- Haskell-Cafe - Haskell.org

Why do I need a 'do'?
by michael rice 30 Apr '09

30 Apr '09

Hi, Why do I need a 'do' in the code below? Michael ================== import System.Random rollDice :: IO Int rollDice = getStdRandom (randomR (1,6)) rollNDice :: Int -> [IO Int] rollNDice 0 = [] rollNDice n = rollDice : rollNDice (n-1) ================= [michael@localhost ~]$ ghci rnd0 GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rnd0.hs, interpreted ) Ok, modules loaded: Main. *Main> rollDice Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. 6 *Main> rollDice 3 *Main> rollNDice 3 <interactive>:1:0: No instance for (Show (IO Int)) arising from a use of `print' at <interactive>:1:0-10 Possible fix: add an instance declaration for (Show (IO Int)) In a stmt of a 'do' expression: print it *Main>

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default values in a record structure
by Vasili I. Galchin 30 Apr '09

30 Apr '09

Hello, Is there anyway when defining a dat type record struct to indicate default values for some of the fields? Regards, Vasili

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A question about laziness and foreign resources (and libxml2)
by Graham Fawcett 30 Apr '09

30 Apr '09

Hi folks, I'm having trouble reasoning about laziness and FFI resources. I've written a little C wrapper function to augment Text.XML.LibXML; given a Ptr Node, it will return the node's tag-name: foreign import ccall unsafe _getName :: Ptr Node -> IO CString Given LibXML's 'withNode' function, I can write: withNode :: Node -> (Ptr Node -> IO a) -> IO a getName1 :: Node -> IO String getName1 n = withNode n $ \n' -> _getName n' >>= peekCString But I'm tempted to write this instead: getName2 n = withNode n return >>= _getName >>= peekCString In a strict language, I'd observe that the (Ptr Node) in getName2 was escaping the lexical scope of the withNode call, and so getName2 would be unsafe (since it might get GC'd upon leaving that scope). But in Haskell, I can't reason out whether that's the case. Is 'withNode n return' a risky proposition? Is lexical scope indeed the right test here? If not, how might you reason about the safety of this code? Thanks, Graham

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INAP 2009: 1st CFP
by Pedro Salgueiro 29 Apr '09

