- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by wren ng thornton 25 Jul '11

25 Jul '11

On 7/24/11 10:09 PM, Sebastian Fischer wrote: >> >> because list is a (the?) free monoid. > > Yes, all free monoids are isomorphic (to lists). > > Sebastian For completeness... The free monoid over a set S is the set of all finite sequences of elements drawn from S. Often this is written with the Kleene star: S^*. Finite lists are a particular representation for the free monoid (namely with all operations associated to the right), though this representation may not be "isomorphic enough" for certain analyses (e.g., algorithmic complexity). Note that the free semigroup over a set S is the set of all finite non-empty sequences of elements drawn from S. We can name this by S^+. Other than the fact that S^+ does not contain the empty sequence, the other important difference is that, while both S^* and S^+ are both closed under the operation, only S^* is co-closed under the operation (i.e., not all elements of S^+ can be decomposed into two elements of S^+ joined by the operator). -- Live well, ~wren

1 0

Re: [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by Maciej Wos 25 Jul '11

25 Jul '11

Personally, I have nothing against mempty (although I agree that mid makes more sense), but I don't like mappend at all. I wonder what happened to the idea of using <> instead of mappend (that's what I always do). I think a <> b <> c looks so much better than a `mappend` b `mappend` c and it solves the name problem altogether. -- Maciej On Jul 24, 2011 3:42 AM, "KC" <kc1956(a)gmail.com> wrote:

7 6

Re: [Haskell-cafe] [Haskell-beginners] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by wren ng thornton 25 Jul '11

25 Jul '11

On 7/24/11 2:29 PM, Julian Porter wrote: > On 24 Jul 2011, at 19:19, KC wrote: >> A lot of math books use "+" or "x" enclosed in a circle to indicate >> that the usual meaning of "+" nor "x" is intended for the meaning of >> the binary operation. > > Er no. Both symbols have extremely precise meanings. $\oplus$ is the direct sum of two modules and $\otimes$ is their tensor product. Well, they mean that if you're off in module/vector land; but they don't mean that everywhere. Another place they're used frequently is for semirings: with $\oplus$ the associative monoid and $\otimes$ the other monoid. While the tourism board for semirings is less well funded than the one for vectors, it's a beautiful locale with rich history. Natural language processing, graph theory, Boolean algebras, and related logics are nothing to scoff at. -- Live well, ~wren

1 0

Re: [Haskell-cafe] ANNOUNCE: Haskell in the Cloud (http://quid2.org)
by Pasqualino "Titto" Assini 25 Jul '11

25 Jul '11

Hi David, no there is no direct relationship to distributed haskell. Quid2's focus is not so much on extending Haskell itself and its runtime to work in a distributed fashion but rather to provide a global naming system for functional values. Very similar to the Web, whose main innovation is to provides a global naming system for all kind of resources that previously had only local names (/dir/myfile versus http://ww.myserver.com/myfile) A description of Quid2 architecture will soon be available and it will be clearer how is meant to work. It will be far from complete, a working functional web will only emerge if many different ideas converge in a coherent whole. Best, titto > On 22 July 2011 15:57, David Barbour <dmbarbour(a)gmail.com> wrote: >> Any relationship to distributed haskell? >> http://www.macs.hw.ac.uk/~dsg/gdh/ >> >> On Fri, Jul 22, 2011 at 3:00 AM, Pasqualino "Titto" Assini >> <tittoassini(a)gmail.com> wrote: >>> >>> Fellow Haskeller, >>> >>> Has your strongly typed, quick checked, formally verified, Oleg blessed, >>> higher order monadic code become a little bit too predictable? >>> >>> Are you feeling a bit bored, emotionally drained and disillusioned with >>> perfection? >>> >>> Longing for the days when Men were Men (and Women were Women for that >>> matter) and squashed bugs out of Assembler code with just their bare hands >>> and a line editor? >>> >>> Enter Quid2 [1]: the half baked, barely tested, totally unsafe and >>> spectacularly unoptimised Haskell in the Cloud system. >>> >>> A delightful blend of Haskell crispiness and JavaScript fluffiness, Quid2 >>> guarantees hours and hours of guilt free hacking enjoyment. >>> >>> Let a soft lamb(a)da rhythm [2] take you over as you gingerly step onto >>> the Haskell Cloud, it's time to party as if it were 1991 all over again! >>> >>> Yours truly, >>> >>> titto >>> >>> [1] http://quid2.org >>> >>> [2] http://www.youtube.com/watch?v=i8mz9uOvFQA >>> >>> -- >>> Dr. Pasqualino "Titto" Assini >>> http://quid2.org/ >>> >>> >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> >> > > > > -- > Dr. Pasqualino "Titto" Assini > http://quid2.org/ > -- Dr. Pasqualino "Titto" Assini http://quid2.org/

1 0

ANNOUNCE: Haskell in the Cloud (http://quid2.org)
by Pasqualino "Titto" Assini 25 Jul '11

25 Jul '11

Fellow Haskeller, Has your strongly typed, quick checked, formally verified, Oleg blessed, higher order monadic code become a little bit too predictable? Are you feeling a bit bored, emotionally drained and disillusioned with perfection? Longing for the days when Men were Men (and Women were Women for that matter) and squashed bugs out of Assembler code with just their bare hands and a line editor? Enter Quid2 <http://quid2.org> [1]: the half baked, barely tested, totally unsafe and spectacularly unoptimised Haskell in the Cloud system. A delightful blend of Haskell crispiness and JavaScript fluffiness, Quid2 guarantees hours and hours of guilt free hacking enjoyment. Let a soft lamb(a)da rhythm <http://www.youtube.com/watch?v=i8mz9uOvFQA> [2] take you over as you gingerly step onto the Haskell Cloud, it's time to party as if it were 1991 all over again! Yours truly, titto [1] http://quid2.org [2] http://www.youtube.com/watch?v=i8mz9uOvFQA -- Dr. Pasqualino "Titto" Assini http://quid2.org/

3 3

ANN: shelltestrunner 1.0
by Simon Michael 24 Jul '11

24 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

1 0

Cloud Haskell
by Tom Murphy 24 Jul '11

24 Jul '11

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

4 4

Re: [Haskell-cafe] Monad for binary tree data structure
by wren ng thornton 24 Jul '11

24 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

1 0

Re: [Haskell-cafe] question regarding the $ apply operator
by wren ng thornton 24 Jul '11

24 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

1 0

OpenGL vs OpenGLRaw
by Yves Parès 24 Jul '11

24 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.

4 3