- Haskell-Cafe - Haskell.org

PDF library?
by Clifford Beshers 24 Jan '07

24 Jan '07

I don't suppose anyone has any Haskell code that understands the PDF format, do they?

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embedding haskell into html
by Forest Liu 24 Jan '07

24 Jan '07

I am new to haskell, and now working on embedding haskell into html. Thus we will write webapp using haskell as server-side language like php. Here I explain my plan and ask some questions, looking for experienced ones to discuss with. It is not proper embedding lines of haskell code into html (like php). Because we would not know what to output, as it is not a procedural language as php. I think the proper way is using a single tag containing a haskell expression (usually a function name) each time of needing haskell. I call it "hasp" standing for "HASkell Page" :). It should look like: <html> <head><title><?hasp genTitle?></title></head> <body> ...some html... <?hasp getDataFromDB?> ...some html... </body> </html> And the functions ("genTitle", "getDataFromDB") should be implemented in "/same_dir_to_hasps/hs/same_name_to_each_hasp.hs" . It just like the templates in php. There are two possible ways to implementing the mechanism: compiled-CGI, and interpreting(or Just-In-Time Compilation) 1. Complied-CGI: use a preprocessor to combine the hasp templates and hs implementations, then compiled by ghc to generate cgi-bins. That should be easy for me to implement. 2.Interpreting (or JIT-Compilation) I prefer this method because modern web-scripting programmers should be familiar with it. The browser request for somepage.hasp, and Apache knows giving it to hasp interpreter to handle. I am facing to the dilemma between interpreting or JIT-Compilation. The best situation is a perfect haskell interpreting procedure written in haskell. I am trying hs-plugins, and I hope it will help me a lot. Or any other "haskell interpreting procedure written in haskell" you can tell me? The trouble about JIT-Compilation is that hasp-handler must call ghc through OS shell commandline using System.Cmd, and that is not proper in Apache cgi handler(lots of stuff to configure). BTW, if the "haskell interpreting procedure written in haskell" does not work well, I could use cmdline "ghc -e some_expr file.hs" to evaluate the expr, though an ugly way. And in that case, we would face to the problem of "external cmd of CGI" again. -- Sincerely, Forest Liu(��S)

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catch the stdout when execute a shell cmd
by Forest Liu 24 Jan '07

24 Jan '07

I use "system" in System.Cmd to execute a shell cmd, then how can I catch its stdout? -- Sincerely, Forest Liu(��S)

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GADTs are expressive
by Jim Apple 24 Jan '07

