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Haskell in Artificial Intelligence
by Christos Chryssochoidis 16 Oct '08

16 Oct '08

Greetings, I'm interested in doing a survey about the use of Haskell in the field of Artificial Intelligence. I searched in Google, and found in the HaskellWiki, at www.haskell.org/haskellwiki/Haskell_in_industry, two organizations that use Haskell and do work related to AI. Besides that, I haven't found much else. Could somebody from the Haskell community give me some pointer to a project or system related to AI that uses Haskell? Thank you very much in advance. - CC

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ledger manual available
by Simon Michael 16 Oct '08

16 Oct '08

> From: Simon Michael <simon(a)joyful.com> > Date: October 16, 2008 11:36:15 AM PDT > To: haskell-cafe(a)haskell.org, ledger-cli(a)googlegroups.com > Subject: ledger manual available > > This seems to have fallen off the ledger site: the latest ledger > manual in easily-browsable form. To be clear, this documents the > mature ledger, not the young hledger released yesterday. But it's a > good read, gives the big picture and some of it is directly > applicable to hledger. > > one big page: http://joyful.com/repos/ledger/doc/ledger.html > many pages: http://joyful.com/repos/ledger/doc/ledger/ > pdf: http://joyful.com/repos/ledger/doc/ledger.pdf > >

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Cabal package with library and tests
by Mauricio 16 Oct '08

16 Oct '08

Hi, I'm writing a Cabal package, using main=defaultMain in Setup.hs. It has a library, and I want to also build a few executables so I can test the library. How am I supposed to do that? My attempt was to create 'executable' sections for those tests. However, I don't know how to include the main library in the modules used by those tests. I tried to insert my own package in a 'build-depends' line, but that didn't work. Thanks for your tips, Maurício

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Where did House/hOp go?
by Svein Ove Aas 16 Oct '08

16 Oct '08

I'd finally gotten to the point in learning haskell at which they might be interesting to look at, and then.. they were gone. Does anyone know what happened to these projects? They used to be open-source, so the code should still be around somewhere, right?

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Re: [Haskell-cafe] Very silly
by Juan Carlos Arevalo Baeza 16 Oct '08

16 Oct '08

Andrew Coppin wrote: > Some guy told me that templates are "the best feature in the > language", and proceeded to show me a huge chunk of highly > complex-looking code which is approximately equivilent to > > join :: Array x -> Array x -> Array x > > I was unimpressed. Now I'm curious to know what chunk of code he showed you. FWIW, that declaration you put there is done in C++ like so: template < typename X > Array<X> join(Array<X> const&, Array<X> const&); Nothing more. I like both languages for very different reasons. C++ for its insane amount of flexibility, and Haskell for its expressiveness. And I've encountered equally harsh roadblocks in both whenever I try to get "too crazy" while using them. > Actually, that's a lie. I was impressed that such a low-level language > could manage even that much abstraction. But I still prefer the > Haskell way... The dynamic range of the C++ language is probably unparalleled. In particular, templates enable implementing high level abstractions in a way such that we're still discovering how far it can go. But of course, it's not just templates: it's templates when united to all other language features like overloading. Maybe I can take a crack at my own comparison of simple polymorphism between C++ and Haskell. C++: template < typename A > struct B { A m; }; // Polymorphic type definition template <> struct B<bool> { int m; }; // Polymorphic type specialization template < typename A > struct B<B<A> > { A m; }; // Polymorphic type partial specialization template < int N > struct B<int[N]> { int m[N]; }; // Non-type polymorphic parameters template < typename A > struct C { typedef B<B<A> > type; }; // Polymorphic type aliases (type functions) must use trickery and bad syntax B<int> const b = { 8 }; // Declare object of polymorphic type template < typename A > A get(B<A> const& b) { return b.m; } // Polymorphic function using polymorphic type int get(B<int> const& b) { return b.m + 1; } // Specialization of polymorphic function (actually, it is an overload - functions can't be specialized, but this is nearly equivalent) Haskell: data B a = B { m :: a } -- Polymorphic type definition // Polymorphic type specializations not available // Non-type polymorphic parameters not available (must emulate using Peano and such) type C a = B (B a) -- Polymorphic type aliases (type functions) let b = B 8 :: B Int in -- Declare object of polymorphic type get :: B a -> a; get b = m b -- Polymorphic function using polymorphic type (declaration and definition separate) // Specialization of polymorphic function not available without putting the function in a type class C++ templates are clearly more flexible, but they are also a lot clunkier to use (and both things are very much related). If you look at the specializations, and understand them, you'll see that it's easy to make a specialization that is incompatible with the main parametric (template) type definition. Sometimes, that's a good thing. I've created main templates that purposefully fail to compile when used, as a means to detect types that need their own particular specialization. But it's easy to see how they can have... unexpected consequences. For instance: B<B<int> > const bint = { 7 }; B<int> const bintresult = get(bint); // Compiler error! Can you see why? A specialization is to blame. But the main hurdle is that a template's definition is partially untyped. Type-checking of the innards of a template is finalized at the time the template is invoked, at which point it is too late for the compiler to root-cause any failures it encounters. Thus, using complex template-based libraries can be quite inconvenient if you don't get things right on your first try. For instance: C++: template < typename A > bool compare(B<A> const& b1, B<A> const& b2) { return b1.m == b2.m; }; // Another polymorphic function B<SomeType> val; compare(val, val); // This gives out horrible errors if SomeType doesn't have the equality operator defined Haskell: compare :: Eq a => B a -> B a -> Bool; compare b1 b2 = m b1 == m b2 -- Another polymorphic function let val = ... :: B SomeType in compare val val // This Complains if SomeType is not an instance of Eq This is a silly example, and the C++ errors you get are not quite so bad. But they can get really, really bad. The new C++ standard they are finalizing will "fix" this by adding concepts (akin to type classes) and concept_maps (akin to instances). It is interesting to realize, though, that in C++, templates must all be resolved at compile time, whether they use concepts or not. There's never any dictionary-passing happening anywhere, and all polymorphic functions must be specialized for all the different types that are used with them. For runtime polymorphism, C++ uses OO classes which are completely separate. OO classes are in some way like Haskell classes, but the dictionaries must be embedded in the objects themselves, whereas Haskell classes pass the dirctionaries into polymorphic functions separately. If a class is a salad, and the dictionary is the dressing, then C++ serves the dressing in the salad, and Haskell serves it on the side. The main consequence for this is that, in C++, class instances must be attached at the point of declaring the type, whereas in Haskell, you can do it after the fact. Maybe this is not quite so clear or useful after all. One-liners can be quite obtuse. And the craziness with templates shines when you combine them with various other features of the language. You can even make things like Boost.Proto (http://boost-sandbox.sourceforge.net/libs/proto/doc/html/index.html) which is a library for creating arbitrary DSLs in C++. JCAB

