July 2011 - Haskell-Cafe

ANN: shelltestrunner 1.0
by Simon Michael 23 Jul '11

23 Jul '11

I'm pleased to announce the 1.0 release of shelltestrunner! Home page: http://joyful.com/repos/shelltestrunner Install: $ cabal install shelltestrunner shelltestrunner tests command-line programs or arbitrary shell commands. It reads simple declarative tests specifying a command, some input, and the expected output, error output and exit status. Tests can be run selectively, in parallel, with a timeout, in color, and/or with differences highlighted. Projects using it include hledger, yesod, and berp. shelltestrunner is free software released under GPLv3+. I started shelltestrunner two years ago, inspired by John Wiegley's ledger tests. John Macfarlane, Bernie Pope and Trygve Laugst?l have contributed code. The hackage page shows the libraries it relies on - most notably, Max Bolingbroke's test-framework. The site uses hakyll, pandoc & hamlet. New in 1.0: * New home page/docs * The `>>>=` field is now required; you may need to add it to your existing tests * Input and expected output can now contain lines beginning with `#` * Multiple tests in a file may now have whitespace between them * The `-i/--implicit` option has been dropped * New `-d/--diff` option shows test failures as a unified diff when possible, including line numbers to help locate the problem * New `-x/--exclude` option skips certain test files (eg platform-specific ones) * Passing arguments through to test-framework is now more robust * Fixed: parsing could fail when input contained left angle brackets * Fixed: some test files generated an extra blank test at the end

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Cloud Haskell
by Tom Murphy 23 Jul '11

23 Jul '11

Is anyone using Cloud Haskell yet? I'm really excited by the possibilities. Tom

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Re: [Haskell-cafe] Monad for binary tree data structure
by wren ng thornton 23 Jul '11

23 Jul '11

On 7/23/11 1:31 AM, Александр wrote: > Hello, > > I built binary tree with: > > data Tree a = Empty > | Node a (Tree a) (Tree a) > deriving (Eq, Ord, Read, Show) > > How can i make Monad type class instance for this tree? And can i make it on not? Of course you can. One way of thinking about monads is that it's just performing substitution, aka tree grafting. In this case, because the "variables" being substituted for are located within the tree, we need to make sure to distribute the substructure of the original tree over the result of substituting for the variable. Thus, we can define: data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Eq, Ord, Read, Show) -- This one should be obvious. instance Functor Tree where fmap f Empty = Empty fmap f (Node x l r) = Node (f x) (fmap f l) (fmap f r) -- | The tree @graft gl gr t@ is the result of grafting @gl@ in -- place of all left @Empty@ leaves and @gr@ in place of all -- right @Empty@ leaves; with the empty tree mapped to itself. graft :: Tree a -> Tree a -> Tree a -> Tree a graft gl gr = go where go Empty = Empty go (Node x Empty Empty) = Node x gl gr go (Node x Empty r ) = Node x gl (go r) go (Node x l Empty) = Node x (go l) gr go (Node x l r ) = Node x (go l) (go r) instance Monad Tree where return x = Node x Empty Empty Empty >>= _ = Empty Node x l r >>= f = graft (l >>= f) (r >>= f) (f x) And we can verify the correctness by using QuickCheck or lazy SmallCheck: import Control.Monad ((<=<)) import Test.QuickCheck instance Arbitrary a => Arbitrary (Tree a) where arbitrary = do b <- elements [True,False] if b then return Empty else do x <- arbitrary l <- arbitrary r <- arbitrary return (Node x l r) -- | Extensional function equality. (f === g) x = f x == g x prop_mapIdentity = fmap id === id prop_mapCompose f g = (fmap f . fmap g) === fmap (f . g) prop_leftUnit f = (return <=< f) === f prop_rightUnit f = (f <=< return) === f prop_assoc f g h = (f <=< (g <=< h)) === ((f <=< g) <=< h) Of course, there are other monads as well. In particular, for any semigroup on A there is a monad for Tree A. For these instances we would want to define a merging function: -- | The first argument is an associative operation for resolving -- conflicts in merging. Non-conflicts are resolved by assuming -- an identity element for the operation, thus lifting it into a -- monoid. merge :: (a -> a -> a) -> Tree a -> Tree a -> Tree a merge _ Empty tr = tr merge _ tl Empty = tl merge f (Node x xl xr) (Node y yl yr) = Node (f x y) (merge f xl yl) (merge f xr yr) Of course, there are only two semigroups which are parametric in A, hence there are only two more Monad instances we can define this way. newtype Tree2 a = T2 (Tree a) deriving (Eq, Ord, Read, Show) instance Monad Tree2 where return x = T2 $ Node x Empty Empty T2 Empty >>= _ = T2 Empty T2(Node x xl xr) >>= f = case f x of T2 Empty -> T2 Empty T2 (Node y yl yr) -> T2 $ Node y (merge' yl (T2 xl >>= f)) (merge' yr (T2 xr >>= f)) where merge' xs (T2 ys) = merge const xs ys instance Arbitrary a => Arbitrary (Tree2 a) where arbitrary = arbitrary >>= (return . T2) And we can similarly define Tree3 with (flip const) in place of const. All the other monads formed this way (with any semigroup operation in place of const) would require using a type class for restricted monads since we need to restrict the element type to support the semigroup operation we want to use. -- Live well, ~wren