29 Apr '09

[apologies for multiple posts; please distribute] ------------------------------------------------- First Call for Papers INAP 2009 18th International Conference on Applications of Declarative Programming and Knowledge Management November 5-7, 2009 Evora, Portugal http://www.di.uevora.pt/inap2009/ http://inap.dialogengines.com/ Organized by the Portuguese AI Society (APPIA), the INAP Committee and the Society of Logic Programming (GLP e.V.) == Overview == Declarative Programming is a family of advanced paradigms for the modeling and solving of complex problems. These specification and implementation methods have attracted more and more attention over the past years, e.g. in the domains of databases and natural language processing, for modeling and the processing of combinatorial problems, and for establishing systems for the web. == INAP 2009 == INAP is a communicative and dense forum for intensive discussion of applications of important technologies related to Prolog, Logic and Constraint Programming as well as closely related advanced software. It comprehensively covers the impact of programmable logic solvers in the Internet Society, its underlying technologies, and leading edge applications in industry, commerce, government, and societal services. INAP 2009 continues a tradition of successful workshops cast around the applications of declarative programming, which were held in Kobe (1997), Tokyo (1995, 1996, 1998 - 2001), Potsdam (2004), Fukuoka (2005) and Wuerzburg (2007). We invite the submission of high quality papers on the described topics, especially, but not exclusively, on different aspects of Declarative Programming, Constraint Processing and Knowledge Management as well as their use for Distributed Systems and the Web: - Knowledge Management, e.g. Data Mining, Decision Support, Deductive Databases - Distributed Systems and the Web, e.g. Agents and Concurrent Engineering, Semantic Web - Constraints, e.g. Constraint Systems, Extensions of Constraint (Logic) Programming - Theoretical Foundations, e.g. Deductive Databases, Nonmonotonic Reasoning - Systems and Tools for Academic and Industrial Use - Knowledge-based Web Services - Logic Solvers and Applications == Workshop Format == The technical program of the workshop will include invited presentations (to be announced), regular technical sessions with presentations of the accepted papers, system demonstrations and a panel discussion. == Conference Venue == The conference will be held at the University of Evora, Portugal in November 5-7, 2009. Evora is a nice and quiet historical city located in the south of Portugal that can be reached from Lisbon by train or coach in under 2 hours. It is a small city of 60.000 inhabitants, 120 km inland from Lisbon and classified by Unesco as World Heritage. The University of Evora was established in the 16th Century and is the 2nd oldest Portuguese University. The social program is promising since the region is very rich in historical sites (Stone Age, Roman, Medieval and Renaissance remains) and also offers a very special gastronomy. See http://en.wikipedia.org/wiki/%C3%89vora for more information. == Important Dates == Paper Submission Deadline: June 29, 2009 Notifications to Authors: August 17, 2009 Camera-ready Version Deadline: September 14, 2009 INAP 2009 Workshop: November 5-7, 2009 == Submission Guidelines == Participants should submit a paper (maximum 15 pages, PDF format), describing their work in topics relevant to the workshop. Accepted papers will be presented during the workshop. At least one author of an accepted contribution is expected to register for the workshop, and present the paper. All submissions should include the author's name(s), affiliation, complete mailing address, and email address. Authors are requested to prepare their submissions, following the LNCS/LNAI Springer format. Please see: http://www.springer.de/comp/lncs/authors.html for further details. The submission should be submitted through the electronic submission site: http://www.easychair.org/inap2009/ The deadline for receipt of submissions is June 29, 2009. Papers received after this date will not be reviewed. Eligible papers will be peer-reviewed by at least three members of the Program Committee. Authors will be notified via email of the results by August 17, 2009. Authors of accepted papers are expected to improve their paper based on reviewers' comments and to send a camera ready version of their manuscripts by September 14, 2009. Accepted papers will be included in the workshop proceedings, which will be distributed to the participants. As in previous editions, we plan to publish selected papers in a proceedings volume in the Springer Lecture Notes in Artificial Intelligence (LNAI) series. == Organizing Committee == Vitor Nogueira vbn AT di.uevora.pt Salvador Abreu spa AT di.uevora.pt Pedro Salgueiro pds AT di.uevora.pt Universidade de Evora Portugal == Program Committee == Salvador Abreu, University of Evora, Portugal (co-chair) Sergio Alvarez, Boston College, USA Philippe Codognet, CNRS/JFLI, Tokyo, Japan Vitor Santos Costa, University of Porto, Portugal Daniel Diaz, University of Paris I, France Ulrich Geske, University of Potsdam, Germany Gopal Gupta, UT Dallas, USA Petra Hofstedt, Technical University of Berlin, Germany Ulrich Neumerkel, Technical University of Vienna, Austria Vitor Nogueira, University of Evora, Portugal Enrico Pontelli, New Mexico State University, USA Irene Rodrigues, University of Evora, Portugal Carolina Ruiz, Worcester Polytechnic Institute, USA Dietmar Seipel, University of Wuerzburg, Germany (co-chair) Terrance Swift, CENTRIA, Portugal Hans Tompits, Technical University of Vienna, Austria Masanobu Umeda, Kyushu Institute of Technology, Japan Armin Wolf, Fraunhofer FIRST, Berlin, Germany Osamu Yoshie, Waseda University, Japan == Contact Information == inap2009 AT di.uevora.pt Universidade de Evora Departamento de Informatica Largo dos Colegiais, 2 7004-516 Evora - PORTUGAL

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Cabal's default install location
by Thomas Davie 29 Apr '09

29 Apr '09

There seems to be an assumption amongst the community that a user's home directory is the most useful place for cabal to install to by default. A few people have challenged that. I wanted to find out which one most people do actually prefer, so please go and vote on this poll. http://noordering.wordpress.com/2009/04/21/cabals-default-install-location/ It's no more than a straw poll, I don't know how protected it is against ballot stuffing, but I'm pretty confident that Haskellers are trustworthy enough not to play with it that way. I'll tell you all the result next week. Bob

