June 2010 - Haskell-Cafe

TypeFamillies and UndecidableInstances - why?
by Maciej Piechotka 22 Jun '10

22 Jun '10

When I tried to do something like: > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE TypeFamilies #-} > > class Test a where > type TestMonad a :: * -> * > from :: a b -> TestMonad a b > to :: TestMonad a b -> a b > > data Testable a b = Testable (a b) > > instance (Test a, Functor (TestMonad a)) => Functor (Testable a) where > f `fmap` Testable v = Testable $! (to . fmap f . from) v > It asks for adding UndecidableInstances as: test.hs:11:0: Constraint is no smaller than the instance head in the constraint: Functor (TestMonad a) (Use -XUndecidableInstances to permit this) In the instance declaration for `Functor (Testable a)' What is undecidable? a is bound so TestMonad a should be bound so Functor (TestMonad a) should be valid. Is it a bug/missing feature in ghc or do I fail to see something? Regards

4 4

ANN: srec-0.0.0
by Tom Hawkins 22 Jun '10

22 Jun '10

A little library for reading s-record files: http://hackage.haskell.org/package/srec -Tom

2 2

TH instance code.
by Andy Stewart 22 Jun '10

22 Jun '10

Hi all, I have below duplicate code, but i don't know how to use TH instance code. ------------------------------> duplicate code start <------------------------------ instance Variable PageType where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: PageType) $ fromVariant x instance Variable Int where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Int) $ fromVariant x instance Variable (Maybe Char) where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Maybe Char) $ fromVariant x instance Variable (Maybe Int) where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: Maybe Int) $ fromVariant x instance Variable ProcessID where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: ProcessID) $ fromVariant x instance Variable GWindowId where toVariant = toVariant . show fromVariant x = fmap (\v -> read v :: GWindowId) $ fromVariant x ------------------------------> duplicate code end <------------------------------ Any TH expert help? Thanks, -- Andy

4 5

Minix
by Henk-Jan van Tuyl 22 Jun '10

22 Jun '10

L.S., I just read about Minix and found a discussion [1] about Andrew Tanenbaum (the creator of Minix) wanting to try drivers written in Haskell on Minix. It has been four years since, is there currently a way to (cross)compile Haskell programs for Minix? Haskell well suited to write reliable software and Minix is all about reliability; they seem to match perfectly. It would increase visibility of Haskell if it can be used on Minix systems. Regards, Henk-Jan van Tuyl [1] http://groups.google.com/group/fa.haskell/browse_thread/thread/43f22847305b… -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

2 1

Different choice operations in a continuation monad
by Sebastian Fischer 22 Jun '10

22 Jun '10

Dear Café, `MonadPlus` instances are usually required to satisfy certain laws, among them the monad laws and monoid laws for `mzero` and `mplus`. Additionally one may require that (>>=f) is a monoid morphism, that is: mzero >>= f = mzero (a `mplus` b) >>= f = (a >>= f) `mplus` (b >>= f) The list monad satisfies these additional laws, the `Maybe`-Monad does not satisfy the second, distributive, law: ghci> (return False `mplus` return True) >>= guard :: [()] [()] ghci> (return False `mplus` return True) >>= guard :: Maybe () Nothing Instead of the distributive law, the `Maybe` monad satisfies a different law: return x `mplus` a = return x that is, `return` annihilates the `Maybe`-Monad regarding `mplus`. This "cancellation law" is incompatible with the distributive law because (together with other laws) it implies that the result of the above example expression is `Nothing` whereas the distributive law implies that it is `Just ()`. We can lift the `Maybe` type into a continuation monad: > newtype CMaybe r a = CMaybe ((a -> Maybe r) -> Maybe r) > > instance Monad (CMaybe r) where > return x = CMaybe (\k -> k x) > CMaybe ca >>= f = CMaybe (\k -> ca (\x -> let CMaybe cb = f x in cb k)) > > instance MonadPlus (CMaybe r) where > mzero = CMaybe (\_ -> mzero) > CMaybe ca `mplus` CMaybe cb = CMaybe (\k -> ca k `mplus` cb k) Unlike the `Maybe`-monad, the `CMaybe`-monad satisfies the distributive law, not the cancellation law. Can you define an associative operation orElse :: CMaybe r a -> CMaybe r a -> CMaybe r a with identity `mzero` that satisfies the cancellation law? Cheers, Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

