January 2006 - Haskell-Cafe

In for a penny, in for a pound.
by David F. Place 10 Jan '06

10 Jan '06

Hi All, Regarding the Fannkuch Shootout Entry: If we are willing to specialize flop in the way shown on the wiki, another 8% can be gained by similarly specializing rotate: rotate 2 (x1:x2:xs) = x2:x1:xs rotate 3 (x1:x2:x3:xs) = x2:x3:x1:xs rotate 4 (x1:x2:x3:x4:xs) = x2:x3:x4:x1:xs rotate 5 (x1:x2:x3:x4:x5:xs) = x2:x3:x4:x5:x1:xs rotate 6 (x1:x2:x3:x4:x5:x:6:xs) = x2:x3:x4:x5:x:6:x1:xs rotate 7 (x1:x2:x3:x4:x5:x:6:x7:xs) = x2:x3:x4:x5:x:6:x7:x1:xs rotate 8 (x1:x2:x3:x4:x5:x:6:x7:x8:xs) = x2:x3:x4:x5:x:6:x7:x8:x1:xs rotate 9 (x1:x2:x3:x4:x5:x:6:x7:x8:x9:xs) = x2:x3:x4:x5:x: 6:x7:x8:x9:x1:xs rotate 10 (x1:x2:x3:x4:x5:x:6:x7:x8:x9:x10:xs) = x2:x3:x4:x5:x: 6:x7:x8:x9:x10:x1:xs rotate n (x:xs) = rot' n xs where rot' 1 xs = x:xs rot' n (x:xs) = x:rot' (n-1) xs Cheers, David -------------------------------- David F. Place mailto:d@vidplace.com

6 7

Re: [Haskell-cafe] Intersection types for Haskell?
by Brian Hulley 10 Jan '06

10 Jan '06

José Miguel Vilaça wrote: > Hi > > If I understand your problem than the following is a solution: > > -------------------------------------------------- > > {-# OPTIONS -fglasgow-exts #-} > > class Foo a b where > g :: a -> b > > type A = {- change the following -} Int > type B = {- change the following -} Char > > instance Foo A B where > g a = {- change the following -} ' ' > > type C = {- change the following -} Float > type D = {- change the following -} String > > instance Foo C D where > g c = {- change the following -} "" > > > f :: (Foo a b, Foo c d) => a -> c -> (b, d) > f x y = (g x, g y) Thanks for the workaround. However this does not seem to be quite so general as intersection types, because it would only allow me to define f for some specific g ie the g of Foo, rather than for any general function... Regards, Brian Hulley

1 0

RE: [Haskell] Haskell DB bindings (was Re: ANN: HDBC (HaskellDatabase Connectivity)
by Bayley, Alistair 10 Jan '06

10 Jan '06

[Moving to café, too] > There are three active database libraries: HDBC, HSQL and Takusen. It > is quite disappointing from my point of view. Recently there was the > same situation with the GUI libraires. The Haskell Community is quite > small to waste efforts, developing different libraries for the same > things. When I started with HSQL there were only two database > libraries: HaSQL for ODBC and libpq for PostgreSQL. They both are > dead, I think. I decided that it is useful to have one abstract API > that can cover all database bindings. I imagine something like JDBC, > ADO or DBI for Haskell. If you guys would like this to happen then > lets discuss what we want. I would be happy to work on single project > that can satisfy all needs. > > Cheers, > Krasimir > ... The Haskell Community is quite > small to waste efforts, developing different libraries for the same > things. Yes, that could be considered undesirable. However, there is the choice for users over which API to choose that best supports their needs. In the use of an enumerator, Takusen has chosen quite a different design space from HSQL and HDBC. Whether or not that proves to be a good idea, or attractive to users, remains to be seen :-) HaskellDB+HSQL seems to be to be the most mature, and actively maintained. Although I'm keen to progress Takusen, I find it hard at present to take the necessary time, so work seems to stall. I'd have no bone to pick with HaskellDB+HSQL being promoted as the default database API, but I still want to work on Takusen, because I think there are some ideas worth exploring. When I started Takusen, I was specifically interested in using HaskellDB to interface to Oracle. At the time, HaskellDB did not use HSQL, and HSQL lacked a unifying interface, so I started work on a low-level library to Oracle (I was also keen to learn to use the FFI). The design later evolved towards the enumerator approach (Oleg persuaded me that it was a good idea) and the HaskellDB goal was never reached. Now that HSQL supports Oracle, I can choose to use HaskellDB with Oracle, so I suppose my original goal has been satisfied. Alistair ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

2 1

RE: Re[2]: [Haskell-cafe] I/O and utf8
by Bayley, Alistair 10 Jan '06

10 Jan '06

> From: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Bulat Ziganshin > > i have the question about this issue - i also want to provide > autodetection mechanism, which relies on first bytes of text files to > set proper encoding. what is the standard rules to encode utf8/utf16 > encoding used for text in file in these first bytes? Are you asking about the byte-order-mark in UTF encodings? http://www.unicode.org/faq/utf_bom.html#BOM Note that UTF8 files typically lack the BOM, as UTF8 is meant to be backwards-compatible with US7ASCII, I think. Windows Notepad is one of the few programs that will insert it if a text file is saved as UTF8. Alistair. ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

