- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] implementing try for RWST ?
by tpledger＠ihug.co.nz 17 Apr '07

17 Apr '07

Jeremy Shaw wrote: : | However, I think this is buggy, because changes | to 's' and 'w' will be lost if 'm' raises an | exception. : That's determined by the way you stack your monad transformers when declaring the type: adding error handling to a writer monad, or adding writing to an error handling monad. For a concrete example, see the result types in the following. The first has the Either inside the tuple, and the second has the tuple inside the Either. Prelude> :t Control.Monad.Writer.runWriter . Control.Monad.Error.runErrorT Control.Monad.Writer.runWriter . Control.Monad.Error.runErrorT :: Control.Monad.Error.ErrorT e (Control.Monad.Writer.Writer w) a -> (Either e a, w) Prelude> :t either Left Right . Control.Monad.Writer.runWriterT either Left Right . Control.Monad.Writer.runWriterT :: Control.Monad.Writer.WriterT w (Either a) a1 -> Either a (a1, w) Regards, Tom

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Re: [web-devel] A light-weight web framework
by Ricardo Herrmann 16 Apr '07

16 Apr '07

The maintenance nightmare happens when someone uses the embedded language to specify business logic, and that's entirely the web-{developer,designer}'s fault. Thus, the problem is not that these languages shouldn't be powerful enough. IMHO, a safe approach would be simply not allowing I/O inside templates (hey, sounds familiar ;-) On 4/5/07, Maurice Codik <maurice.codik(a)gmail.com> wrote: > > A few things, some of which I sort of mentioned in my previous email: > > - If I'm already going to commit some time to learn a templating language, > why dont I just spend that same amount of time learning the little bit of > haskell I need to make the template work? If thats too much to ask, I can > just spit out HTML, and have the programmer put in the dynamic parts for me. > Both of these scenarios seem to be a more efficient use of time. > > - Who is the target audience? If its a big organization where there are > multiple designers and multiple devs, then your approach may work just fine. > If its the single developer, then something like what David suggested would > work even better. If its a small team (which may or may not include a > full-time designer), then something like what I suggested would work best. > For a web framework for haskell, I would guess that the latter two are much > more likely. > > - Embedding a real programming language in a template already gives you > power to do what ever you need to do. What if you need to implement some > logic that the template language doesnt support? In those cases, you're > usually out of luck and have to move that logic into a controller, where it > doesnt really belong (assuming its actual display logic, not business > logic). > > - It's really just a matter of taste. Any web framework thats worth using > should be flexible in its support of view technologies, but come with one > thats a sensible default. > > Maurice > > On 4/5/07, Joel Reymont <joelr1(a)gmail.com> wrote: > > > > Do you see anything wrong with the approach I suggested, though? > > > > On Apr 5, 2007, at 6:16 PM, Maurice Codik wrote: > > > > > That's not necesarily true. Templates where there is mostly markup, > > > but let you embed code into them using special tags (ex, <% code % > > > >) are extremely popular and work fairly well. They also keep the > > > template language simple because there is already a full-powered > > > programming language thats embedded into it. Good examples of this > > > method are ERB templates in Rails, JSPs, Perl Mason templates, etc. > > > > -- > > http://wagerlabs.com/ > > > > > > > > > > > > > > > -- > http://blog.mauricecodik.com > _______________________________________________ > web-devel mailing list > web-devel(a)haskell.org > http://www.haskell.org/mailman/listinfo/web-devel > > -- Ricardo Guimarães Herrmann "Any sufficiently complicated C or Fortran program contains an ad hoc, informally specified, bug-ridden, slow implementation of half of Common Lisp" "Any sufficiently complicated Lisp or Ruby program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Haskell"

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Fwd: [Haskell-cafe] Tutorial on Haskell
by Ryan Dickie 16 Apr '07

