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[Offtopic] Re: Re: A better syntax for qualified operators?
by Cale Gibbard 01 Oct '06

01 Oct '06

> "What a beautiful world this could be..." ;-)) (*) > > Cheers, > Ben > (*) Donald Fagen (forgot the name of the song) I think I.G.Y. (International Geophysical Year) is it: On that train all graphite and glitter Undersea by rail Ninety minutes from New York to Paris (More leisure time for artists everywhere) A just machine to make big decisions Programmed by fellows with compassion and vision We'll be clean when their work is done We'll be eternally free yes and eternally young What a beautiful world this will be What a glorious time to be free

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irrefutable patterns for existential types / GADTs
by apfelmus＠quantentunnel.de 01 Oct '06

01 Oct '06

Ross Paterson wrote: > The story so far: > apfelmus: why are there no irrefutable patterns for GADTs? > Conor: because you could use them to write unsafeCoerce > Ross: how about irrefutable patterns (or newtypes) for existential types? > Simon: Try giving the translation into System F + (existential) data types I'd like to add that I see no problem with coerce :: Eq a b -> a -> b coerce ~Refl x = x as long as we have coerce _|_ x === _|_ The wish is that f = \refl -> Just . coerce refl = \~Refl x -> Just x should satisfy f _|_ x === Just _|_ f _|_ x =/= _|_ and likewise for any other constructor than Just. Regards, apfelmus

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Re: Why is type 'b' forced to be type 'm a' and not possibly 'm a -> m a'
by tpledger＠ihug.co.nz 01 Oct '06

01 Oct '06

Vivian McPhail wrote: ... > I need the arg a to be evaluated before it gets > passed to a1 and a2. This definition does the right thing > when type 'a' is a function type, because it is not a > value, but with something like 'm a -> (m a -> m a) -> m > a' with Forkable (a -> b) the first arg gets evaluated > twice, to be more concrete: > > With > > (and golden white) eggs > > I want the 'eggs' that is passed to 'golden' to be the > same as the 'eggs' that is passed to 'white', i.e. ... Could you reduce the need for Forkable instances, by rewriting '(and golden white) eggs' as 'and golden white =<< eggs'? Or would the same piece of code also have to handle combinations such as monadic 'and golden white' and non-monadic eggs? [BTW, thanks for giving me a pretext to use the phrase non-monadic eggs!] > Tom suggested that I might be able to use the Reader monad > , but I'm not clear as to how I could do this. Please ignore that. I only mentioned it in case the sole purpose of fork was to propagate a String, which you've now explained is not so. Regards, Tom

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Greetings...
by tpledger＠ihug.co.nz 01 Oct '06

01 Oct '06

Seth Gordon wrote: > I thought I should check and see if anyone > on this list has used Haskell to munge a > ten-million-row database table, and if > there are any particular gotchas I should > watch out for. Are you sure you want to target the data directly? Another approach, that might have a better chance of a quick win within your time frame, is to use Haskell to generate SQL code. That could still reduce the amount of code you maintain by hand. Regards, Tom

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Re: Greetings
by Paul Johnson 01 Oct '06

01 Oct '06

I've done some stuff with maybe 50k rows at a time. A few bits and pieces: 1: I've used HSQL (http://sourceforge.net/project/showfiles.php?group_id=65248) to talk to ODBC databases. Works fine, but possibly a bit slowly. I'm not sure where the delay is: it might just be the network I was running it over. One gotcha: the field function takes a field name, but its not random access. Access the fields in query order or it crashes. 2: For large data sets laziness is your friend. When reading files "getContents" presents an entire file as a list, but its really evaluated lazily. This is implemented using unsafeInterleaveIO. I've never used this, but in theory you should be able to set up a query that returns the entire database as a list and then step through it using lazy evaluation in the same way. 3: You don't say whether these algorithms are just row-by-row algorithms or whether there is something more sophisticated going on. Either way, try to make things into lists and then apply map, fold and filter operations. Its much more declarative and high level when you do it that way. Let us know how you get on. Paul.