24 Jan '07

To show how expressive GADTs are, the datatype Terminating can hold any term in the untyped lambda calculus that terminates, and none that don't. I don't think that an encoding of this is too surprising, but I thought it might be a good demonstration of the power that GADTs bring. {-# OPTIONS -fglasgow-exts #-} {- Using GADTs to encode normalizable and non-normalizable terms in the lambda calculus. For definitions of normalizable and de Bruin indices, I used: Christian Urban and Stefan Berghofer - A Head-to-Head Comparison of de Bruijn Indices and Names. In Proceedings of the International Workshop on Logical Frameworks and Meta-Languages: Theory and Practice (LFMTP 2006). Seattle, USA. ENTCS. Pages 46-59 http://www4.in.tum.de/~urbanc/Publications/lfmtp-06.ps @incollection{ pierce97foundational, author = "Benjamin Pierce", title = "Foundational Calculi for Programming Languages", booktitle = "The Computer Science and Engineering Handbook", publisher = "CRC Press", address = "Boca Raton, FL", editor = "Allen B. Tucker", year = "1997", url = "citeseer.ist.psu.edu/pierce95foundational.html" } -} module Terminating where -- Terms in the untyped lambda-calculus with the de Bruijn representation data Term t where Var :: Nat n -> Term (Var n) Lambda :: Term t -> Term (Lambda t) Apply :: Term t1 -> Term t2 -> Term (Apply t1 t2) -- Natural numbers data Nat n where Zero :: Nat Z Succ :: Nat n -> Nat (S n) data Z data S n data Var t data Lambda t data Apply t1 t2 data Less n m where LessZero :: Less Z (S n) LessSucc :: Less n m -> Less (S n) (S m) data Equal a b where Equal :: Equal a a data Plus a b c where PlusZero :: Plus Z b b PlusSucc :: Plus a b c -> Plus (S a) b (S c) {- We can reduce a term by function application, reduction under the lambda, or reduction of either side of an application. We don't need this full power, since we could get by with normal-order evaluation, but that required a more complicated datatype for Reduce. -} data Reduce t1 t2 where ReduceSimple :: Replace Z t1 t2 t3 -> Reduce (Apply (Lambda t1) t2) t3 ReduceLambda :: Reduce t1 t2 -> Reduce (Lambda t1) (Lambda t2) ReduceApplyLeft :: Reduce t1 t2 -> Reduce (Apply t1 t3) (Apply t2 t3) ReduceApplyRight :: Reduce t1 t2 -> Reduce (Apply t3 t1) (Apply t3 t2) {- Lift and Replace use the de Bruijn operations as expressed in the Urban and Berghofer paper. -} data Lift n k t1 t2 where LiftVarLess :: Less i k -> Lift n k (Var i) (Var i) LiftVarGTE :: Either (Equal i k) (Less k i) -> Plus i n r -> Lift n k (Var i) (Var r) LiftApply :: Lift n k t1 t1' -> Lift n k t2 t2' -> Lift n k (Apply t1 t2) (Apply t1' t2') LiftLambda :: Lift n (S k) t1 t2 -> Lift n k (Lambda t1) (Lambda t2) data Replace k t n r where ReplaceVarLess :: Less i k -> Replace k (Var i) n (Var i) ReplaceVarEq :: Equal i k -> Lift k Z n r -> Replace k (Var i) n r ReplaceVarMore :: Less k (S i) -> Replace k (Var (S i)) n (Var i) ReplaceApply :: Replace k t1 n r1 -> Replace k t2 n r2 -> Replace k (Apply t1 t2) n (Apply r1 r2) ReplaceLambda :: Replace (S k) t n r -> Replace k (Lambda t) n (Lambda r) {- Reflexive transitive closure of the reduction relation. -} data ReduceEventually t1 t2 where ReduceZero :: ReduceEventually t1 t1 ReduceSucc :: Reduce t1 t2 -> ReduceEventually t2 t3 -> ReduceEventually t1 t3 -- Definition of normal form: nothing with a lambda term applied to anything. data Normal t where NormalVar :: Normal (Var n) NormalLambda :: Normal t -> Normal (Lambda t) NormalApplyVar :: Normal t -> Normal (Apply (Var i) t) NormalApplyApply :: Normal (Apply t1 t2) -> Normal t3 -> Normal (Apply (Apply t1 t2) t3) -- Something is terminating when it reduces to something normal data Terminating where Terminating :: Term t -> ReduceEventually t t' -> Normal t' -> Terminating {- We can encode terms that are non-terminating, even though this set is only co-recursively enumerable, so we can't actually prove all of the non-normalizable terms of the lambda calculus are non-normalizable. -} data Reducible t1 where Reducible :: Reduce t1 t2 -> Reducible t1 -- A term is non-normalizable if, no matter how many reductions you have applied, -- you can still apply one more. type Infinite t1 = forall t2 . ReduceEventually t1 t2 -> Reducible t2 data NonTerminating where NonTerminating :: Term t -> Infinite t -> NonTerminating -- x test1 :: Terminating test1 = Terminating (Var Zero) ReduceZero NormalVar -- (\x . x)@y test2 :: Terminating test2 = Terminating (Apply (Lambda (Var Zero))(Var Zero)) (ReduceSucc (ReduceSimple (ReplaceVarEq Equal (LiftVarGTE (Left Equal) PlusZero))) ReduceZero) NormalVar -- omega = \x.x@x type Omega = Lambda (Apply (Var Z) (Var Z)) omega = Lambda (Apply (Var Zero) (Var Zero)) -- (\x . \y . y)@(\z.z@z) test3 :: Terminating test3 = Terminating (Apply (Lambda (Lambda (Var Zero))) omega) (ReduceSucc (ReduceSimple (ReplaceLambda (ReplaceVarLess LessZero))) ReduceZero) (NormalLambda NormalVar) -- (\x.x@x)(\x.x@x) test4 :: NonTerminating test4 = NonTerminating (Apply omega omega) help3 help1 :: Reducible (Apply Omega Omega) help1 = Reducible (ReduceSimple (ReplaceApply (ReplaceVarEq Equal (LiftLambda (LiftApply (LiftVarLess LessZero) (LiftVarLess LessZero)))) (ReplaceVarEq Equal (LiftLambda (LiftApply (LiftVarLess LessZero) (LiftVarLess LessZero)))))) help2 :: ReduceEventually (Apply Omega Omega) t -> Equal (Apply Omega Omega) t help2 ReduceZero = Equal help2 (ReduceSucc (ReduceSimple (ReplaceApply (ReplaceVarEq _ (LiftLambda (LiftApply (LiftVarLess _) (LiftVarLess _)))) (ReplaceVarEq _ (LiftLambda (LiftApply (LiftVarLess _) (LiftVarLess _)))))) y) = case help2 y of Equal -> Equal help3 :: Infinite (Apply Omega Omega) help3 x = case help2 x of Equal -> help1