16 Apr '07

Blast.. i didn't hit "reply all" so here's a forward of my mail to the group... --ryan ---------- Forwarded message ---------- From: Ryan Dickie <goalieca(a)gmail.com> Date: Apr 16, 2007 4:24 PM Subject: Re: [Haskell-cafe] Tutorial on Haskell To: Simon Peyton-Jones <simonpj(a)microsoft.com> I can tell you what me and my colleagues would be interested in (though none of us are actually going). We code a lot of math. You may call it scientific computing. Haskell seems like a natural fit for the task. In particular we are interested in: 1) the type system 2) concurrency (can these be set to run on a large system) 3) simple relation between what equations we write on paper, and what equations we write in haskell. I'm still a n00b to Haskell. For us languages like matlab, maple, etc. do not fit the job very well and run too slowly. C/C++ is usually what i use but it can be a pain. Python, etc... well its good for the glue i suppose. Haskell might fit that niche. On 4/16/07, Simon Peyton-Jones <simonpj(a)microsoft.com> wrote: > > Friends > > I have agreed to give a 3-hr tutorial on Haskell at the Open Source > Convention 2007 > http://conferences.oreillynet.com/os2007/ > > I'm quite excited about this: it is a great opportunity to expose Haskell > to a bunch of smart folk, many of whom won't know much about Haskell. My > guess is that they'll be Linux/Perl/Ruby types, and they'll be practitioners > rather than pointy-headed academics. > > One possibility is to do a tutorial along the lines of "here's how to > reverse a list", "here's what a type is" etc; you know the kind of > thing. But instead, I'd prefer to show them programs that they might > consider *useful* rather than cute, and introduce the language along the > way, as it were. > > So this message is to ask you for your advice. Many of you are exactly > the kind of folk that come to OSCON --- except that you know Haskell. So > help me out: > > Suggest concrete examples of programs that are > * small > * useful > * demonstrate Haskell's power > * preferably something that might be a bit > tricky in another language > > For example, a possible unifying theme would be this: > http://haskell.org/haskellwiki/Simple_unix_tools > > Another might be Don's cpu-scaling example > http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10<http://cgi.cse.unsw.edu.au/%7Edons/blog/2007/03/10> > > But there must be lots of others. For example, there are lots in the blog > entries that Don collects for the Haskell Weekly Newsletter. But I'd like > to use you as a filter: tell me your favourites, the examples you find > compelling. (It doesn't have to be *your* program... a URL to a great blog > entry is just fine.) Of course I'll give credit to the author. > > Remember, the goal is _not_ "explain monads". It's "Haskell is a great > way to Get The Job Done". > > Thanks! > > Simon > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: `Expect'-like lazy reading/Parsec matching on TCP sockets
by Scott Bell 16 Apr '07

16 Apr '07

Ooops! It seems that this doesn't behave well with a -threaded RTS. I get an EOF on handles that I know for a fact shouldn't be receiving them. It still works well without -threaded, but does anyone know why I'm getting this behavior? hGetContentsTimeout :: Handle -> Int -> IO String hGetContentsTimeout h t = do hSetBuffering stdin NoBuffering ready <- hWaitForInput h t if (not ready) then return [] else do c <- hGetChar h s <- unsafeInterleaveIO (hGetContentsTimeout h t) return (c:s) (I did add EOF checking, but all that did was return the end of the list earlier than I wanted)

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Type classes and type equality
by Neil Mitchell 16 Apr '07

16 Apr '07

Hi, I'm looking for a type class which checks whether two types are the same or not. My first guess is: class Same a b where same :: a -> b -> Bool instance Same a a where same _ _ = True instance Same a b where same _ _ = False In Hugs this seems to work with overlapping instances (not requiring unsafe overlapping instances). GHC requires {-# LANGUAGE MultiParamTypeClasses, IncoherentInstances #-} So my question is if this is safe? Will the compiler always pick the "right one"? Is there a better way to do this? The alternative I thought of is using Typeable, but this is not supported particularly well on Hugs (no deriving Typeable) and would require modifications to the existing data structures (additional derivings) so is not such a good choice. Thanks Neil

