- Haskell-Cafe - Haskell.org

closures with side effects
by dkarapet 26 Jun '06

26 Jun '06

I have been trying to understand closures in haskell and how they relate to side effects. I have been looking around but all I find are trivial examples with no side effects. Please let me know if you know of any examples.

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Scoped data declarations
by Christophe Poucet 26 Jun '06

26 Jun '06

Dear, Yesterday, while discussing with Cale and SamB on I suddenly came up with the crazy idea of scoped data declarations. After some brief discussion to check the validity, I finally came to the conclusion that they should be feasible. In addition, I don't think that they would require a high amount of changes in current compilers. Basically if you have something like: module Main where foo = let data Foo = Foo deriving Show in Foo\ main :: IO () main = print foo One can see this as having an extra hidden module that defines Foo but that does not export it. The only change that is then required is that while compiling Foo, the hidden-ness of Foo must be removed. For instance, if one were to load this into, say, ghci (this is fictive of course): # ghci Main.hs > :t foo foo :: Codeloc2.Foo There were initially some objections to this, because it is no longer feasible to actually write the type of the function foo. But if one looks at current GHC, this objection is already there: module A(foo) where data Foo = Foo deriving Show foo = Foo module Main where import A main = print foo As Excedrin then pointed out, importing this Main into ghci, gives foo :: Foo.Foo And this notation can not be written in Main either, because Foo is hidden in A. Therefore, I would like to note that scoped data declarations are just like hidden data-declarations with two extra requirements: 1) Generate source-location-based submodule names 2) Add an extra import rule for those hidden modules in the subexpressions of where the data-declaration is being originally defined. Comments are welcome, of course :) Cheers! Christophe (vincenz)

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Re: [Haskell-cafe] Re: Functional progr., infinity, and the Universe
by voigt.16734551＠bloglines.com 26 Jun '06

26 Jun '06

--- paul.hudak(a)yale.edu wrote: Jerzy Karczmarczuk wrote: > > OK, I think that this subject matured enough to rest in peace... > > I would have to agree with that, although... Since the subject is not going to rest, why not also jump in? > Well, each partial list is finite. I think quite a few people would agree that a finite list is one ending in []. So 1:_|_ is a partial list, but not a finite one. 1:[] is a finite list. > But the limit of a chain IS the maximal element of the set of all > elements comprising the chain, since the LUB, in the case of a chain, is > unique, and thus we don't have to worry about choosing the "least" > element (i.e. it reduces to the upper bound, or maximal element). That doesn't quite make sense, IMHO. The chain _|_, 1:_|_, 1:1:_|_, 1:1:1:_|_, ... has the limit (or upper bound) "ones", but "ones" does not appear in the chain. An upper bound of a set may not appear in the set. But the maximal element of a set by definition ("element of") necessarily is in the set. So you cannot use the two notions interchangably. BTW, if the limit of a chain would always appear in the chain (by virtue of being the maximal element of the set of all elements comprising the chain), then every chain would be stationary eventually. That would quite simplify denotational semantics, wouldn't it? > So I'd say that Brian has at least come close to discovering God :-) And come to the conclusion, in a later mail, that both: A. "the list [0,1,2,3,4,5,..] is longer than [1,2,3,4,5,..]" and B. "ie [0,1,2,3,4,..] is the same length as [1,2,3,4,5,..]" I think not. Obviously not. I know that this conclusion was qualified by "assuming that they all grow at the same rate", which of course has no counterpart in the denotational semantics setting discussed above. So it's not a wrong statement, just one that cannot really be formulated. Well, anyway, didn't want to be nitpicking. But I think it's important that if we want to think about Haskell's semantics denotationally (which in itself is somewhat problematic), we should at least be careful not to blur concepts. And asserting that the limit of a chain is always an element of that chain is a dangerous oversimplification that does not work out. Ciao, Janis Voigtlaender.

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parsec, how to feed single tokens?
by Marc Weber 25 Jun '06

25 Jun '06

Hi. I want to write a haskellquickfind app to get a list of files using human readable hash values (eg the first character of directory/filenames) Eg hquickfind vtl should print /var/tmp/local My implementation should look like this: Every folder/file is token and I want to run a parser. to consume /var Then I want to run it with all subdirs in var where it should fail on anything but t.* ... My Problem : I can't access runP directly to feed the parser with one token and get the result (failure, need more tokens).. Have I missed a function or isn't parsec meant to be used this way? Would you recommend writing your own small parser for this? Another question: Can you use another SourcePos state than SourcePos? Marc

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Haskell developer is needed
by pvainuk＠aol.com 25 Jun '06

