- Haskell-Cafe - Haskell.org

Printing the empty list.
by Joshua Ball 02 Jul '11

02 Jul '11

GHCi seems to be clever about some things: If I try to print the empty list in ghci, I encounter no problems: Prelude> [] [] Prelude> show [] "[]" Prelude> print [] [] Even though the type of the list is clearly unknown, it must be picking SOME type. (why does it print [] instead of "")? If I write a program in a file and load it in main = print [] Then I get the ambiguous type variable error that I would expect. Why doesn't ghci generate this error at the prompt? -- Borrow my books: http://goo.gl/UBbSH

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Period of a sequence
by michael rice 02 Jul '11

02 Jul '11

MathWorks has the function seqperiod(x) to return the period of sequence x. Is there an equivalent function in Haskell? Michael

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Re: [Haskell-cafe] ~ type operator
by Henning Thielemann 02 Jul '11

02 Jul '11

On 24.06.2011 11:10, Daniel Fischer wrote: > On Friday 24 June 2011, 10:26:42, Guy wrote: >> What does the ~ type operator mean? I've sometimes seen types such as (a >> ~ b) in error messages, but can't understand what GHC is trying to tell >> me. > > Type equality, (a ~ b) means that a and b are the same type (rather, that > the compiler can prove they're the same). This type operator is needed when using type families. With functional dependencies you can assert a bijection between types by writing class C a b | a -> b, b -> a where but using type functions you have to tell the compiler somehow that one type function reverses the mapping of another function. This is where (~) is needed. You can use this type operator, when LANGUAGE TypeFamilies is enabled. I assume that the use of (~) in type error messages reflects that GHC's type checker is now internally based on type families. There are also other circumstances, where GHC uses non-Haskell-98 syntax in its messages, e.g. it uses explicit 'forall' quantifiers in error messages also if they are disabled in the affected module.

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ANNOUNCE: Hs2lib-0.4.8
by Phyx 02 Jul '11

02 Jul '11

What is it? ======== A preprocessor and library which allow you to create dynamic libs from arbitrary annotated Haskell programs with one click. It also allows you to use the generated lib in C, C++ and C# just by including the generated header files. At a minimum it can be considered the inverse of c2hs. Where to get it? ============ You can get it from Hackage (http://hackage.haskell.org/package/Hs2lib) or by using cabal (cabal install Hs2lib). Documentation, Mailing List, Source, etc ======================= Go to http://mistuke.wordpress.com/category/hs2lib/ for information. What's New? ========= - Currently Supported: * Generates Marshaling information for arbitrary data types. Types with kind other then * have to be fully applied. (e.g. Maybe String). A specialized Marshaling instance is then generated for each of these instances. * Generates FFI exports for any function you mark, with FFI compatible type * Properly supports marshaling of lists. [Int] * Supports Ptr, FunPtr and StablePtr. Which introduces the possibility to have caches/sessions. * Supports Callbacks via Higher order functions (everything is auto generated from the function type) * Re-exports existing exports found in source file * Honors existing Storable instances found for types it needs * Avoids unsafePerformIO as much as possible, except for in one specific instance. * Generates Initialization functions for you * Hides unnecessary exports using a DEF file * Allows you to override default type conversions (e.g. String –> CWString) * Provides helper includes for C, C++ and C# (placed into wherever cabal places extra includes. %AppData%\cabal\Hs2lib-0.4.8 for windows) And more Details ===== A more detailed documentation will follow in the next few weeks. Why 0.4.8? -------------- This project has been in development for a very long time. Nearly two years on and off. It was developed mainly to facilitate the creation of Visual Haskell 2010. This is just the first public release of this tool.

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Lambdabot plugin system description
by Richard Wallace 02 Jul '11

02 Jul '11

Hey all, I'm curious if there are any papers or anything else describing the plugin system in Lambdabot. If not I'll dig through the code, but my Haskell isn't yet that strong so a higher level introduction would be very helpful. Thanks, Rich

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compare iteratee with python's yield
by yi huang 02 Jul '11

02 Jul '11

I just read several tutorials on iteratee, i find that iteratee is similar to python's generator, both allow streamlined data processing. For example, i can implement enumFile and printChunks in python like this: EOF = None def enum_file(bufsize, filename): with open(filename) as input: while True: data = input.read(bufsize) if not data: break yield data yield EOF def print_chunks(print_empty, generator): for chunk in generator: if chunk==EOF: print 'EOF' return if len(chunk)==0 and not print_empty: continue print chunk print_chunks(True, enum_file(2, "data")) But i find iteratee far more complicated than python's generator, is that because iteratee can do something python's generator can't, or i simply need to be more familar with functional programming style. -- http://www.yi-programmer.com/blog/

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Re: [Haskell-cafe] Parsing cabal files to calculate average number of dependencies
by Gwern Branwen 01 Jul '11

01 Jul '11

On Fri, Jul 1, 2011 at 4:49 PM, L Corbijn <aspergesoepje(a)gmail.com> wrote: > Is this including or exluding 'or'-ed dependency lists like > http://hackage.haskell.org/package/hugs2yc ? Excluding, it seems. When I run the script on that tarball: $ tar --wildcards "*.cabal" -Oxf `find . -name "*.tar.gz" | g hugs2yc` | runhaskell /home/gwern/deps.hs PackageName "mtl" PackageName "uniplate" PackageName "yhccore" PackageName "ycextra" PackageName "parsec" PackageName "directory" PackageName "filepath" No version of base or containers appears. (mtl appears in both branches and also the general build-depends list.) -- gwern http://www.gwern.net

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Is there Functor instance of Enumerator'
by yi huang 01 Jul '11

01 Jul '11

Say i want to compose Enumerator ByteString m b and Iteratee Builder m b, so I try to transform the enum to Enumerator Builder m b, providing function ByteString -> Builder. It's like implement a Functor instance for Enumerator. But i failed, there are no way to make type system happy. Am I right that there is no Functor instance of Enumerator? -- http://www.yi-programmer.com/blog/

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Twitter API?
by Dmitri O.Kondratiev 01 Jul '11

01 Jul '11

Hi, Does anybody work with any Haskell Twitter API library? In particular supporting OAuth authentication? Thanks!

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Patterns for processing large but finite streams
by Eugene Kirpichov 01 Jul '11

01 Jul '11

Hi, I'm rewriting timeplot to avoid holding the whole input in memory, and naturally a problem arises: How to represent large but finite streams and functions that process them, returning other streams or some kinds of aggregate values? Examples: * Adjacent differences of a stream of numbers * Given a stream of numbers with times, split it into buckets by time of given width and produce a stream of (bucket, 50%,75% and 90% quantiles in this bucket) * Sum a stream of numbers Is this, perhaps, what comonads are for? Or iteratees? -- Eugene Kirpichov Principal Engineer, Mirantis Inc. http://www.mirantis.com/ Editor, http://fprog.ru/

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