February 2006 - Haskell-Cafe

haskell question
by IBRAHIM MOSHAYA 17 Feb '06

17 Feb '06

please can you help me with this function: Significant Figures I need to write a function to return a given number (1st argument) to a given number of significant figures (2nd argument). The function should be called sigFig and have a type signature of the form: sigFig :: (???) => a -> Int -> a For instance: *Main> sigFig 123.456 2 120.0 *Main> sigFig 123.456 4 123.5 *Main> sigFig 0.987 2 0.99 *Main> sigFig 0.432 2 0.43 The type variable a will need to be constrained, but you should aim for the most general implementation you can (replacing ??? above with the appropriate constraint(s)). You may make free use of Prelude functions. Referring to [Jon03] you may wish to consider the use of functions / operators including: /, div, mod, ^, ^^, **, round, fromIntegral, realToFrac thanks alot

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Re: [Haskell-cafe] Decidable type systems? (WAS: Associated TypeSynonyms question)
by Robin Green 17 Feb '06

17 Feb '06

Miles Sabin <miles(a)milessabin.com> wrote: > Can someone explain to me why decidability is of any practical > interest at all? procedure which might never terminate and one which might take > 1,000,000 years to terminate? Actually, why push it out to 1,000,000 > years: in the context of a compiler for a practical programming > language, a decision procedure which might take an hour to terminate > might as well be undecidable But are there any decidable type checking algorithms that have been seriously proposed or used which would take far too long to terminate for real code? If not, then decidability is the only thing that matters. -- Robin

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library sort
by Radu Grigore 16 Feb '06

16 Feb '06

Is there a sort function in the libraries that come with GHC (6.4)? My search at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html has failed, but I can't believe there is none. -- regards, radu http://rgrig.blogspot.com/

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Cyclical Type Synonyms
by Tom Hawkins 16 Feb '06

16 Feb '06

I want to define a type for a function that returns its own type... type F a = a -> (F a,a) But the compiler gives me an error: "Cycle in type synonym declaration: ...". Why is this a restriction? Of course I can create a user defined type, but then I need an extra mechanism to call the embedded function: data F a = F (a -> (F a ,a)) call :: F a -> a -> (F a, a) call (F f) a = f a -Tom

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Haskell' Status
by Isaac Jones 15 Feb '06

15 Feb '06

I'll try to occasionally post an announcement of the the status of Haskell'[1], the next Haskell standard, so that you can all be aware of my thinking, and our current place in the timeline. There is a list of proposals and a "strawman" categorization of them on the wiki[2]. The categorization reflects someone's current thinking about what's in and out, but it's really just for discussion at this phase. The timeline for this effort is also on the wiki[4]. You'll notice that it's very aggressive; we plan to announce something at the next Haskell Workshop in September. Our current spot in the timeline is that we're meant to be brainstorming, discussing, and refining proposals. I've just posted some leading questions [3] in order to focus the discussion a bit. I can't help but notice that there is a lot of excitement (and support?) for a larger standard; one that would have a bigger impact. I think that part of the Haskell' effort should be to make a plan for moving forward. At the very least we should have a set of recommendations for what the community needs to work on between this and the next standard; what are the most promising proposals and what needs to happen to make them a reality. I think that the wiki will be a great resource for any future standard, and we should work to make it as nice as possible. Please reply to the Haskell' mailing list, and email me if you have any questions. peace, isaac [1] Haskell' Wiki: http://hackage.haskell.org/trac/haskell-prime [2] list of proposals and strawman categorization: http://hackage.haskell.org/trac/haskell-prime/report/9 [3] plan for moving discussion forward http://www.haskell.org//pipermail/haskell-prime/2006-February/000582.html [4] The TimeLine is here. Ascii text of the timeline follows (might not make sense unless you use a proportional font: http://hackage.haskell.org/trac/haskell-prime/wiki/TimeLine Write Report----Review---- | | Edit | Discuss / Refine--|---------- | | | | | | | brainstorm----------- | | | | | | | | | | | setup | | | | | | | | | | | | | ------------------------------------------------------------------- 10 | 11 | 12 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 2005 2006 ------------------------------------------------------------------- | | | | | | Face-To-Face? | Strawman Trial Decision Announce @HW

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Re[2]: strict Haskell dialect
by Bulat Ziganshin 15 Feb '06