1 0

Haskell DB bindings
by oleg＠pobox.com 10 Jan '06

10 Jan '06

[moved from the main Haskell-list: I'm afraid the HDB discussion on the Haskell mailing list had already been long] Tim Docker wrote on the Haskell mailing list > The differences between HDBC and HSQL have been recently discussed. > Where does Takusen fit into this picture? From the above, it sounds > like it has quite a different API. Are all 3 of these actively > maintained? Takusen is built around an enumerator -- left fold enumerator, to be precise. That is, a database query is considered to be `left-folding' over the result table. Early termination is supported -- the iteration can be finished at any time the iteratee decided it has had enough. Cursors -- derivatives (pun intended) of the enumerator -- are supported as well, and we have a withCursor function (although, due to laziness, we haven't implemented the type-based region management for cursors. But we have talked about it!) The best supported databases are Oracle and Sqlite. For them, we implement buffer pre-allocation, pre-fetching (useful for minimizing the communication overhead if rows are small), parameter binding, handling date-time datatypes. Parameter binding is not yet supported for PostgreSQL. All interfaces support DML/DDL statements, transactions and isolation modes (if the back-end supports that), transparent translation from the DB type to a Haskell type. The user doesn't need to do anything for the translation. Database NULLs are fully handled: if the Haskell datatype is a Maybe datatype, NULL is permitted and translated into Nothing. Otherwise, NULL raises an exception. Nulls are distinguished from empty strings, provided the database server does so. Takusen has two layers. The low-level API (which is not actually meant for the end user) reconciles the differences among various DB interfaces. Currently supported are Oracle, Sqlite, PostgreSQL, Stub (that is, no DB -- useful for testing), and the ntwdblib interface to MSSqlServer (Microsoft encourages a newer interface though). Krasimir Angelov wrote: > Can you explain how do you manage the lifetime of handles in Takusen? > It isn't clear to me where are the advantages of enumerators. The handles are not visible to the user. The user cannot close the handle (or forget to close the handle) because the user is not given the handle in the first place. Just as we left-fold over a list, we are not given the pointer to the ``current list''. We are merely given the current element of the list. Perhaps the following links might be of help: http://www.eros-os.org/pipermail/e-lang/2004-March/009643.html http://pobox.com/~oleg/ftp/Haskell/misc.html#fold-stream http://pobox.com/~oleg/ftp/papers/LL3-collections-enumerators.txt

1 0

Is there a notion for identity?
by Tim Walkenhorst 09 Jan '06

09 Jan '06

{- Disclaimer: I'm rather new to Haskell and completely new to this board. I may not use the correct terminology in all cases, but I hope my intention becomes clear when you have a look at the code-snippets. -} Hey ho, Is there any way two find out whether two variables refer to the same cell? That is I'm looking for a notion of identity (compareable to == in Java) as opposed to equality (== in Haskell). The motivation is that I need to find out whether two infinite structures are identical. Consider this Haskell implementation of the simple typed lambda calculus (STLC) with the primitive Type "TyUnit" and the primitive value "TmUnit": data Type = TyUnit | TyArrow Type Type deriving (Show,Eq) data Term = TmUnit | TmVar String | TmAbs String Type Term | TmApp Term Term deriving Show {- Context is some LIFO structure which stores Argument-Type bindings: -} data Context = {- ... -} ctxEmpty :: Context addBinding :: Context -> String -> Type -> Context getBinding :: Context -> String -> Type typeof :: Context -> Term -> Type typeof _ TmUnit = TyUnit typeof ctx (TmVar s) = getBinding ctx s typeof ctx (TmAbs s ty1 t) = let ctx' = addBinding ctx s ty1 ty2 = typeof ctx' t in TyArrow ty1 ty2 typeof ctx (TmApp t1 t2) = let (TyArrow ttp tte) = typeof ctx t1 tta = typeof ctx t2 {- (1) problem here: -} in if tta==ttp then tte else error "..." -- eval omitted... The STLC does not provide a notion for recursion as it stands. My idea was to simulate recursion by using infinte structures of type Type. Consider this definition of the omega-combinator: omegaType :: Type omegaType = TyArrow omegaType omegaType omega, omegaAbs :: Term omega = TmApp omegaAbs omegaAbs omegaAbs = TmAbs "x" omegaType (TmApp (TmVar "x") (TmVar "x")) Now it would be nice if "typeof ctxEmpty omega" would return omegaType. Unfortunately it doesn't. Instead it loops on line ((1)), since omegaType and omegaType will never be compared to be equal using (==). If we had a function `sameAddress` we could write "tta `sameAddress` ttp || tta==ttp" instead of "tta==ttp" in line ((1)). If I interpret the chapter about infinite lists in the bird book correctly, this should be sound. omegaType will be represented by a cyclic graph in the runtime system and will therefore always refer to the same cell. The issue becomes a little more complicated if we consider: omegaType, omegaType' :: Type omegaType = TyArrow omegaType omegaType omegaType' = TyArrow omegaType' omegaType' omega, omegaAbs, omegaAbs' :: Term omega = TmApp omegaAbs omegaAbs' omegaAbs = TmAbs "x" omegaType (TmApp (TmVar "x") (TmVar "x")) omegaAbs' = TmAbs "x" omegaType' (TmApp (TmVar "x") (TmVar "x")) In this case it wouldn't be clear whether " omegaType `sameAddress` omegaType' " should be True or False, as a clever compiler might detect that both are identical and make them refer to the same address (, correct me if I'm wrong). This might be a reason that there is no `sameAddress`. My questions are: Is there any way to implement "sameAddress" in Haskell? Or is it at least possible to implement it with GHC using some compiler-specific notation? Thanks, Tim