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Announce: Package rdtsc for reading IA-32 time stamp counters
by Martin Grabmueller 24 Jan '07

24 Jan '07

Hallo all, version 1.0 of package rdtsc has just been released. This small package contains one module called 'Rdtsc.Rdtsc'. This module provides the function 'rdtsc' for accessing the 'rdtsc' machine register on modern IA-32 processors. This is a 64-bit counter which counts the number of processor cycles since the machine has been powered up. Using this instruction, you can make very precise time measurements which are independent of the actual CPU frequency. But note that you can get strange results sometimes on a superscalar processor. A small program using this library looks like this: module Main where import Rdtsc.Rdtsc main = do t1 <- rdtsc t2 <- rdtsc putStrLn ("Cost of rdtsc (ffi call): " ++ show (t2 - t1)) Note that the Haskell foreign function interface imposes some additional overheads. On my machine, it takes about 950 cycles to call this function twice and to compute the difference, whereas in C the overhead is only about 84 cycles. The documentation for the package is available here: http://uebb.cs.tu-berlin.de/~magr/projects/rdtsc/ Download the source using darcs: darcs get http://uebb.cs.tu-berlin.de/~magr/darcs/rdtsc/ I would really like to get feedback on this little package, which already has been quite useful for me. Have fun and Happy Haskell Hacking in th New Year, Martin

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Re: [Haskell-cafe] Strings in Haskell
by Alexy Khrabrov 24 Jan '07

24 Jan '07

I wonder if that's another reason OCaml is used in a(t least one) hedge fund -- why Jane St. preferred OCaml to Haskell, I wonder? Was it the state of affairs then that OCaml was more efficient (? -- WAGuess), and would they prefer Haskell now? I'm trying to make sense out of OCaml objects out of that already infamously annoying "Practical OCaml" book, and class object <blah>... doesn't look like much, not to say that "class object" sounds about as bad as most English in that book. (Written by an English major... What a decline in US education! :) I come from ML background, so Haskell laziness and OCaml objects are all new to me. But my Haskell book, Haskell School of Expression, is so much better written, that I'm reading it much faster. I'm CC'ing Yaron as his e-mail comes up in my Gmail context adwords on the word "Haskell." :) I'm interested in financial data mining and market modeling -- are there any good application of FP there, say in Lisp? Cheers, Alexy P.S. Somebody with an old-fashioned mail client please feel free to change the subject to "Financial Engineering with FP," gmail seems to etch its subjects in stone. :) On 1/23/07, Martin Jambon <martin_jambon(a)emailuser.net> wrote: > On Mon, 22 Jan 2007, Alexy Khrabrov wrote: > > > Greetings -- I'm looking at several FP languages for data mining, and > > was annoyed to learn that Erlang represents each character as 8 BYTES > > in a string which is just a list of characters. Now I'm reading a > > Haskell book which states the same. Is there a more efficient Haskell > > string-handling method? Which functional language is the most > > suitable for text processing? > > In OCaml, strings are compact sequences of bytes. And you can pass them > as-is to C functions: > > http://caml.inria.fr/pub/ml-archives/caml-list/2002/08/e109df224ff0150b3020… > > > Martin > > -- > Martin Jambon > http://martin.jambon.free.fr >

