September 2006 - Haskell-Cafe

variadic functions and typeCast
by Michael Shulman 02 Oct '06

02 Oct '06

Hi all, I am writing, for my own amusement, a more general version of the trick to implement variadic functions in Haskell outlined at http://okmij.org/ftp/Haskell/vararg-fn.lhs. (If someone else has done this already, please point me to it!) Code is attached at the end of the message. My question concerns the use of `typeCast', which is defined as follows: class TypeCast x y | x -> y, y -> x where typeCast :: x -> y instance TypeCast x x where typeCast = id This is taken from the paper "Strongly typed heterogeneous collections" by Oleg Kiselyov, Ralf Lammel, and Keean Schupke. As observed there, the TypeCast instance declaration must occur in a separate module; otherwise the compiler is too eager about type simplification. (I also get the same errors if VarArg is interpreted by GHCi, even if TypeCast is in a separate module.) I think I understand the reason for this, but I find it a little surprising; it wasn't clear to me from the documentation that fundeps introduce a new dependence on module boundaries. Anyway, my main question about typeCast is this: why is it needed here at all? If I omit it entirely, the code compiles fine, but then using it gives error messages like the following: Prelude VarArg> build 'h' 'i' :: String <interactive>:1:0: No instances for (VarFold [a] [a] [Char], VarAccum Char [a]) arising from use of `build' at <interactive>:1:0-4 ... I don't understand why the type-checker is unable to infer that the type variable `a' should be specialized to `Char' here, since the only available instance of VarFold whose third type is [Char] has the first type also being [Char]. I've given the compiler all the type hints I can think of. Can someone explain this to me? I should say that if I add a functional dependency "l -> a" to the class VarAccum, then this particular example works. However, I have other examples in mind for which l doesn't functionally determine a, so I don't want to do that. And I don't see why it's necessary. Here's the code: class VarAccum a l where accum :: a -> l -> l class VarFold b l r where varFold :: (l -> b) -> l -> r instance (TypeCast b c) => VarFold b l c where varFold f xs = typeCast (f xs) instance (VarFold b l r, VarAccum a l) => VarFold b l (a -> r) where varFold f xs x = varFold f (accum x xs) -- This is the type of variadic functions from lots of As to a B. type a :--> b = forall r. (VarFold b [a] r) => r infixr 0 :--> instance VarAccum a [a] where accum x xs = (x:xs) varArg :: forall a b. ([a] -> b) -> (a :--> b) varArg f = varFold (f . reverse) ([] :: [a]) build :: forall a. a :--> [a] build = varArg (id :: [a] -> [a]) Thanks! Mike

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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: 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 30 Sep '06

30 Sep '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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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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Is Haskell a 5GL?
by David Curran 30 Sep '06

30 Sep '06

Sorry if this comes across as the rant it is. If you are interested in doing useful stuff rather then navel gazing please stop here. Where are compute languages going? I think multi core, distributed, fault tolerant. So you would end up with a computer of the sort envisioned by Hillis in the 80s with his data parallel programs. The only language that seems even close to this model is Erlang. What am I missing about the ability of Haskell to distribute across processors or a network? Say instead of fault tolerant it is fault avoiding. Can proving programs correct (with Haskell) really reduce our workload? http://www.seas.upenn.edu/~sweirich/wmm/03-benton.pdf Finally is Haskell a language for programming or a mental gymnasium that might be the proving ground for concepts in the next popular language? To quote from a post on the topic "Old functional programming ideas " on programming.reddit.com "Church-Turing equivalence tells us that all models of recursive computing have the same formal power. But it tells us nothing about which models are the most effective way for humans to understand and express definitions of functions. For some reason I'd expect researchers in programming languages to have a lot of opinions on this subject. But they don't seem to talk about it much. Instead, a cynical and mean-spirited person might come to the conclusion that PL researchers (such as Wadler) are actually just mathematicians, who happen to have discovered a new name for their specialty which comes with a lot more funding. They certainly seem quite comfortable working in a notation that almost no programmers understand or seem to be learning. If a responsible scientist wanted to counter this cynical, mean-spirited, and generally Luddite and Philistine argument, what would he or she say?"

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Re: [Haskell-cafe] smallest double eps
by Chad Scherrer 30 Sep '06

30 Sep '06

> > 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. > Then Joachim wondered if you wanted > 1+x /= 1 > which is around x=2.2e-16. > But not you claim to be looking for the smallest positive number that > a Double can represent. Which is a totally different beast. The > smallest possible Double depends on if you want to accept > denormalized numbers or not. If you don't, then it's about x=4.5e-308. > > Now what is the number you are looking for? > > -- Lennart > This is the point I was confused about also. Joachim seemed to be correcting what might have been a misstatement of the problem.

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smallest double eps
by Tamas K Papp 29 Sep '06

29 Sep '06

Hi, Is there a built-in constant in Haskell (or, if it is compiler-specific, in ghc) that gives the smallest positive floating point number x such that 1+x /= x? Some languages refer to that as double.eps or similar. I need it for numeric algorithms. Thanks, Tamas

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