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Greetings...
by Seth Gordon 01 Oct '06

01 Oct '06

I've finally gotten enough round tuits to learn Haskell, and now that I've done some of the exercises from _The Haskell School of Expression_ and I finally (think I) understand what a monad is, the language is making a lot more sense to me (although my code is not always making so much sense to the compiler :-). My employer (MetaCarta) makes a search engine that can recognize geographic data. My group within MetaCarta is responsible for building the "Geographic Data Module" within our software. To do this, we slurp a heap of geographic and linguistic data from a variety of sources, normalize it, and then use some algorithms (that I'm not allowed to describe) to generate the module. This seems like the sort of task that cries out for a functional-programming approach, and that's what we use, sorta: a lot of the code that I'm responsible for is SQL, with chains of "CREATE TEMP TABLE X AS [insert very complicated query here]", some C++ for the parts that would be very time-consuming or impossible to implement in SQL, and shell scripts to tie everything together. I told my tech lead that I want to try porting some of this code to Haskell in the hope that it would run faster and/or be easier to read. He said I should spend two work days on the project and then be prepared to convince my co-workers that further research in this vein is (or is not) worth doing. So before I embark on day 1 of the project, I thought I should check and see if anyone on this list has used Haskell to munge a ten-million-row database table, and if there are any particular gotchas I should watch out for. adTHANKSvance....

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Re: [Haskell-cafe] smallest double eps
by Chad Scherrer 01 Oct '06

01 Oct '06

Tamas, You might want to read Joachim's post more carefully - he's trying to help you, and I think he makes a good point. -Chad > > Am Freitag, den 29.09.2006, 19:30 -0400 schrieb Tamas K Papp: > > > the smallest positive floating point number x such that 1+x /= x? > > That would be the smallest positive number, woudn't it? > > > > Do you mean the smalles postive number x with 1+x /= 1? > > Hi Joachim, > > Specifically, I would be happy with the smallest Double that makes the > statement true. I need it as a convergence bound for an iterative > algorithm. Anyhow, thanks for the clarification, but I would be > happier with an answer. > > Tamas > > > ------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > End of Haskell-Cafe Digest, Vol 37, Issue 92 > ******************************************** > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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cutting long strings into lines
by Andrea Rossato 01 Oct '06

01 Oct '06

Hello! I've been trying for quite some time to find an elegant solution to cut long strings into lines, but the only solution I was able to come up is the following piece of ugly code. Is there a library function for that? What kind of approach would you suggest? Thanks for your kind attention. Andrea Here's the code: -- does the actual job wrapString str = foldr addNL "" $ rmFirstSpace $ concat $ splitS (getIndx $ indx str) str -- gets the indexes of the spaces within a string indx = findIndices (\x -> if x == ' ' then True else False) -- gets the indexes of where to split the string into lines: lines -- must be between 60 and 75 char long getIndx :: [Int] -> [Int] getIndx = takeFirst . checkBound . (delete 0) . nub . map (\x -> if x > 60 && x `rem` 60 >= 0 && x `rem` 70 <= 10 then x else 0) -- groups indexes when their distance is too short checkBound = groupBy (\x y -> if y - x < 10 then True else False) -- takes the first index of a group of indexes takeFirst = map (\(x:xs) -> x) -- split a string given a list of indexes splitS _ [] = [] splitS (x:xs) (ls) = [take x ls] : splitS (map (\i -> i - x) xs) (drop x ls) splitS _ ls = [ls]:[] -- remove the first space from the begging of a string in a list of strings rmFirstSpace = map (\(x:xs) -> if x == ' ' then xs else x:xs) -- used by foldr to fold the list of substrings addNL s s1 = s ++ "\n" ++ s1 try with putStrLn $ wrapString longString where: longString = "The Haskell XML Toolbox (HXT) is a collection of tools for processing XML with Haskell. The core component of the Haskell XML Toolbox is a domain specific language, consisting of a set of combinators, for processing XML trees in a simple and elegant way. The combinator library is based on the concept of arrows. The main component is a validating and namespace aware XML-Parser that supports almost fully the XML 1.0 Standard. Extensions are a validator for RelaxNG and an XPath evaluator."