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Re: [Haskell-cafe] Tutorial on Haskell
by Neil Bartlett 16 Apr '07

16 Apr '07

Well, given that concurrency is a hot topic at the moment, how about something based on STM? E.g. perhaps some kind of instant messaging server? Or "Twitter" except scalable. By ruthlessly eliminating features, you could get the core of one of these down to something that could be built in three hours. But please, no Santa Clauses ;-) Neil > Friends > > I have agreed to give a 3-hr tutorial on Haskell at the Open Source Convention 2007 > http://conferences.oreillynet.com/os2007/ > > I'm quite excited about this: it is a great opportunity to expose Haskell to a bunch of smart folk, many of whom won't know much about Haskell. My guess is that they'll be Linux/Perl/Ruby types, and they'll be > practitioners rather than pointy-headed academics. > > One possibility is to do a tutorial along the lines of "here's how to reverse a list", "here's what a type is" etc; you know the kind of thing. But instead, I'd prefer to show them programs that they might consider *useful* rather than cute, and introduce the language along the way, as it were. > > So this message is to ask you for your advice. Many of you are exactly the kind of folk that come to OSCON --- except that you know Haskell. So help me out: > > Suggest concrete examples of programs that are > * small > * useful > * demonstrate Haskell's power > * preferably something that might be a bit > tricky in another language > > For example, a possible unifying theme would be this: > http://haskell.org/haskellwiki/Simple_unix_tools > > Another might be Don's cpu-scaling example > http://cgi.cse.unsw.edu.au/~dons/blog/2007/03/10 > > But there must be lots of others. For example, there are lots in the blog entries that Don collects for the Haskell Weekly Newsletter. But I'd like to use you as a filter: tell me your favourites, the examples you find compelling. (It doesn't have to be *your* program... a URL to a great blog entry is just fine.) Of course I'll give credit to the author. > > Remember, the goal is _not_ "explain monads". It's "Haskell is a great way to Get The Job Done". > > Thanks! > > Simon > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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type inference futility
by Paul Wankadia 16 Apr '07

16 Apr '07

The following code has ambiguity, but I can't figure out how to get around it. Am I missing something trivial? Am I going in the wrong direction? Thank you in advance for your time and for any help that you can offer. > data MehQueue = MehQueue > > class MehBase a where new :: IO a > instance MehBase MehQueue where new = return MehQueue > > class (MehBase a) => HasShift a where shift :: a -> IO a > instance HasShift MehQueue where shift a = return a > > main :: IO () > main = do > x <- new > shift x > return () Please note that I intend to extend this example with MehStack, HasPush and HasPop. You can probably guess where I'm going with all this.

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ghci Crashing...
by Gaetano Caruana 16 Apr '07

16 Apr '07

Hi all. I am new to this list ;) Today I have encountered something new that has never happened to me in the past 6 months that I have been using ghci and Haskell. I wrote this function: type Line = (Point,Point) type Point = (Float,Float) circleLineIntersection :: Float -> Float -> Float -> Line -> Maybe [Point] circleLineIntersection h k r ((x1,y1),(x2,y2)) = do let perpdist = calcPerpDist (h,k) ((x1,y1),(x2,y2)) noSolution = perpdist > r oneSolution = perpdist == r twoSolutions = perpdist < r in do if noSolution then Nothing else let dx = x2 - x1 dy = y2 - y1 dr = sqrt (dx * dx + dy * dy) bigD = x1 * y2 - x2 * y1 x1 = ( bigD * dy + sign (dy) * dx * sqrt( r * r * dr * dr - bigD * bigD ) ) / (dr * dr) y1 = - bigD * dx + abs(dy) * sqrt( r * r * dr * dr - bigD * bigD ) / (dr * dr) x2 = ( bigD * dy - sign (dy) * dx * sqrt( r * r * dr * dr - bigD * bigD ) ) / (dr * dr) y2 = - bigD * dx - abs(dy) * sqrt( r * r * dr * dr - bigD * bigD ) / (dr * dr) in if oneSolution then do Just [(x1,y1)] else do Just [(x1,y1),(x2,y2)] Compiling is fine... When I run the function like this.... circleLineIntersection 1 1 1 ((1,1),(2,2)) it simply outputs.... Just [( No CPU usage. I am using fglasgow-exts Thanks for your help. Gaetano