3 4

Parsec Question
by Gerd M 09 Jan '06

09 Jan '06

I'm trying to use parsec for parsing a custom input stream. As far as I understood the manual correctly I need to define the primitive parser: type MyParser a = GenParser (SourcePos,Tok) () a mytoken :: (Tok -> Maybe a) -> MyParser a mytoken test = token showToken posToken testToken where showToken (pos,tok) = show tok posToken (pos,tok) = pos testToken (pos,tok) = test tok The problem is, since SourcePos is an abstract datatype, how can I actually run this parser without explicitly using values of type SourcePos in the input stream? Many thanks in advance! _________________________________________________________________ Express yourself instantly with MSN Messenger! Download today it's FREE! http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/

3 3

Re: [Haskell] Re: haskell.org Public Domain
by Brian Sniffen 09 Jan '06

09 Jan '06

> > It might be nice to at least include some disclaimers of warranty. I'm not a lawyer. But those US copyright lawyers I've spoken with have expressed doubts about anybody's ability to put things into the public domain. Certainly, if you put it in the public domain, you can't also disclaim a warranty. I suspect that some license that is a superset of the following would be best: * The documentation of GHC * The documentation of Hugs * The Haskell 98 report I think that works out to be "BSD, with an option for authors to copyleft pages or code as they wish." On the other hand, I also think MoinMoin has lots to offer over Mediawiki---not least ease of maintenance and improvements such as license markers. -- Brian T. Sniffen bts(a)alum.mit.edu or brian.sniffen(a)gmail.com http://www.evenmere.org/~bts

1 0

Expanding do notation
by David F. Place 09 Jan '06

09 Jan '06

Hi All, Is there a program for expanding 'do' notation? I am trying to understand why the following code (from the Fannkuch entry) doesn't hold onto the list 'p' causing a space leak. main = do n <- getArgs >>= return . read . head let p = permutations [1..n] mapM_ (putStrLn . concatMap show) $ take 30 p putStr $ "Pfannkuchen(" ++ show n ++ ") = " putStrLn . show $ foldl' (flip (max . steps 0)) 0 p If I add a line which refers to 'p' at the end, there is a space leak. print (head p) Thanks. Cheers, David ---------without extra line ./Latest 9 +RTS -K256m -c -sstderr 123456789 ... Pfannkuchen(9) = 30 244,215,012 bytes allocated in the heap 613,972 bytes copied during GC 3,920 bytes maximum residency (1 sample(s)) 931 collections in generation 0 ( 0.06s) 1 collections in generation 1 ( 0.00s) 1 Mb total memory in use INIT time 0.00s ( 0.01s elapsed) MUT time 2.20s ( 2.72s elapsed) GC time 0.06s ( 0.13s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.26s ( 2.86s elapsed) %GC time 2.7% (4.5% elapsed) Alloc rate 111,006,823 bytes per MUT second Productivity 97.3% of total user, 76.9% of total elapsed --------with extra line ./Latest 9 +RTS -K256m -c -sstderr ... Pfannkuchen(9) = 30 [1,2,3,4,5,6,7,8,9] 244,216,248 bytes allocated in the heap 19,804,816 bytes copied during GC 10,586,416 bytes maximum residency (6 sample(s)) 931 collections in generation 0 ( 0.96s) 6 collections in generation 1 ( 1.28s) 18 Mb total memory in use INIT time 0.00s ( 0.01s elapsed) MUT time 2.30s ( 2.72s elapsed) GC time 2.24s ( 3.15s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 4.54s ( 5.88s elapsed) %GC time 49.3% (53.6% elapsed) Alloc rate 106,180,977 bytes per MUT second Productivity 50.7% of total user, 39.1% of total elapsed -------------------------------- David F. Place mailto:d@vidplace.com

10 18

Where does memory go?
by Joel Reymont 09 Jan '06

09 Jan '06

I compiled a simple one-liner: main = print "Blah". This is the GC report: 5,620 bytes allocated in the heap 0 bytes copied during GC 0 collections in generation 0 ( 0.00s) 0 collections in generation 1 ( 0.00s) 1 Mb total memory in use Where did the memory go? What is 5K and what is (more suprisingly) 1Mb? Thanks, Joel -- http://wagerlabs.com/

4 3