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Re: [Haskell-cafe] smallest double eps
by Victor Bandur 01 Oct '06

01 Oct '06

-------- Forwarded Message -------- From: Victor Bandur <bandurvp(a)mcmaster.ca> Reply-To: bandurvp(a)mcmaster.ca To: Brandon Moore <brandonm(a)yahoo-inc.com> Subject: Re: [Haskell-cafe] smallest double eps Date: Sat, 30 Sep 2006 20:17:05 -0400 Hi all, I'm new to this mailing list, so my response may be a little out of place, but I think either what's being asked is what is the smallest x such that 1 + x /= 1 (machine epsilon,) or the largest such that 1+x /= x. The bounds seem to be confused. Victor On Sat, 2006-30-09 at 16:10 -0700, Brandon Moore wrote: > Bryan Burgers wrote: > >> >>> Hang on, hang on, now I'm getting confused. > >> >>> First you asked for the smallest (positive) x such that > >> >>> 1+x /= x > >> >>> which is around x=4.5e15. > >> >> > >> >> 1 + 0 /= 0 > >> >> > >> >> 0 is smaller than 4.5e15 > >> >> > >> >> So I don't understand this at all... > >> > > >> > But then 0 isn't positive. > >> > >> Why not? > >> In any case every positive number nust satisfy the above inequation > >> so what > >> about 0.1, which is certainly smaller than 4500000000000000? > People are confusing equality and inequality - > the nontrivial thing here is to find the smallest positive x > that satisfies the equation 1 + x == x. > > In math, every positive number must satisfy the above inequation, that > > is true. But as Chad said, the smallest number in Haskell (at least > > according to my GHC, it could be different with different processors, > > right?) that satisfies the equation is 2.2e-16. > And you've changed the subject - the stuff above was talking about > x + 1 /= x, you're demonstrating solutions to a different problem, > finding the smallest > x such that 1 + x == 1. That's the number often called epsilon. > >> 1 + 2.2e-16 /= 1 > > True > >> 1 + 2.2e-17 /= 1 > > False > Let's stop confusing ourselves about this. > > Brandon > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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cant get trivial c2hs to work
by Anatoly Yakovenko 30 Sep '06

30 Sep '06

I am trying to figure out how to use c2hs, so I wrote a wrapper for asin from math.h: $ cat ASin.chs module MySin (mysin) import C2HS #include "math.h" asin::Double -> Double asin xx = {#call fun asin#} xx and this is my main: $ cat Main.hs module Main where import ASin main = do putStrLn $ show $ asin 0.5 `c2hs ASin.chs` generates this: $ cat ASin.hs -- GENERATED by C->Haskell Compiler, version 0.14.5 Travelling Lightly, 12 Dec 2005 (Haskell) -- Edit the ORIGNAL .chs file instead! {-# LINE 1 "ASin.chs" #-}module MySin (mysin) import C2HS asin::Double -> Double asin xx = asin xx foreign import ccall safe "ASin.h asin" asin :: (CDouble -> CDouble) but, when i try to build the wone thing, i get an error that i dont understand: $ ghc -v --make -fffi Main.hs Glasgow Haskell Compiler, Version 6.4.2, for Haskell 98, compiled by GHC version 6.4.2 Using package config file: /usr/lib/ghc-6.4.2/package.conf Hsc static flags: -static *** Chasing dependencies: Chasing modules from: Main.hs *** Deleting temp files Deleting: ASin.chs:2:0: parse error on input `import' So, what am i missing?

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