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Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Hi Yitz, Thanks for your input. Since I'm trying to learn Monads, let's look at this as a teaching moment. The example code (see below), which I pulled off YAMT (Yet Another Monad Tutorial ;-)), is the source of my 'comb' function. I understand the code as it now stands, and I understand that the Prelude (>>=) would replace the 'comb'. Adding whatever statements are needed, how would you "specialize" the (>>=) to Maybe and solve this particular problem. Michael --- On Wed, 4/29/09, Yitzchak Gale <gale(a)sefer.org> wrote: From: Yitzchak Gale <gale(a)sefer.org> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 4:30 AM Michael Rice wrote: > -- comb is a combinator for sequencing operations that return Maybe > comb :: Maybe a -> (a -> Maybe b) -> Maybe b > comb Nothing _ = Nothing > comb (Just x) f = f x comb is essentially the same as something in the Prelude: it is just (>>=) specialized to Maybe. (>>=) :: Monad m => m a -> (a -> m b) -> m b > Now what am I misunderstanding in the code below? > lst = [('A',65),('B',66),('C',67),('D',68)] Brandon S. Allbery wrote: > ...it defaulted to [(Char,Integer)]. This is a manifestation > of the Monomorphism Restriction... While it may be debatable whether the Monomorphism Restriction is helpful in compiled code, it is unquestionably a major nuisance at the GHCi prompt, for this and other reasons. I highly recommend that you create a .ghci file in your home directory containing the line: :set -XNoMonomorphismRestriction In my opinion, MR should be off by default in GHCi. -Yitz

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Re: [Haskell-cafe] chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Oops, here's the code. Michael =========================== {- Author: Jeff Newbern Maintainer: Jeff Newbern <jnewbern(a)nomaware.com> Time-stamp: <Mon Nov 10 11:59:14 2003> License: GPL -} {- DESCRIPTION Example 1 - Our first monad Usage: Compile the code and execute the resulting program. It will print Dolly's maternal grandfather. -} -- everything you need to know about sheep data Sheep = Sheep {name::String, mother::Maybe Sheep, father::Maybe Sheep} -- we show sheep by name instance Show Sheep where show s = show (name s) -- comb is a combinator for sequencing operations that return Maybe comb :: Maybe a -> (a -> Maybe b) -> Maybe b comb Nothing _ = Nothing comb (Just x) f = f x -- now we can use `comb` to build complicated sequences maternalGrandfather :: Sheep -> Maybe Sheep maternalGrandfather s = (Just s) `comb` mother `comb` father fathersMaternalGrandmother :: Sheep -> Maybe Sheep fathersMaternalGrandmother s = (Just s) `comb` father `comb` mother `comb` mother mothersPaternalGrandfather :: Sheep -> Maybe Sheep mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father -- this builds our sheep family tree breedSheep :: Sheep breedSheep = let adam = Sheep "Adam" Nothing Nothing eve = Sheep "Eve" Nothing Nothing uranus = Sheep "Uranus" Nothing Nothing gaea = Sheep "Gaea" Nothing Nothing kronos = Sheep "Kronos" (Just gaea) (Just uranus) holly = Sheep "Holly" (Just eve) (Just adam) roger = Sheep "Roger" (Just eve) (Just kronos) molly = Sheep "Molly" (Just holly) (Just roger) in Sheep "Dolly" (Just molly) Nothing -- print Dolly's maternal grandfather main :: IO () main = let dolly = breedSheep in do print (father dolly) -- END OF FILE --- On Wed, 4/29/09, Yitzchak Gale <gale(a)sefer.org> wrote: From: Yitzchak Gale <gale(a)sefer.org> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 29, 2009, 4:30 AM Michael Rice wrote: > -- comb is a combinator for sequencing operations that return Maybe > comb :: Maybe a -> (a -> Maybe b) -> Maybe b > comb Nothing _ = Nothing > comb (Just x) f = f x comb is essentially the same as something in the Prelude: it is just (>>=) specialized to Maybe. (>>=) :: Monad m => m a -> (a -> m b) -> m b > Now what am I misunderstanding in the code below? > lst = [('A',65),('B',66),('C',67),('D',68)] Brandon S. Allbery wrote: > ...it defaulted to [(Char,Integer)]. This is a manifestation > of the Monomorphism Restriction... While it may be debatable whether the Monomorphism Restriction is helpful in compiled code, it is unquestionably a major nuisance at the GHCi prompt, for this and other reasons. I highly recommend that you create a .ghci file in your home directory containing the line: :set -XNoMonomorphismRestriction In my opinion, MR should be off by default in GHCi. -Yitz