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Re: [Haskell-cafe] question regarding the $ apply operator
by wren ng thornton 23 Jul '11

23 Jul '11

On 7/23/11 9:37 AM, Ting Lei wrote: > > I know the Reverse Polish is not a couple of hundred years old. > I have an impression of reading something about people writing natural > deduction systems using only dots in place of parenthesis. And it is > said that it was "natural" in those pre-historic times. The dot notation was used in Principia Mathematica (1910--1913), where differing numbers of dots represented differing levels of grouping (N.B., not quite the same as depth of grouping, i.e. count of parentheses, as indicated by expressions such as p . = . q V r where, because the right dot is required for grouping the disjunct, we must also introduce the left dot in order to ensure that the p, the equality symbol, and the disjunctive expression are all on the same level). The dots were also used to represent logical conjunction. However parentheses, brackets, and braces do show up occasionally without much explanation as to their meaning. Whitehead and Russell claim to have taken the dot notation from Peano. In fact, the dot notation is the origin of the period in the current notation for binding forms like lambda, Pi, Sigma, forall, and exists; as well as, IIRC, the origin of the colon for typing expressions and set comprehensions. The PM notation for quantifications were (x) . P (exists x) . P and more-modern philosophical texts retain this notation sans the period. It is by far the most baffling notation IMO. The other popular notation in modern philosophical texts is to replace the period by bracketing, leading to forall x[P] exists x[P] Note that an apparent variant latter with the forall elided, is actually a notation for expressing an open term P (N.B., not an abstraction) which dates back to Frege. -- Live well, ~wren

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OpenGL vs OpenGLRaw
by Yves Parès 23 Jul '11

23 Jul '11

Hello Café, Where do you people stand on using OpenGLRaw instead of the higher-level layer? I saw that the ports of the nehe tutorial use directly OpenGLRaw, and I wondered why that choice had been made.

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Reactive Programming in Machine Learning
by bob zhang 23 Jul '11

23 Jul '11

Hi all, I am doing a survey on combining Functional Reactive Programming and Machine Learning. Has anyone did relevant research on this topic? Any discussion or link is appreciable. Best,bob

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Monad for binary tree data structure
by Александр 23 Jul '11

23 Jul '11

Hello, I built binary tree with: data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Eq, Ord, Read, Show) How can i make Monad type class instance for this tree? And can i make it on not? i try: instance Monad Tree where return x = Node x Empty Empty Empty >>= f = Empty (Node x Empty Empty) >>= f = f x But i can't make (>>=) for Node x left right. Thank you.

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question regarding the $ apply operator
by Ting Lei 23 Jul '11

23 Jul '11

Hi I have a naive question regarding the basic use of the $ operator, and I am confused why certain times it doesn't seem to work. e.g. The following works: applySkip i f ls = (take i) ls ++ f (drop i ls) But the following doesn't: applySkip i f ls = (take i) ls ++ f $ drop i ls which gives me an error: The first argument of ($) takes one argument, but its type `[a0]' has none In the expression: (take i) ls ++ f $ drop i ls In an equation for `applySkip': applySkip i f ls = (take i) ls ++ f $ drop i ls FYI, I am using GHC 7.0.3 for this. Thanks in advance for comments. Ting

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Github support for cabal files
by Joris Putcuyps 23 Jul '11

23 Jul '11

Hello everybody I'm looking for a way to avoid having two files with almost the same information on github: README.markdown and package.cabal. As far as I can tell, a new github README parser[1] can be written, but it only supports README files with different extensions and requires either ruby or a script which converts stdin into html. Another solution would be that haddock supports a README file, just like github, as the index.html of the cabal package. Github houses a lot of cabal packages so more people could benefit from this solution. Greetings Joris [1]: https://github.com/github/markup

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ghc on arm status
by Sergey Mironov 23 Jul '11

23 Jul '11

Hi. I was searching for info about building ghc on ARM arch. I already know about [1] approach, and also saw debian binaries [2], but I am afraid I have to compile ghc by myself this time, since our system uses incompatible libc, so my question is addressed to gentoo users and sympathetic :) Did anybody see any working (currently or in the past) dev-lang/ghc arm ebuild? Thanks, Sergey [1] - http://hackage.haskell.org/trac/ghc/wiki/ArmLinuxGhc [2] - http://packages.debian.org/sid/ghc6

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