5 16

Is there good place to post Haskell alorithms/data structures that follow Steven Skiena's book on algorithm design and also Haskell code snippets that follow some of Knuth's books?
by Casey Hawthorne 21 Jun '10

21 Jun '10

Is there good place to post Haskell alorithms/data structures that follow Steven Skiena's book on algorithm design and also Haskell code snippets that follow some of Knuth's books? These code snippets don't seem to fit with Hackage. Is there a Hatorial? -- Regards, Casey

3 2

Ph.D. positions on HW acceleration of Haskell
by Rami Mukhtar 21 Jun '10

21 Jun '10

========================================================================= Vacancies for Ph.D. positions at NICTA with the University of Sydney, Australia ========================================================================= Background ~~~~~~~~~~ NICTA has established a project with a goal of developing technology to map high level descriptions of computer vision algorithms to heterogeneous parallel hardware architectures. NICTA scholarships are available at the University of Sydney for outstanding students in the areas listed below. For further information, please contact either: Dr Rami Mukhtar (rami.mukhtar _AT_ nicta.com.au) or Dr Philip Leong (philip.leong _AT_ sydney.edu.au). Further information concerning these scholarships is available from: http://www.nicta.com.au/education/scholarships/sydney_research_lab/scholars… Reconfigurable Computing Architectures for Acceleration of Functional Languages ---------------------------------------------------------------------- This project would provide a candidate with the opportunity to develop new reconfigurable computing hardware architectures that are well suited for accelerating computations on collections of data. The work would be driven by recent advances on computations of collections made in the functional language community. The outcomes of this research would be to develop new hardware accelerator architectures, supporting tools chains, and undertake performance analysis. The candidate would be part of a larger team working to develop a technology for mapping computer vision algorithms to heterogeneous parallel hardware architectures. Hardware Acceleration of Functional Languages --------------------------------------------- This project would provide a candidate with the opportunity to develop new methods for mapping functional languages to implementations that target hardware accelerators, including FPGAs and other reconfigurable computing architectures. The outcomes of this research would be to develop algorithms, methodologies and related software frameworks that make this possible. The candidate would be part of a larger team working to develop a technology for mapping computer vision algorithms to heterogeneous parallel hardware architectures. The information in this e-mail may be confidential and subject to legal professional privilege and/or copyright. National ICT Australia Limited accepts no liability for any damage caused by this email or its attachments.

1 0

Functional programming techniques in small projects
by Markus Dönges 21 Jun '10

21 Jun '10

Hello Community, I am a student from the University of Duisburg-Essen, Germany, and at the moment I am writing my bachelor thesis about "Post-Mass-Prodcution Softwaresupply/ -development in case of an University administration". This approach deals with student-driven software development in which functional programming techniques (ie, Haskell) and agile development approaches matter. I am looking for opinions and statements on how small (agile) software development projects may benefit from functional programming techniques (perhaps in contrast to imperative programming techniques). Since I am at the very start, I would appreciate further literature advices. In addition, does anybody know particular people who are familiar with this topic? Any answer is appreciated :-) Regards, Markus

2 1

Generics and type families
by Louis Wasserman 21 Jun '10

21 Jun '10

What is the relationship between -XGenerics and -XTypeFamilies? Can I automatically create a data family instance based on its generic decomposition? Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

1 0

HaskellWiki and Wikipedia
by Roman Beslik 21 Jun '10

21 Jun '10

Hi all. There are some notions which are not described in HaskellWiki but described in Wikipedia, e.g. "catamorphism". When clicking on a link [[catamorphism]] that leads to "create a new page" it would be nice to show link to a corresponding Wikipedia page. Also "search in Wikipedia" on the search results page. -- Best regards, Roman Beslik.

6 14