March 2007 - Haskell-Cafe

Proposal: simple interface to libraries Haddock
by David House 03 Mar '07

03 Mar '07

Hey all, I'm proposing something simple that should make it easier for both Haskell programmers and Haskell tool writers to find the documentation for a function. We need a simple URL for the Haddock docs of any function. PHP has this already: just hit php.net/funcname for the documentation for funcname(). My proposal is similarly simple: http://docs.haskell.org/funcname A CGI script would find funcname in the index and redirect to the URL of the docs for funcname. The only contentious issue is what to do in case of ambiguous lookups; i.e. would docs.haskell.org/lookup be Data.Map.lookup, or Data.List.lookup? My suggestion is that we present a page saying 'which function did you mean?'. We should also allow a way to be unambiguous in the URL: http://docs.haskell.org/Data.Map.funcname Pros: 1) It's really easy to find the documentation for any function. 2) It's really easy to write a tool that finds documentation for a function (including Firefox keywords, Emacs/Vim integration, GHCi integration and so on). Any comments? -- -David House, dmhouse(a)gmail.com

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instance Eq Exception??
by Isaac Dupree 03 Mar '07

03 Mar '07

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 It seems the Exception datatype is currently an instance of Eq, that does not even satisfy reflexivity (for Dynamic exceptions). Does anybody use this? What is it good for? Why is it there? What if we delete the instance because it doesn't make sense? Exception is not an instance of Ord currently; it also has an instance Show that does make a little sense - for Dynamic exceptions, it gives (in GHC, not Hugs) the information of what the exception's type looks like. Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.3 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFF6bSNHgcxvIWYTTURArNRAJwMmp/keolibaZggANLCe5IMdit7ACcDZkp 1WP7Fkdj36ZEeHxHFem1py8= =gkfv -----END PGP SIGNATURE-----

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Re: [Haskell-cafe] MPTCs and rigid variables
by David House 03 Mar '07

03 Mar '07

On 03/03/07, Bryan Burgers <bryan.burgers(a)gmail.com> wrote: > As far as I know, you should be defining the type of bar in the class > declaration for Foo, not in an instance declaration. I think I over-reduced my problem. Here's a more complex issue that can't be resolved by moving bar to be a method of the class: {-# OPTIONS_GHC -fglasgow-exts #-} class Foo a b | a -> b where foo :: Foo b c => a -> Maybe c instance Foo String () where foo _ = Nothing instance Foo Int String where foo 4 = Just (); foo _ = Nothing -- -David House, dmhouse(a)gmail.com

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An HSH case study
by John Goerzen 03 Mar '07

03 Mar '07

Some of you may be interested in my recent blog post[1], where I discuss porting hg-buildpackage[2] from a set of shell scripts to a set of Haskell programs that use HSH. To my surprise, I achieved a 20% *reduction* in source lines of code, while doing the exact same tasks. I guess I have to consider switching more code to HSH. Purely in the interests of science, of course ;-) [1] http://changelog.complete.org/posts/587-A-better-environment-for-shell-scri… [2] http://hg.complete.org/hg-buildpackage

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ANN: HSH 1.0.0
by John Goerzen 03 Mar '07

03 Mar '07

Hello, I'm pleased to announce the first release of HSH. HSH is designed to let you mix and match shell expressions with Haskell programs. With HSH, it is possible to easily run shell commands, capture their output or provide their input, and pipe them to/from other shell commands and arbitrary Haskell functions at will. HSH makes it easy to run shell commands. But its real power is in piping. You can pipe -- arbitrarily -- between external programs, pure Haskell functions, and Haskell IO functions, mixing and matching at will. Here are a few examples to get you started: runS $ "echo /etc/pass*" -> "/etc/passwd /etc/passwd-" run $ "ls -l" -|- "wc -l" -> 12 run $ "ls -l" -|- wcL -> 12 wcL is a pure Haskell function: wcL :: [String] -> [String] wcL inp = [show $ genericLength inp] More information, including downloads and further examples, is at the HSH homepage: http://software.complete.org/hsh -- John

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Very Basic IO question
by Joe Olivas 03 Mar '07

03 Mar '07

I am VERY new to Haskell, and just getting my feet wet with functional programming in general. I've been going over a few examples online, but I can't figure out the behavior I'm seeing on a very basic example: --- module Main where import System.IO main :: IO () main = do putStrLn "Please enter your name: " name <- getLine putStrLn ("Hello, " ++ name ++ ", how are you?") --- This example works as I would expect. The prompt displays with a new line, the input is entered and echoed: --- Please enter your name: joe Hello, joe, how are you? However, changing 'putStrLn' to 'putStr' does not do what I would expect. The prompt doesn't get displayed until after there is input: --- joe Please enter your name: Hello, joe, how are you? Why does it exhibit this behavior? I'm using a ghc 6.6 built from source from 20070227. The regular 6.6 release had the same behavior. Any help would be greatly appreciated. -Joe -- Using Opera's revolutionary e-mail client: http://www.opera.com/mail/

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Hi I need help for very simple question!
by iliali16 03 Mar '07

