- Haskell-Cafe - Haskell.org

RE: [Haskell-cafe] Re: Module.T naming style
by Simon Marlow 18 Jul '05

18 Jul '05

On 15 July 2005 11:53, Malcolm Wallace wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: > >> I hope you weren't including GHC in "most compilers" :-) GHC's >> implementation of mutually recursive modules is conformant with >> Haskell 98 (see Section 5.7). > > It depends on how you interpret this phrase: > "may require that imported modules contain additional information" > If you count writing a separate .hs-boot file as being the additional > information that is somehow "contained" within the module, then I > suppose nhc98 is also conformant with the report. True, I considered mentioning this detail, but decided against it - the fact that the type signatures aren't actually in the same file isn't really important. We follow the spirit of the specification. You guys should really document that nhc98 support mut. rec. modules too. > The only difference with nhc98 is that you hand-write a .hi file, > rather than a .hs-boot file, but the syntax is very similar to plain > Haskell. Also, the hand-written .hi file will be replaced with a > machine-generated one after compilation, but that is only a problem > if the user inadvertently removes all the .hi files. Hand-writing .hi files is how GHC used to work (up to version 0.29, IIRC). It's not entirely satisfactory because you can get into situations where you have to 'make' several times to get to a fixed point, and you can construct examples that never reach a fixed point. And 'make' complains about recursive dependencies a lot. What does hmake do? It used to be Really Fun(TM) when GHC was built like this... you could improve GHC's performance by simply typing 'make' a couple more times to let the inter-module optimisations propagate a bit further :) Cheers, Simon

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IO platform for testing
by yin 18 Jul '05

18 Jul '05

Hello all, first I have to apologise for my English. So, I don't fully understand haskells IO, but I need some testing 'main' function: ./bundle01 <cmd> <arg1> <arg2> ... I more apoarches, but no one worked. Now I tried this: m01_mod :: Int -> Int -> Int b01_mod a b = a mod b main = do (cmd:ss) <- getArgs putStrLn ( if (cmd == "mod") then do show (b01_mod (read (ss!!1)) (read (ss!!2))) else do "unknown command!") Please, send me somethung like this (I need to understand it). I need runing more <cmd>s and with various argument counts and types. All functions "in a bundle" should be in one file. Thanks to you all :) Matej 'Yin' Gagyi

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flushing trace debug output before getting other output
by Terrence Brannon 18 Jul '05

18 Jul '05

I'm working on exercise 6.16 in Thompson's "Haskell; the Craft of Functional Programming" My goal for the moment is to call printPicture to see my results: type Picture = [[Char]] printPicture :: Picture -> IO () printPicture = putStr . concat . map (++"\n") And everything is encouraging. The return type of my function syncs up: *Main> :t superimposeImage imgA imgB superimposeImage imgA imgB :: [[Char]] Now, the only issue is that I have put a number of trace statements in my program whose output is *interspersed* with the return data. The return data is simply lists of characters, where each character is "." or "#" *Main> printPicture $ superimposeImage imgA imgB siPointLogic i1 i2 0 15 .siPointLogic i1 i2 1 15 .siPointLogic i1 i2 2 15 .siPointLogic i1 i2 3 15 .siPointLogic i1 i2 4 15 .siPointLogic i1 i2 5 15 .siPointLogic i1 i2 6 15 .siPointLogic i1 i2 7 15 .siPointLogic i1 i2 8 15 .siPointLogic i1 i2 9 15 .siPointLogic i1 i2 10 15 .siPointLogic i1 i2 11 15 When I remove my various trace statements, I get what I want: *Main> printPicture $ superimposeImage imgA imgB ................ ................ ................ ................ ...........##... So the question is, how can I leave the trace statements in the code and yet get the printPicture output after all the debugging output has output? It only makes sense for it to work that way. The trace output occurs during the program and in particular, during the program *prior* to the call to printPicture. Therefore all of the trace output should have finished *prior* to printPicture outputting it's rows. -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.