25 Jun '06

An enthusiastic Haskell Developer with good C++ skills is needed to assist with the rollout of a new system. This is a high profile project and the successful candidate will work in a very strong business oriented team where they will gain experience in wide variety of products and systems. Essential skills include: Haskell language, C++, office technology, Windows environment. Some finance knowledge is desired, but not essential. The ideal candidate with in depth understanding of computer science basics must be passionate about Haskell. You will need a 1st class computer science degree or demonstrate sound work experience. A successful candidate must be prepared to work under the pressure in the very dynamic environment extended hours. This is an excellent opportunity for a talented developer to join a well established company with global presence and to work on the most cutting edge technologies and to really progress your career. Location: HQ, London, UK Please send your CV to pvainuk(a)aol.com

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ReadP and MonadFix
by Gracjan Polak 24 Jun '06

24 Jun '06

Hi all, A question for hot summer day: Text.ParserCombinators.ReadP.ReadP is an instance of Monad. Could it be an instance of MonadFix too? I'm not that sharp in Haskell to write it myself, but it seems I could make use of such a beast. :) Anybody willing to share? This will also present the advantage of Lazy over Eager Parser Combinators, mentioned in some other thread. -- Gracjan

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Binary IO
by jeff lasslett 24 Jun '06

24 Jun '06

Greetings, I am considering writing -in Haskell of course - a small program to translate binary files to human readable text. The trouble is that I can find no easily digestible tutorial style info on how to do binary IO in Haskell. I have read about some of the libraries that people have created to do binary IO, but the documentation wasn't sufficient to get me started (perhaps because I'm a bit dense, and also relatively new to Haskell). What I would like is to see a short example showing some code that reads from a binary file. I would also like to know what the most widely used library for doing binary IO in Haskell is. I would greatly appreciate it if someone could post a small example or two illustrating how to do binary IO in Haskell using the most widely used binary IO lib (if there is such a thing). Failing that, I would appreciate a link to some Haskell code that does binary IO that I could study. Lastly, if there is a good tutorial on doing binary IO in Haskell then I would appreciate a link to that as well. Thanks in advance, Jeff Lasslett

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newbie - IO issues (of course)
by Geoffrey King 24 Jun '06

24 Jun '06

I have been work my way through "Haskell The Craft of Functional Programming", all was fine until IO (chapter 18). That is causing me bafflement. I am trying to write a totally trivial program that reads a series of integers from the console until it gets a zero, then returns the series of integers as a list. I am down to randomly editing my code, never a good sign. Any clues would be appreciated, My code so far, which clear doesn't work, is: getInt :: IO Int getInt = do putStr "Enter number (zero to quit)" line <- getLine return (read line :: Int) anIntList :: [Int] anIntList = do let n = getInt if n == 0 then return [] else return (n : anIntList) ps: even a simple version that returns an Int i can't get to work. anInt :: Int anInt = do n <- getInt return n

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Lambda abstraction analogous to imperative pseudo-code?
by Clifford Beshers 23 Jun '06

23 Jun '06

The Wikipedia article on lambda abstractions (http://en.wikipedia.org/wiki/Lambda_abstraction) has a statement that does not resonate with me: A lambda abstraction is to a functional programming <http://en.wikipedia.org/wiki/Functional_programming> language such as Scheme <http://en.wikipedia.org/wiki/Scheme_programming_language> what pseudo-code <http://en.wikipedia.org/wiki/Pseudo-code> is to an imperative programming <http://en.wikipedia.org/wiki/Imperative_programming> language. Does anyone else find this to be a peculiar statement? If you think it is accurate, could you provide an alternate explanation and/or example to the one in the article?

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Functional programming for processing of large raster images
by oleg＠pobox.com 23 Jun '06

23 Jun '06

Recently Vo Minh Thu wondered if Haskell (or, I generalize, functional programming) can be of much use for computer graphics programming. I'd like to point out a project that experimentally shown that functional programming is of great help for processing of large raster images (24-bit PPM files). The paper describing the system has been accepted for `Science of Computer Programming' and is in press: Combining Partial Evaluation and Staged Interpretation in the Implementation of Domain-Specific Languages Christoph A. Herrmann, Tobias Langhammer The code for the system is available online http://infosun.fmi.uni-passau.de/cl/metaprog/imagefilter2-2005-10-30.tgz The previous [slightly obsolete] version of the paper, presented at the MetaOCaml'04 workshop, is available online http://www.fmi.uni-passau.de/forschung/mip-berichte/MIP-0410.html The paper includes a few pictures and the table with benchmark, so we can observe the benefits. Now, with offshoring and native MetaOCaml fully implemented, we can gain bigger benefits. In this project, functional programming does what it is probably best suited: to develop a `compiler' -- a compiler from a filter specification to quite efficient code. The code is specialized to all available static information. To be precise, the authors implement an interpreter -- which is then specialized to the source program and so interpretative overhead is removed and the code is optimized. Staging an interpreter automatically gives a compiler. Futamura projections are not only the fascinating, but useful, too.

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