- Haskell-Cafe - Haskell.org

Outstanding context : (Num b, Random b)
by "horsh 26 Feb '04

26 Feb '04

Hi, I am very new to haskell. Could anyone please explain why these two things are not equivalent: m2 = do a <- (drawInt 1 10) print a drawInt :: Int -> Int -> IO Int drawInt x y = getStdRandom (randomR (x,y)) m1 = do b <- getStdRandom (randomR (1,10)) print b the second one produces *** Binding : m1 *** Outstanding context : (Num b, Random b) in Hugs. thanks horsh

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Perl-ish =~ operator
by ozone＠algorithm.com.au 24 Feb '04

24 Feb '04

In my effort to turn Haskell into a language more like Perl (muahaha)[1], I got a bit fed up and implemented something like Perl 5's =~ binding operator (a.k.a. "regex" operator); I thought maybe somebody else here might find it useful. Perl has the concept of 'contexts': a function does something different depending on what type its caller expects back from the function. Sounds like a perfect abuse of type classes, to me :). Code follows: --- {-# OPTIONS -fglasgow-exts #-} {- Need this for "instance Foo [String]" declarations -} module PLRE where -- Perl-Like Regular Expressions import Text.Regex -- Perl-Like =~ operator, which changes behaviour depending on its calling -- context class RegExContext a where (=~) :: String -> String -> a instance RegExContext Bool where s =~ re = case matchRegex (mkRegex re) s of Nothing -> False Just x -> True instance RegExContext [String] where s =~ re = case matchRegex (mkRegex re) s of Nothing -> [] Just x -> x boolContextTest string regEx = case string =~ regEx of True -> print True False -> print False stringListContextTest string regEx = case string =~ regEx of (a:x) -> print ("First match: " ++ a) _ -> error "No subexpression matches" --- Some test output for you: *PLRE> boolContextTest "foo" "^f" True *PLRE> boolContextTest "foo" "^g" False *PLRE> stringListContextTest "foo" "^(.)" "First match: f" *PLRE> stringListContextTest "goo" "^(.)" "First match: g" *PLRE> stringListContextTest "" "^(.)" *** Exception: No subexpression matches Note that you have a fairly severe restriction if you want to use =~ in your code: the Haskell compiler must be able to determine a concrete type for the context that =~ is used in. i.e. if stringListContextTest was defined as: stringListContextTest string regEx = case string =~ regEx of (a:x) -> print a _ -> error "No subexpression matches" The compiler can't concretise a type for 'a', and it'll complain about not having an instance for RegExContext [a] (which is fair enough). Even with this restriction, I'm sure it'll still be useful. It shouldn't be a bit leap to define other Perl-ish operators in this fashion, such as !~, or even s/.../. Have the appropriate amount of fun! 1. Actually, I wanted to turn Haskell into a language more suitable for text processing, but that doesn't sound as evil. -- % Andre Pang : trust.in.love.to.save

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RE: [Haskell-cafe] More help for the newb
by Matthew Morvant 21 Feb '04

21 Feb '04

Thanks for the catching the typo. However, I keep getting type errors. The error that I get seems self-explanaotry, by telling me what to change the type to. But when I change it I get another error that seems to tell me to change it back. -----Original Message----- From: Philippa Cowderoy [mailto:flippa@flippac.org] Sent: Saturday, February 21, 2004 10:02 AM To: Matthew Morvant Subject: Re: [Haskell-cafe] More help for the newb On Sat, 21 Feb 2004, Matthew Morvant wrote: > 36 code3 :: [b] -> [Char] > > 37 code3 = do{char '!'; digit; char 's'; many (do char 's'); return "Suc"} > +++ do{char '%'; char 's'; digit; many (do digit): return "Suc"} > You appear to have a colon instead of a semicolon, which isn't good. Consider "many digit" rather than "many (do digit)", too. -- flippa(a)flippac.org

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Thinner secure email?
by Justin Walsh 21 Feb '04

21 Feb '04

Re: my last goof up Hi, I know, being a totally ignorant Haskell newbie, that I should soon get to know my pecking order. But this e-paper shuffle is so damned annoying. i.e. "Hello World" In .txt mode is 11 bytes. In NoteTab Lite .txt (with url; say "Ripose.com.au") is 28 bytes In WordPad .doc 4,603 kbytes In MS Word .doc 19,456 kbytes. I would have though that the first project might have been the email itself. X does not seem to have been considered here. Why is so much rubbish data going down the line? Am I asking too many question? On the squeak-dev list I would be getting; Well $#1*? stop winging and fix it. I would, if I knew that Haskell is the right tool to use, if a tool is really needed at all. Does anybody get my reasoning? At my age 64.5 I'm too old to start another language, then find out that it's another dead end. I'm not trolling (I forgot to mention, I hate trolls too). With respect Justin walsh --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com) Version: 6.0.585 / Virus Database: 370 - Release Date: 2/11/04

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5 Kb limit with notetab lite attachment
by Justin Walsh 21 Feb '04

