October 2008 - Haskell-Cafe

Re: What I wish someone had told
by John Lato 17 Oct '08

17 Oct '08

Richard O'Keefe wrote: > On 17 Oct 2008, at 9:53 am, Daryoush Mehrtash wrote: > >> So does this mean that the reason for complexity of generics is the >> Java inheritance? > > No. The reason for the complexity of generics in Java is that > they weren't designed into the language in the first place. > It took several attempts and quite a lot of work to come up with > a version of generics that was 100% interoperable with earlier > JVMs *and* still worth having. > > You could have something very like Java generics but without > the strangeness if you designed it into the language from the > beginning, as Betrand Meyer did with Eiffel. Of course, what > _he_ didn't design in from the beginning was lambdas, now > present as "agents". Eiffel has its own kinds of strangeness. > Or C# generics, which are built into the IL (in .Net 2.0 and above). In many ways, C# is a very nice language, and having generics fully supported in reflection is pretty cool. I do have some words to say about various ASP.Net libraries, however...

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howto mix <- within do?
by Larry Evans 17 Oct '08

17 Oct '08

The attached code produces error: <-- cut here -- runghc -dcore-lint do_with_assignment.proto.hs do_with_assignment.proto.hs:30:2: Couldn't match expected type `[]' against inferred type `IO' Expected type: [t] Inferred type: IO () In the expression: putStr "v0=" In a 'do' expression: putStr "v0=" >-- cut here-- However, similar code here: http://www.nabble.com/List-as-input-p19987726.html apparently does work. Is there some compiler option I need to use? The compiler is ghc: ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2

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Invalid declaration names in TH that typecheck
by Sean Leather 17 Oct '08

17 Oct '08

[I posted this on the Template Haskell list, but nobody seemed interested. Thus, I'm re-posting it here for higher coverage.] Apparently, I can create a function declaration (FunD) with a name containing an invalid sequence of characters. It passes typechecking and compiles. It even shows up in GHCi! The function name is "ep:#:", and the ":#:" is derived from an infix type constructor. In GHCi (6.8.3), I type "ep", hit <Tab> to get the list of completions, and, sure enough, there it is! When I try to use it, GHCi interprets it as two distinct symbols, "ep" and ":#:", as it should. Template Haskell's claim to fame is that it allows you to do "type-safe compile-time meta-programming," and while this issue is perhaps considered lexical rather than type-level, I'm wondering if there should be some additional error checking (somewhere) to account for this. I admit though, that I wouldn't know how to do this easily within the bounds of the TH API. The Name type is common to functions, infix operators, and everything else. That which is valid for one is not necessarily so for the other. Regards, Sean

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List as input
by leledumbo 17 Oct '08

17 Oct '08

module Main where import Data.List -- quicksort of any list qsort [] = [] qsort (x:xs) = qsort(filter(<x) xs) ++ [x] ++ qsort(filter(>=x) xs) -- optimized quicksort, uses middle element as pivot qsortOpt [] = [] qsortOpt x = qsortOpt less ++ [pivot] ++ qsortOpt greater where pivot = x !! ((length x) `div` 2) less = filter (<pivot) (delete pivot x) greater = filter (>=pivot) (delete pivot x) main = do putStr "Enter a list: " l <- readLn print (qsortOpt l) -- end of code Why do I get ERROR "qsort.hs":17 - Unresolved top-level overloading *** Binding : main *** Outstanding context : (Read b, Show b, Ord b) ? -- View this message in context: http://www.nabble.com/List-as-input-tp19987726p19987726.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Terminal-like Application Design
by Jeff Wheeler 17 Oct '08

17 Oct '08

Hi, I'm a slight Haskell newbie, but I'm trying to write a terminal-like application that accepts simple commands with optional arguments, and can then execute them. Most of these commands will need IO, as later I will want to communicate over USB for most of them. I was hoping, though, that I could get some comments on the initial architecture I've been playing with [1]. I suspect I should be using some sort of monad to represent the commands, but I don't fully understand monads, and am not sure how it would apply in this context. Should I be using a monad here, and if so, how? Thanks in advance, Jeff Wheeler [1] http://media.nokrev.com/junk/cli/

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[fun] HaskellDB Talk trailer
by Ahn, Ki Yung 17 Oct '08

17 Oct '08

There is an impressive HaskellDB Talk trailer on the web. http://www.vimeo.com/1983774 Cheers to the HaskellDB developers :-)

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Arch Haskell News: Oct 4 2008
by Don Stewart 17 Oct '08

