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Announce: Decimal arithmetic library (questions inside)
by Jeremy Shaw 17 Oct '05

17 Oct '05

Announcment: ------------ I am proud to announce the *premature* release of my Decimal arithmetic library: darcs get http://www.n-heptane.com/nhlab/repos/Decimal It implements real decimal arithmetic, for cases where the binary floating point is not acceptable (money, etc). Need Advice: ----------- I am currently stuck trying to figure out how to implement contexts. The decimal operations are supposed to be executed in a context that determines things such as, type of rounding, how many digits of precision, what signals should be considered exceptions, etc. More details at the end of this page: http://www2.hursley.ibm.com/decimal/damodel.html Ideally, the user should be able to have multiple contexts in the same program. So far the best solution I have seen is the solution presented in 'Functional Pearl: Implicit Configurations' to deal with modular arithmetic. http://www.eecs.harvard.edu/~ccshan/prepose/prepose.pdf However, this solution seems a bit over the top -- I would like to implement things such that the finished product could be included in fptools. One option would be to define a unique type for each context. So by default I could provide DecimalFloat, and DecimalDouble. If the user wants a different context then they create a new type. This is a bit annoying however, because the user will have to create a bunch of boilerplate code to add the new type to all the right classes. I could probably automate that with TH however... Another option might be to include the context in each Decimal value. However, to check for context mismatches at compile time will probably require encoding the context into a phantom type, or something of that nature... I have not done a lot of numeric programming, so I am don't really know what people are even expecting or used to. Background: ---------- I plan to use this library as the basis of a Money library. The python folks have done a fine job of decribing why I am doing this which you can read here: http://www.python.org/peps/pep-0327.html I have been working from this spec so far: http://www2.hursley.ibm.com/decimal/decarith.html What's Done So Far: ------------------ I have created instances for Decimal for the type classes: ~ Eq ~ Ord ~ Num (precision/rounding not enforced correctly) ~ Fractional I am also going to investigate adding instances for: ~ Real ~ RealFrac ~ RealFloat ~ Floating The Future: ----------- There is still a fair bit of work to do long term: ~ Handle contexts ~ Enforce precision limits ~ Finish test suite ~ Add additional class instances ~ Optimize for speed But, I want to get the context stuff figured out first, so I don't have to rewrite everything later. Thanks! Jeremy Shaw.

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RE: [Haskell-cafe] pre-built ghc 6.4.1 crashes building FPS
by Simon Peyton-Jones 17 Oct '05

17 Oct '05

Joel A released GHC should never crash. Can you submit a bug report please (see "Report a bug" on GHC's home page)? In doing so, please attach the source code that provokes the crash. Thanks Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Joel | Reymont | Sent: 17 October 2005 14:54 | To: Haskell Cafe | Subject: [Haskell-cafe] pre-built ghc 6.4.1 crashes building FPS | | Folks, | | I downloaded and installed the pre-built ghc 6.4.1 on Windows XP and | Win2k. I tried building FastPackedStrings but ghc crashes soon after | starting. Any tips? | | I'm gonna try with a hand-built ghc but somehow doubt it will help. | | Thanks, Joel | | -- | http://wagerlabs.com/ | | | | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: [Haskell-cafe] Interest in helping w/ Haskell standard
by Simon Marlow 17 Oct '05

17 Oct '05

On 12 October 2005 23:50, Sebastian Sylvan wrote: > (I'm specifically interested in seeing SPJ's records proposal > included, and a new module system). Highly unlikely, IMHO. A new revision of the Haskell standard is not the place for testing new research, rather it's a clear specification of existing well-understood language features. If you want a new record system, or a new module system, now is the time to start designing and implementing them ready for the next standardisation process. Cheers, Simon

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RE: Re[2]: [Haskell-cafe] newtype is superfluous
by Simon Marlow 17 Oct '05

17 Oct '05

On 15 October 2005 19:08, Bulat Ziganshin wrote: > Hello Lennart, > > Saturday, October 15, 2005, 5:03:50 PM, you wrote: > >> Alas, Haskell is more subtle than that. Which is why newtype exists. > >> Try >> case A _|_ of A _ -> 1 >> with the two versions of A to see the difference. > > this have practical usage?? may be for such very special things > strictness annotation will be enough?? The semantics of newtype are really decided by its implementation. The basic idea is that a newtype of T can be represented in the same way as T. Once you know this, it pretty much fixes what semantics you can expect - the semantics of a newtype of T must be the same as T, because the runtime cannot distinguish the two. Cheers, Simon

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Newbie question on Haskell type
by Huong Nguyen 17 Oct '05

17 Oct '05

Hi all, I want to write a small functionto test whether an input is a String or not. For example, isString::(Show a) =>a ->Bool This function will return True if the input is a string and return False if not Any of you have idea about that? Thanks in advance

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Re: [Haskell-cafe] Generic parser without GADTs
by Huong Nguyen 17 Oct '05