03 Mar '07

Hi Haskell People, I have problem implementing one function. I think my idea is write but I have minor mistakes which I cannot get right since I progrem Haskell from very recently. No the function is called intoWords and has to take a string of any size and kind of characters.No I have to produce as output a list of strings.To have a rule which defines an element from the list is the space character which can also be included in the string.So what i think should be smth like. intoWord Hello my name is Smart should produse a list looking like [Hello,my,name,is,Smart] Now I tried to do it recucively to take the string and checks if an element is white space in the list using the isSPace build in function. This is my code please help me if you have any suggestions. Also I don't know how to produce a base case for my recursion. import Char intoWords ::String -> [String] intoWords (x:xs) |isSpace x = intoWords xs |otherwise = xs Thanks in advance for any suggestions. I really want to understand this bloody function couse I've been tring for a week now and I have tried so many ways to produce it that now I am fed up with it and I want to know how should it work since I am curious now. -- View this message in context: http://www.nabble.com/Hi-I-need-help-for-very-simple-question%21-tf3334486.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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numeric minimization in Haskell
by Chad Scherrer 02 Mar '07

02 Mar '07

Does anyone know of any Haskell code for numeric minimization? I was thinking conjugate gradient would be good, but at this point I'd be happy with anything. I've found some code written by Tomasz Cholewo at http://ci.uofl.edu/tom/software/Haskell/ but it requires importing his "Arr.lhs" library, which is not publicly available. The only other thing I've been able to dig up is this www.st.cs.ru.nl/papers/1997/serp97-cgfunctional.ps.gz which suggests Haskell is slow for such problems. I suspect this was an implementation issue, so I don't think their code would be very helpful (though it would be nice to tidy it up and demonstrate the improvement - could it beat the Clean implementation they give?) The other possibility I was considering was using Alberto Ruiz's wrapper for the GSL library http://dis.um.es/~alberto/GSLHaskell/ The only problems with this are (1) requires having GSL available, so it's not as portable, and (2) does everything in terms of lists, which requires a lot of translations to and from lists (I'm using mutable arrays). If there's nothing already written that works together, one of these should give me a start, but I'd like to avoid reinventing the wheel if possible. Thanks! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Takusen and strictness
by Paul Moore 02 Mar '07

02 Mar '07

I'm still fiddling with simple database queries using Takusen. One question I have is regarding strictness. I've just been reading "Haskell IO for Imperative Programmers" and loved the idea that laziness (rather than monads) is what makes IO in Haskell so simple. What I'm not sure about, is whether Takusen offers a way of lazily getting data from a database (and a meta-question - how do I work this out for mysefl?!) With the code {-# OPTIONS -fglasgow-exts #-} import Database.Oracle.Enumerator import Database.Enumerator runSql :: String -> IO [String] runSql s = withSession (connect "USER" "PASSWD" "DB") ( do let iter (s::String) accum = result (s : accum) r <- doQuery (sql s) iter [] return r ) main :: IO () main = do r <- runSql "select username from all_users" putStrLn (unlines r) I can do a simple database query, process the results, and produce output. The sreucture of main is much like an interact loop, but with the input from a database rather than stdin. But, will this read the database lazily, or will it get all the rows into memory at once? How will using result' instead of result (in runSql) affect this? And as I said above, how can I learn to work this out for myself? I know I could just ignore the issue until performance becomes a problem and then profile - but that ignores the question of whether I've got the idiom right. Thanks for any help, Paul.

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Re: [Haskell] Laziness and the IO Monad (randomness)
by Kirsten Chevalier 02 Mar '07

02 Mar '07

[redirecting to haskell-cafe] On 3/1/07, Dave Tapley <dukedave(a)gmail.com> wrote: > However this latter case gets stuck in an infinite loop, terminating on a stack overflow. > > My question asks why this is the case, when laziness should ensure only the first 10 cases need to be computed. > You didn't say which function you had narrowed down the stack overflow to, but I suspect it's here: > firstTen :: IO [Int] > firstTen = do > infiniteNums <- iterateM addRand randNum > return (take 10 infiniteNums) In order for IO to work the way you'd expect, you have to be able to specify which IO operations happen in what order, which is exactly what a do-block within the IO monad specifies. But specifying the order of operations also means specifying in what order evaluation happens. So, the code you've written means "evaluate infiniteNums fully (in order that we might do whatever IO operations it involves before the return statement), then return the first 10 elements of its result." Since you're forcing evaluation of an infinite list, the program loops infinitely. If you find this counter-intuitive, you wouldn't be the first, but there's no other sensible interpretation for the code you've written. Deferring evaluation of infiniteNums in this case would mean that IO would happen in a strange and unpredictable order (or if it actually wouldn't, then the compiler isn't smart enough to know that.) I would solve this problem by writing a version of iterateM that takes the length of the desired list as an argument (e.g., 10 in your example), which sadly breaks modularity, but as I explained above, working within the IO monad necessitates that. Someone more l33t than me may be able to suggest a more elegant solution, though. Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "They killed, they maimed, and they called information for numbers they could easily look up in the book." -- Woody Allen

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