- Haskell-Cafe - Haskell.org

Parsec : Problems with operator precedence
by Erik de Castro Lopo 03 Jan '09

03 Jan '09

Hi all, I'm using Text.ParserCombinators.Parsec.Expr to parse expressions for a Javascript like language. This language has C-like logical operators ('&&' and '||') and bitwise operators ('&' and '|'). Furthermore, the language definition states that bitwise operators have a higher precedence than the logical ones. I therefore have the following (trimmed): import qualified Text.ParserCombinators.Parsec.Expr as E opTable :: [[ E.Operator Char st Expression ]] opTable = [ -- Operators listed from highest precedence to lowest precedence. {- snip, snip -} [ binaryOp "&" BinOpBinAnd E.AssocLeft ], [ binaryOp "^" BinOpBinXor E.AssocLeft ], [ binaryOp "|" BinOpBinOr E.AssocLeft ], [ binaryOp "&&" BinOpLogAnd E.AssocLeft ], [ binaryOp "||" BinOpLogOr E.AssocLeft ] ] binaryOp :: String -> (SourcePos -> a -> a -> a) -> E.Assoc -> E.Operator Char st a binaryOp name con assoc = E.Infix (reservedOp name >> getPosition >>= return . con) assoc but I still get the following parse error: unexpected "|" expecting end of "|" or term on the line: if (name == null || value == null) If I change the above from a logical to a bitwise OR, the parser accepts it quite happily. Any clues as to what I'm doing wrong here? Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- BSD: A psychoactive drug, popular in the 80s, probably developed at UC Berkeley or thereabouts. Similar in many ways to the prescription-only medication called "System V", but infinitely more useful. (Or, at least, more fun.) The full chemical name is "Berkeley Standard Distribution".

4 7

Gitit - Encoding
by Arnaud Bailly 02 Jan '09

02 Jan '09

Hello, I have started using Gitit and I am very happy with it and eager to start hacking. I am running into a practical problem: characters encoding. When I edit pages using accented characters (I am french), the accents get mangled when the page come back from server. The raw files are incorrectly encoded. Where Shall I look for fixing this issue ? Thanks ps: the wiki is live at http://www.notre-ecole.org -- Arnaud Bailly, PhD OQube - Software Engineering web> http://www.oqube.com

6 6

Minor quibble
by Louis Wasserman 02 Jan '09

02 Jan '09

Do CORE pragmas inhibit optimizations going between the code inside and outside of the pragma? This seems the case with certain code of mine, but it's not even alluded to in the GHC documentation. ;_; Louis Wasserman wasserman.louis(a)gmail.com

1 0

Beginner question
by Benjamin Bach 02 Jan '09

02 Jan '09

Supposing I have the following code: > module Main(main) where > main = putStr (show []) I will get these errors from GHC and Hugs respectively: > Main.hs:2:16: > Ambiguous type variable `a' in the constraint: > `Show a' arising from a use of `show' at Main.hs:2:16-22 > Probable fix: add a type signature that fixes these type variable(s) > ERROR "src/Main.hs":2 - Unresolved top-level overloading > *** Binding : main > *** Outstanding context : Show b But if I change my code to the following, it will compile. > module Main(main) where > main = putStr (show [1]) I have no problems typing in "putStr (show [])" in Hugs... runs fine. So what's wrong? I've really tried hard to think of an explanation, but with no luck.. Thanks, Benjamin

3 3

HDBC-ODBC - MS-Access backend problems with inserts
by Günther Schmidt 02 Jan '09

02 Jan '09

Hi, I'm using HDBC extensively in my app and it works very well. Most of the time my app uses an Sqlite database with HDBC and no problems there. The only problem I have is when I use it against an Access database backend in which I want to do inserts. Creating a connection works fine, also running queries. It only budges when I want to do inserts. *RedcomBatch.Export> handleSqlError $ run dbc "insert into mytable values (?)" [toSql "dummytext"] *** Exception: user error (SQL error: SqlError {seState = "[\"HY104\"]", seNativ eError = -1, seErrorMsg = "bindparameter 1: [\"98: [Microsoft][ODBC Microsoft Ac cess Driver]Ung\\252ltiger Genauigkeitswert. \"]"}) *RedcomBatch.Export> Any ideas why? Günther The German part of the error message reads "invalid precision value"

1 0

Re: [Haskell-cafe] representation on persistent store question
by Antoine Latter 02 Jan '09

