April 2009 - Haskell-Cafe

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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HaL4: Local Haskell meeting, Halle/Saale, Germany, June 12
by Henning Thielemann 29 Apr '09

29 Apr '09

Hi all, we like to remind the potential participants of our Local Haskell meeting HaL4 to submit proposals for talks. See the recent announcement in German: http://www.haskell.org/pipermail/haskell/2009-April/021220.html --------------------------------------------- HaL4 : Haskell - Tutorial + Workshop + Party am Freitag, dem 12. Juni 2009, in Halle/Saale --------------------------------------------- Das traditionsreiche HaL-Treffen bietet eine gute Mischung von Haskell-bezogenen Themen aus Forschung, Anwendung und Lehre mit vielen Möglichkeiten zu Diskussion und Unterhaltung bei der anschließenden Party. Der Workshop wird in diesem Jahr ergänzt durch Tutorien für Haskell-Ein- und Umsteiger. Diesmal findet das Treffen in Halle/Saale im Institut für Informatik der Martin-Luther-Universität Halle-Wittenberg statt. Wir planen: 10 - 13 Uhr: Tutorien 15 - 19 Uhr: Fachvorträge 19 - 22 Uhr: Grillparty Wir freuen uns auf rege Teilnahme sowie spannende Vorträge mit heißen Diskussionen und bitten um Vortragsanmeldungen bis zum 8. Mai. Gedacht sind 4 mal je 30 min Vortrag + 30 min Diskussion. Weitere Informationen auf http://www.iba-cg.de/hal4.html Beste Gruesse Henning Thielemann

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chr/ord?
by michael rice 29 Apr '09

29 Apr '09

Hi, My Prelude docs must be out of date because chr and ord don't seem to be there. How do I access these functions? Michael =============== [michael@localhost ~]$ ghci 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. Prelude> chr 65 <interactive>:1:0: Not in scope: `chr' Prelude> ord 'A' <interactive>:1:0: Not in scope: `ord' Prelude>

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

29 Apr '09

Yep, that fixed it.. Thanks, Brandon. Michael --- On Tue, 4/28/09, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: From: Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> Subject: Re: [Haskell-cafe] chr/ord? To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "Tim Wawrzynczak" <inforichland(a)gmail.com>, "Lennart Augustsson" <lennart.augustsson(a)gmail.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Tuesday, April 28, 2009, 11:44 PM On Apr 28, 2009, at 23:32 , michael rice wrote:Thank guys, Now what am I misunderstanding in the code below? lst = [('A',65),('B',66),('C',67),('D',68)] You didn't give a type for lst, so it defaulted to [(Char,Integer)]. This is a manifestation of the Monomorphism Restriction, invoked because lst doesn't take any arguments: it forces lst to have a definite type, so defaulting is used to fix the numeric part to Integer. If you add a type specification for lst, things will work as you expect. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.comsystem administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.eduelectrical and computer engineering, carnegie mellon university KF8NH

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Typing efficient folds
by Keith Battocchi 29 Apr '09

29 Apr '09

I'm trying to write some code to do folds on nested datatypes as in http://web.comlab.ox.ac.uk/people/Jeremy.Gibbons/publications/efolds.pdf but running into trouble getting things to typecheck. Given the types data Nest a = Nil | Cons(a, (Nest (Pair a))) type Pair a = (a,a) and the following function to map over pairs: pair f (a,b) = (f a, f b) the paper indicates that an efficient fold over a nest is defined as follows efold :: forall n m b. (forall a. n a) -> (forall a . (m a, n (Pair a)) -> n a) -> (forall a. Pair (m a) -> m (Pair a)) -> (forall l z. (l b -> m (z b)) -> Nest (l b) -> n (z b)) efold e f g h Nil = e efold e f g h (Cons(x, xs)) = f(h x, efold e f g (g . pair h) xs) However, when I try to compile this, I get the error "Couldn't match expected type `l' against inferred type `z'". I'm new to Haskell so I'm probably missing something obvious, but this code looks to me like it ought to work. Any thoughts on what I'm doing wrong? Thanks, Keith

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Compiling on windows - again
by Gü?nther Schmidt 29 Apr '09

29 Apr '09

Hi guys, I'm sorry, I asked this before ... What is the flag you have to pass during ghc --make .... in order to produce an exe on Windows that doesn't open a DOS window. Günther

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Question about implementing an off-side rule in Parsec 2
by Bas van Gijzel 29 Apr '09

29 Apr '09

Hello everyone, I'm doing a bachelor project focused on comparing parsers. One of the parser libraries I'm using is Parsec (2) and I'm going to implement a very small subset of haskell with it, with as most important feature the off-side rule (indentation based parsing) used in function definitions and possibly in where clauses. But I'm still a bit stuck on how to implement this cleanly. I tried to search for some examples on blogs but I haven't found anything yet. As far as I can see the way to go would be using getState and updateState methods defined in Parsec.Prim and to use the methods in Parsec.Pos to compare the difference in indendation for tokens. But I haven't completely wrapped my head around any state monad yet and I don't understand Parsec enough yet to see how to use the methods Parsec.Pos and state easily. Some examples or pointers to something to read would really be helpful. Thanks in advance, Bas van Gijzel

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Re: [Haskell-cafe] chr/ord?
by michael rice 28 Apr '09

28 Apr '09

Thank guys, Now what am I misunderstanding in the code below? I would think that *Main> comb (Just 65) foo and *Main> comb (lookup 'A' lst) foo would return the same result Just 'A' Michael ===========Haskell code============= import Data.Char lst = [('A',65),('B',66),('C',67),('D',68)] -- 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 foo :: Int -> Maybe Char foo n = Just (chr n) ================Ghci=========== [michael@localhost ~]$ ghci 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. Prelude> :load test1 [1 of 1] Compiling Main ( test1.hs, interpreted ) Ok, modules loaded: Main. *Main> comb (Just 65) foo Just 'A' *Main> comb Nothing foo Nothing *Main> lookup 'A' lst Just 65 *Main> (lookup 'A' lst) Just 65 *Main> comb (lookup 'A' lst) foo <interactive>:1:22: Couldn't match expected type `Integer' against inferred type `Int' In the second argument of `comb', namely `foo' In the expression: comb (lookup 'A' lst) foo In the definition of `it': it = comb (lookup 'A' lst) foo *Main> --- On Tue, 4/28/09, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: From: Lennart Augustsson <lennart.augustsson(a)gmail.com> Subject: Re: [Haskell-cafe] chr/ord? To: "Tim Wawrzynczak" <inforichland(a)gmail.com> Cc: "michael rice" <nowgate(a)yahoo.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Tuesday, April 28, 2009, 10:53 PM You can also use the more general toEnum and fromEnum. -- Lennart (iPhone) On Apr 29, 2009, at 2:13, Tim Wawrzynczak <inforichland(a)gmail.com> wrote: Michael, those functions are not in the Prelude, they're in Data.Char. On Tue, Apr 28, 2009 at 8:08 PM, michael rice <nowgate(a)yahoo.com> wrote: Hi, My Prelude docs must be out of date because chr and ord don't seem to be there. How do I access these functions? Michael =============== [michael@localhost ~]$ ghci 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. Prelude> chr 65 <interactive>:1:0: Not in scope: `chr' Prelude> ord 'A' <interactive>:1:0: Not in scope: `ord' Prelude> _______________________________________________ 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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