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Use GHC API to parse Haskell source code
by Haoyu Bai 29 Apr '09

29 Apr '09

Hi, I'm trying to use GHC API to parse Haskell source code and get an AST but get a "Parse: noRebindableInfo" error. So far my program can parse the simplest code: module HelloWorld where main = do putStrLn $ show "hello" But when I change the show "hello" to show 0, eg, try to parse the following code: module HelloWorld where main = do putStrLn $ show 0 I get a "Parse: noRebindableInfo" error (or exception?) and nothing in the output file. I must did something wrong, but I can't figure out it. The following is my program (highlighted source code here: http://pastebin.com/f556455ff ). I compiled it with "ghc --make -package ghc Parse.hs". The GHC version I used is 6.10.2. After compiled, I run the following command to execute the program: "./Parse hello.hs test.out" where test.out is the output file for the AST. ------------------------------------------------------------------------------------------------------------- module Main where import qualified Parser import StringBuffer import Lexer import FastString import SrcLoc import DynFlags import StaticFlagParser import HscTypes import GHC import GHC.SYB.Utils import System import IO doParse infile dflags = do buf <- hGetStringBuffer infile let loc = mkSrcLoc (mkFastString infile) 1 0 return $ unP Parser.parseModule (mkPState buf loc dflags) parseFile infile = defaultErrorHandler defaultDynFlags $ {-defaultCleanupHandler $-} runGhc (Just "/usr/lib/ghc-6.10.2/") $ do hsc_env <- getSession let dflags0 = hsc_dflags hsc_env let dflags = dflags0{ verbosity = 99 } setSessionDynFlags dflags handleSourceError (\e -> do printExceptionAndWarnings e liftIO $ exitWith (ExitFailure 1)) $ do liftIO $ doParse infile dflags main :: IO () main = do args <- getArgs let [infile, outfile] = args hSetBuffering stdout NoBuffering res <- parseFile infile case res of POk _ x -> bracket (openFile outfile WriteMode) hClose (\h -> hPutStrLn h $ showData Parser 2 x) PFailed loc err -> do hPutStrLn stderr $ "Failure when parsing " ++ show infile ++ ": " -- ++ show err {-exitWith (ExitFailure (-1))-} ------------------------------------------------------------------------------------------------------------- The actual output of the program is: ------------------------------------------------------------------------------------------------------------- Using package config file: /usr/lib/ghc-6.10.2/package.conf hiding package base-3.0.3.1 to avoid conflict with later version base-4.1.0.0 wired-in package ghc-prim mapped to ghc-prim-0.1.0.0 wired-in package integer mapped to integer-0.1.0.1 wired-in package base mapped to base-4.1.0.0 wired-in package rts mapped to rts-1.0 wired-in package haskell98 mapped to haskell98-1.0.1.0 wired-in package syb mapped to syb-0.1.0.1 wired-in package template-haskell mapped to template-haskell-2.3.0.1 wired-in package dph-seq mapped to dph-seq-0.3 wired-in package dph-par mapped to dph-par-0.3 Parse: noRebindableInfo ------------------------------------------------------------------------------------------------------------- Thank you very much! -- Haoyu Bai

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Non-atomic "atoms" for type-level programming
by Claus Reinke 29 Apr '09