02 Jan '09

On Jan 1, 2009 11:50pm, "Galchin, Vasili" <vigalchin(a)gmail.com> wrote: > it is a bioinformatics standard .. . I am writing on this newsgroup in order to try to be objective to get a "correct" and elegant answer .. in any case I am helping on the bioinformatics code (you can see on Hackage). I am trying to finish the 2Bit file format code ... it seems to me that bioinformatics as an area is not clearly defined .... e.g. it is unclear to me whether "offset" is a marshalled/serialized concept or or unmarshalled/unserialized concept ..... this distinction is very important .... I will have to think about more myself! > > > Regards, Vasili > Here's some code using Data.Binary to store data as offsets into a byte array. I haven't tested it too much, so it may have bugs. Maybe there's some inspiration in there. -Antoine >>>> import Data.Binary import Data.Binary.Get import Data.Binary.Put import Data.ByteString.Lazy (ByteString) import qualified Data.ByteString.Lazy as B data TestStruct = TestStruct { property1 :: ByteString , property2 :: ByteString , property3 :: ByteString } deriving Show {- The serialized format looks like (all big-endian): * first offset into data block (Word32) * second offset into data block (Word32) * third offset into data block (Word32) * length of bnary data block (Word32) * binary data block (Arbitrary binary data) -} instance Binary TestStruct where put struct = let data1 = property1 struct data2 = property2 struct data3 = property3 struct dataBlock = data1 `B.append` data2 `B.append` data3 offset1 = 0 offset2 = offset1 + B.length data1 offset3 = offset2 + B.length data2 in do putWord32be $ fromIntegral offset1 putWord32be $ fromIntegral offset2 putWord32be $ fromIntegral offset3 putWord32be $ fromIntegral $ B.length dataBlock putLazyByteString dataBlock get = do offset1 <- getWord32be offset2 <- getWord32be offset3 <- getWord32be dataBlockLength <- getWord32be dataBlock <- B.drop (fromIntegral offset1) `fmap` getLazyByteString (fromIntegral dataBlockLength) let (data1, rest1) = B.splitAt (fromIntegral $ offset2 - offset1) dataBlock (data2, rest2) = B.splitAt (fromIntegral $ offset3 - offset2 - offset1) rest1 data3 = rest2 return $ TestStruct data1 data2 data3 <<<<<

2 1

representation on persistent store question
by Galchin, Vasili 02 Jan '09

02 Jan '09

Hello, Say I have several data structures that are marshalled(using Binary class) and written out linearly on persistence store. I want to calculate the offsets in bytes of these various data structures in a functional language way. What is the "suggested" (elegant) way .... ? Regards, Vasili

2 2

The Psychology of Computer Programming
by Achim Schneider 01 Jan '09

01 Jan '09

http://books.google.com/books?id=tiMPAAAACAAJ&hl=de (by Gerald M. Weinberg) I suggest you procure a copy and read it for a) the lulz (a lot of them) b) insight into your own hack mode c) insight into how to hack recruiting and team lead d) ways to make Haskell even more subversive -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited.

1 0

Stupid question #852: Strict monad
by Brian Hurt 01 Jan '09

01 Jan '09

First off, let me apologize for this ongoing series of stupid newbie questions. Haskell just recently went from that theoretically interesting language I really need to learn some day to a language I actually kinda understand and can write real code in (thanks to Real World Haskell). Of course, this gives rise to a whole bunch of "wait- why is it this way?" sort of questions. So today's question is: why isn't there a Strict monad? Something like: data Strict a = X a instance Monad Strict where ( >>= ) (X m) f = let x = f m in x `seq` (X x) return a = a `seq` (X a) (although this code doesn't compile for reasons I'm not clear on- I keep getting: temp.hs:4:0: Occurs check: cannot construct the infinite type: b = Strict b When trying to generalise the type inferred for `>>=' Signature type: forall a b1. Strict a -> (a -> Strict b1) -> Strict b1 Type to generalise: forall a b1. Strict a -> (a -> Strict b1) -> Strict b1 In the instance declaration for `Monad Strict' as a type error. Feel free to jump in and tell me what I'm doing wrong.) Obviously, there would also be a StrictT monad transformer. The point here is that there are some monads (State being the obvious one) that come in both strict and lazy variants- the two variants could be eliminated, and the strict variant would just become a StrictT (State x) a. Or maybe a StateT x Strict a. Hmm. Which raises the interesting question of what, if any, semantic difference there is between a StrictT (State x) a and a StateT x Strict a. In any case, the idea came when I was thinking about monads being "code regions"- that we can talk about such and such a function being "in" the IO monad or STM monad. The idea was that one could define a "strict" code region you could put code in. Brian

5 10

[HUMOR] Haskell koans
by Gwern Branwen 01 Jan '09

01 Jan '09

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 In the spirit of my previous email on haikus (which turned out fairly well), I'd like to issue a RFK (Request For Koans) to everyone! You can find what I've accumulated already on http://haskell.org/haskellwiki/Koans . There are a few there, but half of them are mine. There is, however, a fairly complete external links section covering not just vaguely-Haskellish koans, but also FP and programming in general. So feel free to chip in with any koans or links to koans, if you know of any. I'm sure I must've missed a few just for #haskell alone. Example: gwern> a young novice spoke to #haskell, asking to be taught about the ways of the Maybe monad. 'What have you brought me to see?' asked the #haskeller. 'Nothing', replied the noob. 'Then take it away!'" - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkldNUkACgkQvpDo5Pfl1oJk4wCeMSsuFMI89oEHTdDCewkHQh5U CvYAn3xfgjbiwt9P9Z2+v8vnRbzztBKH =E7Pd -----END PGP SIGNATURE-----

2 1