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strange behavior in Text.Regex.Posix
by John MacFarlane 23 Jan '07

23 Jan '07

Can anyone help me understand this odd behavior in Text.Regex.Posix (GHC 6.6)? Prelude Text.Regex.Posix Text.Regex> subRegex (mkRegex "\\^") "he\350llo" "@" "he@llo" Why does /\^/ match \350 here? Generally Text.Regex.Posix seems to work fine with unicode characters. For example, \350 is treated as a single character here: Prelude Text.Regex.Posix Text.Regex> subRegex (mkRegex "e.l") "he\350llo" "@" "h@lo" The problem is specific to \350 and doesn't happen with, say, \351: Prelude Text.Regex> subRegex (mkRegex "\\^") "he\351llo" "@" "he\351llo" Is this a bug, or just something I'm not understanding? John

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Re: [Haskell] Hyperlinking source code formatter?
by Simon Marlow 23 Jan '07

23 Jan '07

Conal Elliott wrote: > I'd like to turn source code (mine and others') into a fully hyperlinked > form, in which every name reference links to the name's definition. > Syntax-coloring would be great also. I see Programmatica. Is it in > use, supported, and reasonably easy to install and use? Are there other > options? Thanks, If anyone is thinking of writing such a tool, the GHC API would be a great place to start, incedentally. Cheers, Simon

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Announce: hpaste
by Eric Mertens 23 Jan '07

23 Jan '07

Hello, I am pleased to announce "hpaste", the Haskell Paste-bin. Over the course of this week many of the active #haskell members and I have been developing this application to provide #haskell with a reliable paste bot whose features are tuned to the needs of the channel. Everyone is invited to see hpaste for themselves at http://hpaste.ath.cx:8000 hpaste.org has been registered, and in a few days time will be the permanent home of hpaste. An example of how hpaste was recently used to teach can be seen at http://hpaste.ath.cx:8000/43 A paste-bin is a place for IRC users to paste and annotate code snippets in a stable environment. This keep users from flooding the channel and saves users from having to scroll back constantly to review a paste. hpaste uses haskell syntax coloring (provided by hscolour) and is able to generate a color-coded diff of two pastes to give users an additional tool to share and teach. This project was a wonderful way for me to learn both the old and new HAppS APIs which have both proven to be quite powerful. In addition to providing a web-interface, hpaste is able to announce new pastes to the IRC channel via a bot based on Don Stewart's IRC bot tutorial. darcs has been instrumental in this colaborative effort, and the hpaste source is available with: darcs get --partial http://www.scannedinavian.com/~eric/hpaste Feedback is both encouraged and welcomed! -- Eric Mertens emertens(a)gmail.com

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Strings in Haskell
by Alexy Khrabrov 23 Jan '07

23 Jan '07

Greetings -- I'm looking at several FP languages for data mining, and was annoyed to learn that Erlang represents each character as 8 BYTES in a string which is just a list of characters. Now I'm reading a Haskell book which states the same. Is there a more efficient Haskell string-handling method? Which functional language is the most suitable for text processing? Cheers, Alexy

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