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System.Random
by Ketil Malde 16 Apr '07

16 Apr '07

Hi, I've recently stumbled upon some issues with the System.Random module, and thought I'd try to remedy them. However, I'm not quite sure what an optimal resolution is. Problem 1 is that I often get the same "random" number on consecutive program runs [1]. Looking at the code for initialization (called with '0' argument on program start): mkStdRNG :: Integer -> IO StdGen mkStdRNG o = do ct <- getCPUTime (TOD sec ms) <- getClockTime return (createStdGen (sec * 12345 + ms + ct + o)) Considering that 'getCPUTime' tends to return 0 early in the program, and that we simply ignore the picosecond part of 'getClockTime', it's fairly obvious why we get this result. What is not obvious is the rationale for discarding 'ms', or for that matter, the 12345 multiplication - can anybody shed some light on this? Anyway, on my Linux system, I get (unsurprisingly considering the defintion of time_t) picoseconds in even millions, and successive calls to getClockTime are not successive enough to give the same result in my cases. A better implementation would perhaps use /dev/urandom, but that is possibly not portable enough? The second issue is reading the random state from a string [2]. I realize (as Simon points out) that you can use 'reads' to get the remainder of the string, but as many find it confusing (at least the OP and I :-), I wonder what the rationale is behind this kind of interface? And checking how the state is actually calculated: stdFromString :: String -> (StdGen, String) stdFromString s = (mkStdGen num, rest) where (cs, rest) = splitAt 6 s num = foldl (\a x -> x + 3 * a) 1 (map ord cs) So, it picks six chars, and folds them into an Int for mkStdGen. Now, this number (due to the low multiplier) never exceeds 93549. Since we have an Int of state, why not use the whole range? Why not fold with a multiplier of 256? (All of this is trivial to fix of course, but I worry about changing code I don't understand the rationale behind.) One final question: are there build and install instructions for 'base'? I got the code from darcs, but I haven't find a way to build it and replace the bundled 'base' with a new version. -k References: [1] http://hackage.haskell.org/trac/ghc/ticket/1272 [2] http://www.nabble.com/Re% 3A--Haskell-cafe--System.Random-StdGen-read-fails-on-some-strings--tf3394495.html#a9450043 [3] http://web.archive.org/web/20011027002011/http://dilbert.com/comics/dilbert…

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partial kinds and instances
by Matthew Pocock 16 Apr '07

16 Apr '07

Hi, I saw that it's possible to peal off kinds from the right-hand side when instantiating classes. Is it possible to peel them off from the left-hand side? Or in any order? I have been told in #haskell by people who seems to know that Very Bad Things happen if you do this without also making the kind system more clever. Is there a quick fix that will give me the same effect, without introducing a newtype or re-writing the original type declaration to introduce the extra types in a different order? Illustrative code below. Matthew data MyType e = Singleton e data MyOtherType k v = Empty | Node k v data YetAnotherType k v = OtherEmpty | OtherNode k v type RevYAT v k = YetAnotherType k v class Iterable ite where iterate :: ite e -> [e] instance Iterable MyType where iterate (Singleton e) = [e] instance Iterable (MyOtherType k) where iterate ite = [] --broken but illustrative instance Iterable (RevYAT v) where iterate it = [] --broken again Type synonym `RevMOM' should have 2 arguments, but has been given 1 In the instance declaration for `Iterable (RevYAT v)'

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