29 Apr '09

The recent GHC trac ticket revision reminded me of the old open type-sharing problem with type tags and record labels: - if type-level tags (such as 'data TTrue'/'data TFalse') are declared repeatedly in separate modules, they represent separate types, preventing shared use (your type-level predicate doesn't return my version of 'TTrue'/'TFalse') - if record field labels (as used in your run-of-the-mill extensible record library) have to be declared, that is not only inconvenient, but prevents sharing of field labels over independent code bases (see the old Haskell' ticket #92 for discussion http://hackage.haskell.org/trac/haskell-prime/wiki/FirstClassLabels ; also http://hackage.haskell.org/trac/ghc/ticket/1872 for alternative extensible records libs) Since Haskell has no concept of type sharing, the only way to express this is to declare types in a common import. But there is no way for that common import to predict all possible future uses, and we can't just keep adding more labels/tags to it, forcing all dependencies to be updated and kept in sync. Using Template Haskell, and QuasiQuotes in particular, we can now at least work around this issue, by splitting the atoms:-) - 'undefined :: Tag' becomes 'undefined :: (TT :. Ta :. Tg)' - 'label :: Label' becomes '(Ll :< La :< Lb :< Le :< Ll) :: (Ll :< La :< Lb :< Le :< Ll)' - a common import, Data.Label, provides type letters and combinators: 'data La = La' and 'data a :< b = a :< b' 'data Ta' and 'data a :. b' .. - quasiquoting and Show instances, also provided by Data.Label, try to hide the internal structure of labels and tags, at least at expression level. Since there is no quasiquoting at type-level (why?), generating type synonyms seems the best we can do there.. - since record field labels are constructed from type letters, this would also provide a basis for - type-level numbers (type-level quasiquoting would help, though) - label ordering: http://hackage.haskell.org/trac/ghc/ticket/2104 http://hackage.haskell.org/trac/ghc/ticket/1894 In actual use, this is what tags and labels from Data.Label look like: ------------- -- the usual extensible-records-as-nested-pairs data label := value = label := value deriving Show data field :& record = field :& record deriving Show infixr :& -- no quasiquoting for types:-(, so generate type synonyms instead $(genTypeTag "TTrue") $(genTypeTag "TFalse") -- a type-level predicate, using shared tags TTrue/TFalse class HasField record label tbool | label record -> tbool instance HasField () label TFalse instance HasField ((label:=value):&record) label TTrue instance HasField record label tbool => HasField (field:&record) label tbool -- record field selection, driven by field label types class Select record label value where (!) :: record -> label -> value instance .. -- some example records -- no need to declare field labels, and they will be -- shared with other importers of Data.Label! a = ([$l|this|] := True) :&([$l|that|] := "hi") :&() b = ([$l|that|] := "there") :&([$l|x|] := 100) :&([$l|y|] := 200) :&() -- we don't even need label values for this, -- type tags are sufficient, as long as we don't -- evaluate the (undefined) values of tags c = ([$t|this|] := 'x') :&([$t|y|] := ()) :&() -- testing Show and record selection records = do print a print b print c print $ (a ! [$l|this|]) print $ (c ! [$t|this|]) print $ (a ! [$l|that|]) ++ ", " ++ (b ! [$l|that|]) ------------- *Main> [$l|label|] label *Main> :t [$l|label|] [$l|label|] :: Ll :< (La :< (Lb :< (Le :< Ll))) *Main> [$l|label|] `seq` () () *Main> [$t|label|] label *Main> :t [$t|label|] [$t|label|] :: Tl :. (Ta :. (Tb :. (Te :. Tl))) *Main> [$t|label|] `seq` () *** Exception: Prelude.undefined *Main> :t [$l|100|] [$l|100|] :: L1 :< (L0 :< L0) *Main> records (this := True) :& ((that := "hi") :& ()) (that := "there") :& ((x := 100) :& ((y := 200) :& ())) (this := 'x') :& ((y := ()) :& ()) True 'x' "hi, there" ------------- For example code, see http://community.haskell.org/~claus/misc/Data/Label.hs http://community.haskell.org/~claus/misc/Data/Label/TH.hs http://community.haskell.org/~claus/misc/labels.hs Claus

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Make XNoMonomorphismRestriction the default in GHCi
by Yitzchak Gale 29 Apr '09

29 Apr '09

I hereby propose that XNoMonomorphismRestriction be the default in GHCi. I submitted a ticket: http://hackage.haskell.org/trac/ghc/ticket/3202 Note that I am NOT expressing any opinion about MR in general - just about what the default should be in interactive GHCi sessions only. Please read my comments in the ticket, and respond here or in the ticket. Thanks, Yitz

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