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Re: [Haskell-cafe] IO platform for testing
by Frank-Andre Riess 18 Jul '05

18 Jul '05

Hi there, > There are some other, minor problems -- for indexing you need ss!!0, the > name m01_mod, and you may need an 'import' to make getArgs visible. yup, and you have to get rid of the two "do"s in the application of putStrLn. Regards, Frank-Andre Riess

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RE: [Haskell-cafe] Unicode.hs (was: RE: Re[2]: ANNOUNCE: GHC surveyresults)
by Simon Marlow 17 Jul '05

17 Jul '05

On 30 June 2005 14:36, Dimitry Golubovsky wrote: > It is in CVS now, and I believe will be in 6.4.1 Not planned for 6.4.1, but definitely in 6.6. Cheers, Simon

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O'Haskell
by Monique Louise 16 Jul '05

16 Jul '05

Is there any project for extending GHC or any other Haskell compiler with support to O'Haskell / any object-oriented Haskell extesion? That could be very useful for improving Haskell interoperability with OO languages. Thanks in advance, Monique Monteiro -- ________________________________ Monique Louise B.Monteiro Msc Student in Computer Science Center of Informatics Federal University of Pernambuco

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weired
by wenduan 16 Jul '05

16 Jul '05

The following function which converts a number represents a sum of money in pence didn't work as expected and the result didn't make any sense to me: penceToString :: Price -> String penceToString p = let str = show p len = length str in if len ==1 then "0.0" ++ str else if len ==2 then "0." ++ str else (take (len-2) str) ++ "." ++ (drop (len - 2) str ) *Main> penceToString 234566666667899999999786 "-6710990.94" ?!?! -- X.W.D

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Problem linking against Data.Graph
by Andy Gimblett 16 Jul '05

16 Jul '05

Hi all, I'm something of a Haskell newbie, and this is my first post to this list. I'm trying to do some very basic things with graphs, but can't get started with Data.Graph. In particular, linking fails - I'm wondering if something's wrong with my install. (For the record: ghc-6.4 compiled/installed fresh today on gentoo linux - it _looks_ OK to my Haskell-untrained eyes). Here follows a small example which I believe should compile just fine: ---- Code begins "Main.hs" module Main where import Data.Graph.Inductive import System.IO main :: IO () main = putStrLn "Hello, world!" ---- Code ends As you can see, this basically does nothing! Unfortunately, here's what happens: [csandy@cspcag spa_haskell] ghc -o Main Main.hs Main.o(.text+0x145): In function `__stginit_Main_': : undefined reference to `__stginit_DataziGraphziInductive_' collect2: ld returned 1 exit status I get similar errors whatever I try to do with Data.Graph. Trying to compile ghc-6.4/libraries/fgl/test/test.hs (from the source tree) gives me multiple errors of the above type (though not identical). If anyone can shed any light on this, or tell me what stupid thing I'm doing wrong, I'd really appreciate it. Many thanks, -Andy -- Andy Gimblett Computer Science Department University of Wales Swansea http://www.cs.swan.ac.uk/~csandy/

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weird
by wenduan 16 Jul '05

16 Jul '05

The following function which converts a number represents a sum of money in pence didn't work as expected and the result didn't make any sense to me: penceToString :: Price -> String penceToString p = let str = show p len = length str in if len ==1 then "0.0" ++ str else if len ==2 then "0." ++ str else (take (len-2) str) ++ "." ++ (drop (len - 2) str ) *Main> penceToString 234566666667899999999786 "-6710990.94" ?!?! -- X.W.D

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Functional dependencies and type inference
by Einar Karttunen 15 Jul '05

15 Jul '05

Hello I am having problems with GHC infering functional dependencies related types in a too conservative fashion. > class Imp2 a b | a -> b > instance Imp2 (Foo a) (Wrap a) > > > newtype Wrap a = Wrap { unWrap :: a } > data Foo a = Foo > data Proxy (cxt :: * -> *) > > foo :: Imp2 (ctx c) d => Proxy ctx -> (forall a b. (Imp2 (ctx a) b) => a -> b) -> c -> d > foo p f x = f x The type of "foo (undefined :: Proxy Foo)" is inferred as "forall c. (forall a b. (Imp2 (Foo a) b) => a -> b) -> c -> Wrap c" which shows the outmost functional dependence is working fine. ctx is carried to the inner Imp2. However "foo (undefined :: Proxy Foo) Wrap" will fail complaining that Couldn't match the rigid variable `b' against `Wrap a' `b' is bound by the polymorphic type `forall a b. (Imp2 (ctx a) b) => a -> b' at <interactive>:1:0-32 Expected type: a -> b Inferred type: a -> Wrap a In the second argument of `foo', namely `Wrap' My guess is that GHC cannot see that the functional dependency guarantees that there are no instances which make the inferred type invalid. Any solutions to this problem? - Einar Karttunen

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