks of the challenge. >> >> We hope you like it, and comments are welcome, >> >> Marcos Viera >> Doaitse Swierstra

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 108, Issue 22
by Facundo Domínguez 17 Aug '12

17 Aug '12

I have one unbearable doubt, how do you accent CoCoCo? CóCoCo? CoCóCo? CoCoCó? Facundo > Date: Thu, 16 Aug 2012 21:40:28 +0200 > From: Doaitse Swierstra <doaitse(a)swierstra.net> > Subject: [Haskell-cafe] [Announce] Compositional Compiler > Construction, Oberon0 examples available > To: "haskell-cafe(a)haskell.org Cafe" <haskell-cafe(a)haskell.org>, > Haskell List <haskell(a)haskell.org> > Message-ID: <3B4B9FFC-B1BB-4156-BE80-5E60632EE5CD(a)swierstra.net> > Content-Type: text/plain; charset=us-ascii > > Over the years we have been constructing a collection of Embedded Domain Specific Languages for describing compilers which are assembled from fragments which can be compiled individuallu. In this way one can gradually ``grow a langauge'' in a large number of small steps. The technique replaces things like macro extensions or Template Haskell; it has become feasable to just extend the language at hand by providing extra modules. The nice thing is that existing code does not have to be adapted, nor has to be available nor has to be recompiled. > > Recently we have been using (and adapting) the frameworks such that we could create an entry in the ldta11 (http://ldta.info/tool.html) tool challenge, where one has to show how one's tools can be used to create a compiler for the Oberon0 language, which is used a a running example in Wirth's compiler construction book. > > We have uploaded our implementation to hackage at: http://hackage.haskell.org/package/oberon0. > > More information can be found at the wiki: http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo > > You may take a look at the various Gram modules to see how syntax is being defined, and at the various Sem modules to see how we use our first class attribute grammars to implement the static semantics associated with the various tasks of the challenge. > > We hope you like it, and comments are welcome, > > Marcos Viera > Doaitse Swierstra

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How to simplify the code of Maybe within a monad?
by Magicloud Magiclouds 17 Aug '12

17 Aug '12

Hi, Since Maybe is a monad, I could write code like 'maybeA >> maybeB >> maybeC' to check if all these are not Nothing. Or 'liftM foo maybeD' to avoid ugly 'case of'. But how if here maybe[ABC] are like 'IO (Maybe Int)', or foo is type of 'Int -> IO Int'? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Working with remote HTTP file as ZIP archive
by Dennis Yurichev 17 Aug '12

17 Aug '12

Hi. I want %subj%. Is it possible with lazy evaluation and any existing Haskell libraries do: 1) open remote file via HTTP, read random pieces of it but not to download the whole file? HTTP standard supporting partial reads... 2) open remote file via HTTP as ZIP archive, get list of files, get specific file (whole) or get part of specific file, but again, using lazy eval, not to fetch all stuff. So is it possible? Thanks in advance!

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Parsing pragmas in a Haskell-like language
by Björn Peemöller 17 Aug '12

17 Aug '12

Dear cafe, I'm experimenting with extending the parser for a Haskell-like language by module pragmas. The parser is written using parser combinators. Currently, I adapted the lexer to ignore whitespace and comments, but create lexemes for both the pragma start and end (and the pragma's content, of course). While these lexemes are necessary for parsing the pragmas before the module header, they somehow should be ignored (treated like comments) afterwards. Could anyone give me a hint me how this behaviour (treat pragmas like comments) is achieved in GHC or in haskell-src-exts? Thanks in advance, Björn

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Can pipes solve this problem? How?
by oleg＠okmij.org 16 Aug '12

16 Aug '12

> Consider code, that takes input from handle until special substring matched: > > > matchInf a res s | a `isPrefixOf` s = reverse res > > matchInf a res (c:cs) = matchInf a (c:res) cs > > hTakeWhileNotFound str hdl = hGetContents hdl >>= return.matchInf str [] > > It is simple, but the handle is closed after running. That is not good, > because I want to reuse this function. This example is part of one of Iteratee demonstrations http://okmij.org/ftp/Haskell/Iteratee/IterDemo1.hs Please search for -- Early termination: -- Counting the occurrences of the word ``the'' and the white space -- up to the occurrence of the terminating string ``the end'' The iteratee solution is a bit more general because it creates an inner stream with the part of the outer stream until the match is found. Here is a sample application: run_bterm2I fname = print =<< run =<< enum_file fname .| take_until_match "the end" (countWS_iter `en_pair` countTHE_iter) It reads the file until "the end" is found, and counts white space and occurrences of a specific word, in parallel. All this processing happens in constant space and we never need to accumulate anything into string. If you do need to accumulate into string, there is an iteratee stream2list that does that. The enumeratee take_until_match, as take and take_while, stops when the terminating condition is satisfied or when EOF is detected. In the former case, the stream may contain more data and remains usable. A part of IterDemo1 is explained in the paper http://okmij.org/ftp/Haskell/Iteratee/describe.pdf I am not sure though if I answered your question since you were looking for pipes. I wouldn't call Iteratee pipes.

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Fwd: 'let' keyword optional in do notation?
by David Feuer 16 Aug '12

16 Aug '12

---------- Forwarded message ---------- From: David Feuer <david.feuer(a)gmail.com> Date: Wed, Aug 8, 2012 at 12:22 PM Subject: Re: [Haskell-cafe] 'let' keyword optional in do notation? To: Martijn Schrage <martijn(a)oblomov.com> Changing scoping rules based on whether things are right next to each other? No thanks. On Wed, Aug 8, 2012 at 11:44 AM, Martijn Schrage <martijn(a)oblomov.com> wrote: > On 08-08-12 17:27, Ertugrul Söylemez wrote: > > Vo Minh Thu <noteed(a)gmail.com> wrote: > > This is not a parsing problem, but a scoping one: try to run this > program: > > main = do > let x = y > y = 5 > let a = b > let b = 6 > print (x, y, a, b) > > Cheers, > Thu > > Martijn has actually covered this question: > > Where each sequence of let-less bindings is put in a separate > binding group. I'm no parsing wizard, but I couldn't come up with > any situations in which this would cause ambiguity. To me, the > let-less version is easier on the eyes, more consistent with <- > bindings, and also makes it less of a hassle to move stuff around. > > > To make it more clear, this is the transformation I propose: > > do ... -- not a let-less binding > x1 = exp1 -- \ > .. -- only let-less bindings > xn = expn -- / > ... -- not a let-less binding > > becomes > > do ... > let x1 = exp1 > .. > xn = expn > ... > > So > > main = do > > x = y > y = 5 > a = b > > b = 6 > print (x, y, a, b) > > > would put everything in the same binding group and compile successfully. To > get Thu's example, you would write > > main = do > > x = y > y = 5 > let a = b > let b = 6 > print (x, y, a, b) > > The last let could even be left out. > > Cheers, Martijn > > The suggestion seems sound to me, and the additional 'let' can really be > annoying in cases where you have a lot of 'let' bindings among very few > monadic actions. My current way to deal with this is to move the stuff > to separate computations, but it's certainly not a nice solution: > > myComp = c >>= f > where > f x = ... > where > a = ... > b = ... > > > Greets > Ertugrul > > > > _______________________________________________ > 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 >

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