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Re: Repair to floating point enumerations?
by Henning Thielemann 16 Oct '08

16 Oct '08

David Roundy schrieb: > Why not look for a heuristic that gets the common cases right, rather > than going with an elegant wrong solution? After all, these > enumerations are most often used by people who neither care nor know > how they're implemented, but who most likely would prefer if haskell > worked as well as matlab, python, etc. Although MatLab has a lot of bad heuristics, they fortunately didn't try to be too clever with respect to rounding errors. Floating point enumerations have the same problems in MatLab as in all other languages.

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Rewrite rules
by George Pollard 16 Oct '08

16 Oct '08

Section 8.13.2 of the GHC manual[1] states: > GHC keeps trying to apply the rules as it optimises the program. For > example, consider: > > let s = map f > t = map g > in > s (t xs) > > The expression s (t xs) does not match the rule "map/map", but GHC > will substitute for s and t, giving an expression which does match. If > s or t was (a) used more than once, and (b) large or a redex, then it > would not be substituted, and the rule would not fire. > The part I'm interested in here is (a); if an expression is used more than one then it cannot be substituted for. Is there any way to work around this or force it? The reason I ask is that as a bit of fun (and inspired by Joachim Breitner's blog post [2]) I was going to try writing a rewrite rule for the first time. What I had in mind was this: {-# RULES "index cycled list" forall list n. cycle list !! n = list !! (n `mod` length list) #-} However, in the case he has written about this won't fire, since the LHS cannot be substituted as `cycle list` is used more than once: > let rlist = cycle list > print ( rlist !! (10^9), rlist !! 0 ) I can get it to fire again if I write it like this: > {-# RULES > "!!/cycle" forall list. (!!) (cycle list) = (\n -> list !! (n `mod` length list)) > #-} > > ... > > let rlist = (!!) (cycle list) > print (rlist (10^9), rlist 0) But this is non-obvious and I'd rather have it fire in the first case (i.e. when used naïvely). So, back to my question; is there a workaround or force for this... or does it break too many things if done? [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite-rules.html#… [2] http://www.joachim-breitner.de/blog/archives/308-guid.html

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(OT) Humorous definition for fixed points?
by Corey O'Connor 16 Oct '08

16 Oct '08

I was just reminded of one of the joke definitions of recursion: "recursion: see recursion". Perhaps there is a similar one for fixed points? "To learn about fixed points find the fixed point of the process: Given somebody learn about fixed points from them." Course, my understanding of fixed points is poor and my jokes always have poor delivery. So I'm sure somebody else can do better. -Corey O'Connor

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FFI Newbie: c functions that want pointers to objects
by Mauricio 16 Oct '08

16 Oct '08

Hi, Some functions in C changes data using pointers, like this example: void change_int (int *n) { *n ++; } What is the proper way to handle that? I guess I should wrap it like this: foreign ccall "change_int" change_int :: Ptr CInt -> IO () Can I use that on a CInt in Haskell code? Like this obviously wrong code, but that probably shows what I want: do let myN = 5 change_int myN Thanks, Maurício

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What is this function?
by John Ky 16 Oct '08

16 Oct '08

Hi, I've written this function here: scramble [] = [] scramble [x] = [[z] | z <- scramble x] scramble (x:xs) = [(y:z)|y <- scramble x, z <- scramble xs] and (I think) it roughly does what I want it to: *Main> scramble ([]::[Int]) [] *Main> scramble ([1]::[Int]) [[1],[2]] *Main> scramble ([1,2]::[Int]) [[1,1],[1,2],[2,1],[2,2]] *Main> scramble ([1,2,3]::[Int]) [[1,1,1],[1,1,2],[1,2,1],[1,2,2],[2,1,1],[2,1,2],[2,2,1],[2,2,2]] Three questions: 1. What should I call this function? 2. Is there already one in the standard library that does the same thing? 3. Is it possible to rewrite it with only "scramble []" and "scramble (x:xs)" and not the "scramble[x]"? Thanks -John

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