17 Oct '05

Hi all, You are talking about parsing and data type, so I want to ask you a question relating to my data type. I have a data types and a function as follows: \begin{code} data Type a = C1 { x :: [String] ,y :: Type a} | C2 {x1 :: String} | C3 {y1 :: Bool} deriving Show showType :: Type a -> String showType (C2 a) = show a showType (C3 a) = show a showType (C1 a b) = show "(" ++ (show a) ++ (showType b) ++ (show ")") r1 = C1 { x = ["x","y"] , y = C2{x1 = "x^2 + y^2 < 20"} } r2=C3{y1=False} r3=C2{x1="x+y=z"} \end{code} --Result: Main>showType r1 "\"(\"[\"x\",\"y\"]\"x^2 + y^2 < 20\"\")\"" Main>showType r2 "False" Main>showType r3 "\"x+y=z\"" Now, I want to write a function to see the result in its original type, for example : input: "False" output: False input:"\"x+y=z\"" output:"x+y=z" input:"\"(\"[\"x\",\"y\"]\"x^2 + y^2 < 20\"\")\"" output:(["x","y"]x^2 + y^2 < 20") So, how I should write the function ? Do you have any idea about this ? Thanks. From: oleg(a)pobox.com <oleg(a)pobox.com > Date: Oct 16, 2005 8:50 PM Subject: [Haskell-cafe] Generic parser without GADTs To: haskell-cafe(a)haskell.org Cc: ralf(a)informatik.uni-bonn.de Also inspired by Ralf Hinze's post, I thought of removing GADTs from that code. The result is Haskell98! code, which works well in Hugs. The code seems to be a bit simpler too. Like the original code, the function 'parseAny' correctly discriminates between the list of characters (i.e., strings) and the list of other things. {-- Haskell98! --} class Type a where parse :: ReadS a > hugs /tmp/h.hs Haskell 98 mode: Restart with command line option -98 to enable extensions Main> parseAny "4711" [(ValInt 4711,"")] Main> parseAny "\"4711\"" [(ValString "4711","")] Main> parseAny "[4711, 0]" [(ValList [ValInt 4711,ValInt 0],"")] Main> parseAny "[4711, 'a']" [(ValList [ValInt 4711,ValChar 'a'],"")] Main> parseAny "[\"hello world\"] x" [(ValList [ValString "hello world"]," x")] ______________________________ _________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Generic parser without GADTs
by oleg＠pobox.com 17 Oct '05

17 Oct '05

Also inspired by Ralf Hinze's post, I thought of removing GADTs from that code. The result is Haskell98! code, which works well in Hugs. The code seems to be a bit simpler too. Like the original code, the function 'parseAny' correctly discriminates between the list of characters (i.e., strings) and the list of other things. {-- Haskell98! --} class Type a where parse :: ReadS a newtype Str = Str{unStr :: String} instance Type Char where parse = reads instance Type Int where parse = reads instance Type Str where parse = Str <$> reads instance Type a => Type [a] where parse = parseList parse instance Type Value where parse = parseAny data Value = ValChar Char | ValInt Int | ValString String | ValList [Value] deriving (Show) parseAny = ValChar <$> parse <+> ValInt <$> parse <+> ValString <$> (unStr <$> parse) <+> ValList <$> parse -- The following is Ralf Hinze's code verbatim parseList parsea = readParen False (\ s -> [ xs | ("[", t) <- lex s, xs <- parsel t ]) where parsel s = [ ([], t) | ("]", t) <- lex s ] ++ [ (x : xs, u) | (x, t) <- parsea s, (xs, u) <- parsel' t ] parsel' s = [ ([], t) | ("]", t) <- lex s ] ++ [ (x : xs, v) | (",", t) <- lex s, (x, u) <- parsea t, (xs, v) <- parsel' u] infix 8 <$> infixr 6 <+> (f <$> p) s = [ (f a, t) | (a, t) <- p s ] (p <+> q) s = p s ++ q s Tests: > hugs /tmp/h.hs Haskell 98 mode: Restart with command line option -98 to enable extensions Main> parseAny "4711" [(ValInt 4711,"")] Main> parseAny "\"4711\"" [(ValString "4711","")] Main> parseAny "[4711, 0]" [(ValList [ValInt 4711,ValInt 0],"")] Main> parseAny "[4711, 'a']" [(ValList [ValInt 4711,ValChar 'a'],"")] Main> parseAny "[\"hello world\"] x" [(ValList [ValString "hello world"]," x")]

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Estonia and GADT
by Bulat Ziganshin 16 Oct '05

16 Oct '05

Hello Haskell, 1) where can i find articles from Estonia conference? 2) they all say that GADT is great, but i personally don't "feel" GADTs. can anyone write a paper about it for beginners like me, or may be just collect examples of using GADT in real programs? -- Best regards, Bulat mailto:bulatz@HotPOP.com

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newtype is superfluous
by Bulat Ziganshin 15 Oct '05

15 Oct '05

Hello Haskell, number of type definition statements in Haskell (data, type, newtype) is a bit too large. at least, newtype definition seems to be superfluous - it can be replaced by the same `data` definition: newtype A = A Int and data A = A Int is equal, excluding for internal representation. we can exclude newtype definition and rely on the assumption that compiler is smart enough to optimize `data` definitions with just one constructor containing one field (moreover, i prefer something like `alias` to defining aliases for existing types, and `type` for defining new types, but i think it's a bit too late to complain about it :) ) -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Emptying a list
by poogle＠netspace.net.au 15 Oct '05

15 Oct '05

Before you read any more, let me just say I'm fairly new to Haskell, so forgive me if this is really basic stuff. Hi there, I'm just wondering if there is a command for emptying a list? Also, is there any way to incorporate list operations (concatenation in particular) in a do-statement on lists? Every time I try it gives a type error. Thanks Jeremy Frech ------------------------------------------------------------ This email was sent from Netspace Webmail: http://www.netspace.net.au

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