17 Oct '08

A weekly update about Haskell on Arch Linux. Highlights, * Arch now has 609 Haskell packages in AUR. That’s a record increase of 19 new packages in the last 7 days. Noteworthy, * mp3decoder-0.0.1: “MP3 decoder for teaching.” * yi-0.4.6.2: “The Haskell-Scriptable Editor” * haskell-haskore-0.0.5.1: “The Haskore Computer Music System” Full update list, http://archhaskell.wordpress.com/2008/10/04/arch-haskell-news-oct-4-2008/ -- Don

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What I wish someone had told me...
by John Lato 17 Oct '08

17 Oct '08

I was just thinking about what I wish someone had told me when I started working with Haskell (not that long ago). It would have saved me a great deal of trouble. The Difference Between Interfaces and Type Classes. Many "Introduction to Haskell for the OOper" style tutorials claim that type classes are like Interfaces (for languages with that feature). I now think that, although this is technically true, it's fundamentally misleading. Here's a simple example demonstrating my line of thought: In C# (and presumably Java), this sort of function is common: public IList GetListOfData() In Haskell, a similar function may be GetListOfData :: (Foldable a, Indexable a) => IO a In C#, it doesn't matter what the actual type returned by GetListOfData is, as long is it supports the IList interface. It's not uncommon for GetListOfData to make a choice between several different implementations, depending on the nature of the data to be returned. The following code is perfectly reasonable in C# : // List and MyList are different classes if (something) { return new List(); } else { return new MyList(); } The equivalent won't compile in Haskell, because the actual return type does matter, and *is determined by the calling code*. Our fictional GetListOfData can't return a List or a Mylist depending on some conditional, in fact it can't explicitly return either one at all, because the actual type of the result, as determined by the caller, could be either one, or something else entirely (ignoring the IO bit for the time being). So I've come to the conclusion that stating type classes are like interfaces is misleading to newcomers to Haskell, because it leads people to think they should use type classes for the same sorts of problems OO-languages solve with interfaces. In turn this means new programmers are encouraged to use OO-style architecture in programs, reassured that they're in a "functional" idiom because they're using the "functionally-approved" feature of type classes. I think that if I had understood this earlier, I would have embraced functional idioms more quickly. Incidentally, I'm still horrible at designing functional APIs and modules, but at least now I know it, and I'm no longer trying to force OO interfaces into Haskell. So I've made progress.

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Haskell newbie indentation query.
by Ramaswamy, Vivek 17 Oct '08

17 Oct '08

Hello All~ I have just started with Haskell, and I must confess; I am in love with it. However one area that I am really confused about is indentation. Lets take a look at if-else if- else block. The way I understand it: {------} if something then do something 1 something2 else if nothing then do something3 something4 else do different {-------} The code above gives out an error. I have been programming in python and the above appears fine. But it does not work. What works is: if something then do something1 something2 else if then do something3 something4 else do different I find the above scheme extremely confusing. I tried going to: http://en.wikibooks.org/wiki/Programming:Haskell_indentation after reading I am even more confused. Can somebody please explain how the Haskell indentation works? The else-if and else seem to be aligning up with "then". They should be aligning with "If" in my opinion. Thanks in advance. Regards -Vivek Ramaswamy-

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Re: [Haskell-cafe] What is this function? (RESOLVED)
by John Ky 16 Oct '08

16 Oct '08

Hi Luke, Thankyou so much. I need the sequence function. What I was after was this: class Scramblable a where scramble :: a -> [a] instance Scramblable [MyType] where scramble values = sequence (map scramble values) instance Scramblable MyType where scramble myType = {- blah blah -} I was basically after exhaustively generating lots ASTs given a template AST with lots of leaf values changed. Thanks, -John On Thu, Oct 16, 2008 at 8:39 PM, Luke Palmer <lrpalmer(a)gmail.com> wrote: > 2008/10/16 John Ky <newhoggy(a)gmail.com>: > > Hi, > > > > I've written this function here: > > > > scramble [] = [] > > scramble [x] = [[z] | z <- scramble x] > > scramble (x:xs) = > > [(y:z)|y <- scramble x, z <- scramble xs] > > > > and (I think) it roughly does what I want it to: > > > > *Main> scramble ([]::[Int]) > > [] > > *Main> scramble ([1]::[Int]) > > [[1],[2]] > > So, um, this is nonsense. You've given it only 1, and yet it outputs > a 2, yet there is no mention of addition or the literal 2 anywhere in > your function. > > This function looks a lot like the more sensible: > > scramble' n xs = sequence (replicate n xs) > > >>> scramble' 0 [1,2,3] > [[]] > >>> scramble' 1 [1,2,3] > [[1],[2],[3]] > >>> scramble' 2 [1,2,3] > [[1,1],[1,2],[1,3],[2,1],[2,2],[2,3],[3,1],[3,2],[3,3]] > > Where your examples all had [1,2] as the argument. > > Luke >

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