15 Feb '06

Hello Wolfgang, Friday, February 03, 2006, 1:46:56 AM, you wrote: >> i had one idea, what is somewhat corresponding to this discussion: >> >> make a strict Haskell dialect. implement it by translating all >> expressions of form "f x" into "f $! x" and then going to the standard >> (lazy) haskell translator. the same for data fields - add to all field >> definitions "!" in translation process. then add to this strict >> Haskell language ability to _explicitly_ specify lazy fields and lazy >> evaluation, for example using this "~" sign >> >> what it will give? ability to use Haskell as powerful strict language, >> what is especially interesting for "real-world" programmers. i have >> found myself permanently fighting against the lazyness once i starting to >> optimize my programs. for the newcomers, it just will reduce learning >> path - they don't need to know anything about lazyness WJ> Since laziness often allows you to solve problems so elegantly, I'm really WJ> scared of the idea of a "Strict Haskell"! :-( Is laziness really so "unreal" WJ> that real-world programmers have to see it as an enemy which they have to WJ> fight against? WJ> In fact, I was kind of shocked as I read in Simon Peyton Jones' presentation WJ> "Wearing the hair shirt" [1] that in his opinion "Lazyness doesn't really WJ> matter". i suggest you to write some large program like darcs and try to make it as efficient as C++ ones. i'm doing sort of it, and i selected Haskell primarily because it gives unprecedented combination of power and safety due to its strong but expressive type system, higher-order functions and so on. i also use benefits of lazyness from time to time, and may be even don't recognize each occasion of using lazyness. but when i'm going to optimize my program, when i'm asking myself "why it is slower than C counterparts?", the answer is almost exclusively "because of lazyness". for example, i now wrote I/O library. are you think that i much need lazyness here? no, but that i really need is the highest possible speed, so now i'm fighting against lazyness even more than usual :) well, 80% of any program don't need optimization at all. but when i write remaining 20% or even 5%, i don't want to fight against something that can be easily fixed in systematic way. all other widespread languages have _optional_, explicitly stated lazyness in form of callable blocks, even the Omega goes in this way. and i'm interested in playing with such Haskell dialect in order to see how my programming will change if i need to explicitly specify lazyness when i need it, but have strictness implicitly. i think that newcomers from other languages who wants to implement real projects instead of experimenting will also prefer strict Haskell you may hear that last days Haskell become one of fastest language in the Shootout. why? only because all those programs was rewritten to be strict. it was slow and hard process. and adding preprocessor that makes all code strict automagically will allow to write efficient Haskell programs without reading fat manuals each laguage feature has its time. 15 years ago i could substantially speed up C program by rewriting it in asm. Now the C compilers in most cases generate better code than i can. moreover, strict FP languages now are ready to compete with gcc. But lazy languages are still not compiled so efficient that they can be used for time-critical code. so, if we don't want to wait another 10 years, we should implement easier ways to create strict programs. if you think that lazy programming is great, you can show this in shootout or by showing me the way to optimize code of my real programs. i'm open to new knowledge :) -- Best regards, Bulat mailto:bulatz@HotPOP.com

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Layout processing
by Maurício 15 Feb '06

15 Feb '06

Hi, Is it possible to ask GHC or other Haskell compiler to generate a Haskell source file just after the processing of layout? I.e., I would like to see this: main = do a <- some_function Transformed into this: main = do {a <- some_function; I thought that would be usefull to understand better how layout works. Thanks, Maurício

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Haskell XSLT interpreter?
by S. Alexander Jacobson 15 Feb '06

15 Feb '06

Has anyone written a pure haskell xslt interpreter? If not, how difficult would it be to do so? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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foreign import unsupported in GHCi on amd64?
by Daniel 14 Feb '06

14 Feb '06

Hi, is foreign import expected to work when using GHCi (ghc 6.4.1) on an amd64 linux platform? I'm just (attempting) to play around with some code (Oleg et. al.'s Zipper FS) that has the following lines in it: ZFS/ZFS.hs:522:foreign import ccall "unistd.h select" c_select ZFS/ZFS.hs:573:foreign import ccall "fcntl.h fcntl" fcntl and I get: Compiling ZFS ( ZFS.hs, interpreted ) ghc-6.4.1: foreign import is not implemented for GHCi on this platform. Ticket 290 ( http://cvs.haskell.org/trac/ghc/ticket/290 ) seems to imply the foreign import wrappers work in ghc 6.4. Cheers Daniel

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List-to-outline
by Steve Schafer 14 Feb '06

14 Feb '06

I have some lists of integers; e.g., [0,1,2,2,3,3,3,2,1,2,3,3,1] Think of each integer value as representing the indentation level in a hierarchical outline: e.g., 0 1 2 2 3 3 3 2 1 2 3 3 1 I want to convert the list into a structure that better represents the hierarchy. So, I first define a datatype to represent each node of the new structure: data Node = Nd Int [Node] That is, a node consists of an Int representing the value of the node, followed by a list of its immediate child nodes. (In principle, I can deduce the value of a node simply from the nesting level, of course, but in the real problem I'm trying to solve, each node contains other information that I need to preserve as well.) Next, I define some functions to perform the transformation isChild :: Int -> Node -> Bool isChild i (Nd j _) = (j > i) isChild _ _ = False prepend :: Int -> [Node] -> [Node] prepend i [] = [Nd i []] prepend i ns = (Nd i f):s where (f,s) = span (isChild i) ns unflatten :: [Int] -> [Node] unflatten ns = foldr prepend [] ns Finally, I add some code to display the result in an aesthetically pleasing way: showsNodeTail :: [Node] -> String -> String showsNodeTail [] = showChar '}' showsNodeTail (n:ns) = showChar ' '.shows n.showsNodeTail ns showsNodeList :: [Node] -> String -> String showsNodeList [] = showString "" showsNodeList (n:ns) = showChar '{'.shows n.showsNodeTail ns showsNode :: Node -> String -> String showsNode (Nd i ns) = shows i.showsNodeList ns instance Show Node where showsPrec n = showsNode This all works just fine, and when I enter unflatten [0,1,2,2,3,3,3,2,1,2,3,3,1] I get [0{1{2 2{3 3 3} 2} 1{2{3 3}} 1}] as expected. The reason I'm posting this here is that I have a gnawing suspicion that the unflatten/prepend/isChild functions, and possibly the Node data type as well, are not the most elegant way to go about solving the problem, and that I'm missing another more obvious way to do it. Any suggestions? Thanks, Steve Schafer Fenestra Technologies Corp. http://www.fenestra.com/

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