21 Feb '04

Is this better? Please note that the entire email I first sent is 7,153 bytes and in attachment This one plus attachment is under 5 kb. I am using notetab lite. How does that rank with security i.e. I don't want to scare people with attachments. Note: I am not good at this sort of thing. Subject: Your message to Haskell-Cafe awaits moderator approval > Your mail to 'Haskell-Cafe' with the subject > > Rational Haskell? > > Is being held until the list moderator can review it for approval. > > The reason it is being held: > > Message body is too big: 6180 bytes with a limit of 5 KB >

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More help for the newb
by Matthew Morvant 21 Feb '04

21 Feb '04

I have gotten past the syntax errors and am now into type errors. Hopefully if I get some help with one I can fix the rest on my own. Here is the funct 36 code3 :: [b] -> [Char] 37 code3 = do{char '!'; digit; char 's'; many (do char 's'); return "Suc"} +++ do{char '%'; char 's'; digit; many (do digit): return "Suc"} ERROR "C:\Documents and Settings\mmorvant\Desktop\haskell.hs":37 - Type error in application *** Expression : many do {...} : return "Suc" *** Term : many do {...} *** Type : Parser [Char] *** Does not match : [Char]

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Rational Haskell?
by Justin Walsh 21 Feb '04

21 Feb '04

Please bear with me. Point of departure: My goal is OWL (but replacing the "O" for Ontological with Noological) http://www.bright.net/~jclarke/kant/history.html 2. Noology (as in noumenon or "state of mind") i.e. rational http://en.wikipedia.org/wiki/Ontology Zoology (as in phenomenon or state of body) i.e. Empirical Declaration: None of these OO languages in their current context seem to work for me. Observations: I love AWK, I don't like C++, I Like plain vanilla Prolog and Smalltalk (as procedural and declarative) IDE's, but in there current form useless for my purposes. I'm unsure of Haskell but, feel it has untapped potential at a meta level. So far I have found, that Haskell is a great "what-if" language (the comparison with the spread sheet correct). I am very much a remnant of the "how-to" generation (one cannot get much lower than assembler). I found it difficult to take responsibility and let go of my "goto". But I do need to reach the "when-to" level: my project is Virtual Reality and SAI:(asIn Silicon Assisted Intelligence is gnot AI). Somehow though all of us early (infant) programmers moved pretty seamlessly form goto's and flowcharting to pseudo-code and structure programming techniques: because it works. However in 1989 I did BR and Semantic Modeling (Debenham and Montgomery UTS), realising immediately that it did not work. Since then I still see the Systems Designers equivalent of the flowchart and the goto i.e Entity Relationship Modeling http://www-106.ibm.com/developerworks/rational/library/319.html Why is it so hard for SysDesigners to make the same mind shift we did? Sorry if I sound rude but I'm stuck in an OO rut and I need to reach Process Orientation rather quickly. Any serious suggestions (apart from "just go away")? Justin PS: I don't like dialectical bickering over detail. Please spare me the flaming --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com) Version: 6.0.585 / Virus Database: 370 - Release Date: 2/11/04

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Help a Newbie
by Matthew Morvant 20 Feb '04

20 Feb '04

I am trying (and have been for quite some time) to repair a broken Haskell script. I keep getting stuck on Last generator in do {...} must be an expression. I am betting that this is not uncommon. Can someone please help me understand? code5 :: Parser code5 = do dd <- code3 do ddd <- code3 do dddd <- code3 return "ce" code4 :: Parser code4 = do dd <- code3 do dde <- code3 digit ddd <- many (do digit) q <- code5 return (q : "ss") Thanks in advance for helping Matthew

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RE: [Haskell-cafe] setPermissions bug?
by Simon Marlow 20 Feb '04

20 Feb '04

> On Wed, Feb 18, 2004 at 12:31:34PM -0000, Simon Marlow wrote: > > > I presume that this is a bug in setPermissions? > > > > Yes, it's a bug. However, since > getPermissions/setPermissions doesn't > > really give you enough control in Unix, I suggest you use > the functions > > available from System.Posix instead (fileStatus, fileMode, > setFileMode > > etc.). > > Well, in my case all I want to do is to copy permissions from > one file to > another, so get/setPermissions would have been fine. But yeah, I've > already switched to using setFileMode etc. Actually if you want to copy permissions from one file to another, then get/setPermissions is *not* what you want. It will only copy across the owner's permissions. The problem is that the Permissions type is fully defined and non-abstract; there's no place to store extra platform-specific permission bits. Cheers, Simon

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hOp
by sebc＠macs.hw.ac.uk 19 Feb '04

19 Feb '04

The RTS of GHC is truly an amazing piece of software. By removing the system-specific bits, and just adding a few lines of C and assembly code, we were able to turn it into a standalone micro-kernel that can be extended in our favorite language. A screenshot is available here: http://etudiants.insia.org/~jbobbio/hOp-20040219.png Everything from the base hierarchical library not under the System subdirectory is available. This includes threads and communication primitives and the foreign interface, which means experimenting with writing drivers in Haskell should be reasonably easy. We hope to make a source release soon. -- Sébastien Carlier Jérémy Bobbio

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