- Haskell-Cafe - Haskell.org

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 17 Oct '05

17 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 16 Oct '05

16 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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Examples of using STUArray + help with converting code
by joel reymont 14 Oct '05

14 Oct '05

Folks, Are there any examples on using STUArray and friends? I'm trying to convert the following bit of code which uses deprecated features. I don't understand the syntax needed to create a new double or float array with newArray from Data.Array.MArray. I also don't yet understand how to cast that double array to read ints from it. doubleToInts d = runST ( do arr <- newDoubleArray (1,2) writeDoubleArray arr 1 d i1 <- readIntArray arr 1 i2 <- readIntArray arr 2 return (i1,i2)) Thanks, Joel -- http://wagerlabs.com/ -- http://wagerlabs.com/

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RE: [Haskell-cafe] Re: Interest in helping w/ Haskell standard
by Bayley, Alistair 14 Oct '05

14 Oct '05

> From: Marco Tulio Gontijo e Silva [mailto:marcot@minaslivre.org] > > Em Qui, 2005-10-13 às 09:47 +0100, Bayley, Alistair escreveu: > > ***************************************************************** > Confidentiality Note: The information contained in this message blah blah blah > > Is this kind of notice legal in the context of Haskell-Cafe? Sorry, but it's not under my control. My employer's email system is configured to append this to every message, and I suspect that it's not worth the hassle to try to get it removed. Alistair.

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Re: [Haskell-cafe] Newbie question on Haskell type
by robert dockins 14 Oct '05

14 Oct '05

So this is essentially a parsing problem. You want a user to be able input a string and have it interpreted as an appropriate data value. I think you may want to look at the Parsec library (http://www.cs.uu.nl/~daan/parsec.html) I don't think the direction you are heading will get the results you want. As to typeable, the basic types are mostly all members of Typeable. You can find a pretty good list here: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data.Typeable.ht… Additionally, GHC can create Typeable instances automaticly for user defined datatypes; just add a deriving Typeable clause. data SomeType = C1 | C2 deriving (Show,Eq,Typeable) The restriction is that all types which appear in constructors must also be in Typeable. I believe the DrIFT preprocessor can also create Typeable instances if you are not using GHC. Huong Nguyen wrote: > Hello, > > Thanks for your solution. > > My main purpose is that I want to input a value and check whether this > value is belong to some specific types or not. These types can be some > popular types (such as: String, Char, Int, etc) or some more complex > data structures defined by user. Thus, at first, I try with type String > (even with that simple type, I still face difficulty ;-)) > > I want to ask you which types can be used with Data.Typeable. I read for > over 15 minutes but it is still not clear with me. > For some other complex data types defined by user, what I should do to > use Data.Typeable ? > > Thank you very much. > > On 10/13/05, *robert dockins* <robdockins(a)fastmail.fm > <mailto:robdockins@fastmail.fm>> wrote: > > In GHC you can do this: > > > import Data.Typeable > > > isString :: (Typeable a) => a -> Bool > > isString x = typeOf x == typeOf (undefined::String) > > Why do you want this? It's not the kind of operation one does very > often in Haskell. > > > Huong Nguyen wrote: > > > 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 > > > > > > > ------------------------------------------------------------------------ > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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RE: [Haskell-cafe] Re: Interest in helping w/ Haskell standard
by Bayley, Alistair 14 Oct '05

14 Oct '05

> From: Stephane Bortzmeyer [mailto:bortzmeyer@nic.fr] > > As someone who is not an academic researcher and not a student in CS, > I would like to express a personal opinion; we don't need a new > standard. To me, Haskell needs more libraries, more users (which means > more debugging and more documentations), more implementations, of > course more real applications (darcs did a lot of the success of > Haskell), so we can read their code, push sysadmins to install > Haskell, etc. A significant number of Haskell libraries, both user-contributed and compiler-bundled (in the "standard" library hierarchy), are not written in Haskell98, and rely on a number of (typically well supported) language extensions like multi-parameter type classes + functional dependencies, existential types, hierarchical modules, etc. Some libraries target a specific compiler (often GHC). It should be clear that standardising the popular extensions would make a greater number of compiler-bundled and user-contributed libraries standards compliant, which has the obvious benefit that they can be used with any standardised compiler/interpreter i.e. they become more portable. 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. *****************************************************************

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Re: [Haskell-cafe] Newbie question on Haskell type
by voigt.16734551＠bloglines.com 14 Oct '05

14 Oct '05

--- Cale Gibbard <cgibbard(a)gmail.com wrote: > As an example of this sort of thing, I know that there are only 4 > values of type a -> Bool (without the class context). They are the > constant functions (\x -> True), (\x -> False), and two kinds of > failure (\x -> _|_), and _|_, where _|_ is pronounced "bottom" and > represents something along the lines of nontermination (aborting the > program also counts). Not exactly. There are also (\x -> seq x True) and (\x -> seq x False), neither of which is equivalent to any of the four functions above. Regards, Janis Voigtlaender.

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Interest in helping w/ Haskell standard
by Isaac Jones 14 Oct '05

14 Oct '05

At the end of the Haskell Workshop at ICFP, we had the traditional "Future of Haskell" discussion (chaired by Andres Loeh). One of the main topics was the perceived need of a new standard, because the Haskell 98 standard is quite old already, and Haskell has evolved in the meantime, leading to a situation where almost none of the actually existing Haskell programs is according to the 98 standard. No clear opinion was visible on what form a new standard would take. There was, however, considerable support for the idea to standardize an incremental and moderate extension to Haskell 98 (working name "Haskell 06" or "Industrial Haskell"). This effort would then be separate from discussion about the Real Next Version (dubbed "Haskell 2"). John Launchbury asked for a show of hands of those who would be interested in helping out with the "Haskell 06" standard. I think "helping" means "willing to spend a non-trivial amount of time". That is, it's pretty well expected that most Haskellers will be willing to contribute to discussion on the mailing lists, but we're trying to get a list of those who want to take it to the next level. If you raised your hand, or if you think this describes you, please email John Launchbury at john(a)galois.com. peace, Isaac Jones & Andres Loeh

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