Thanks Bulat! Vasili On Fri, Jun 20, 2008 at 3:20 PM, Bulat Ziganshin wrote:

Hello Vasili,

Friday, June 20, 2008, 11:51:11 PM, you wrote:

...

data Bonzo = A | B |C

...

How do I write the poke functions and call them?

instance Storable Bonzo poke A = poke 0 poke B = poke 1 poke C = poke 4

call as "poke x"

probably, you don't understand differences between OOP classes and type classes. look at http://haskell.org/haskellwiki/OOP_vs_type_classes and papers mentioned there

-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

You just have the arguments to "poke" wrong: poke :: Storable a => Ptr a -> a -> IO () So you are missing the pointer argument poke p Signal = poke p signal_int_value I didn't know about the (#const) syntax. Interesting. Also, alignment of signal should match the alignment of the underlying type (although I think that isn't really used at the moment). -- ryan 2008/6/20 Galchin, Vasili :

I am still reading the web pages. Here is what I tried:

data SigNotify = Signal | None | Thread | ThreadId

instance Storable SigNotify where

sizeOf _ = (#const sizeof (int))

alignment _ = 1

poke Signal = (#const SIGEV_SIGNAL)

poke None = poke (#const SIGEV_NONE)

poke Thread = poke (#const SIGEV_THREAD)

poke ThreadId = poke (#const SIGEV_THREAD_ID)

but I got ...

Couldn't match expected type `Ptr SigNotify' against inferred type `SigNotify' In the pattern: Signal In the definition of `poke': poke Signal = poke (0) In the definition for method `poke'

Basically I want to marshall SigInfo constructors to CInt values. ??

Regards, Vasili

On Fri, Jun 20, 2008 at 3:20 PM, Bulat Ziganshin wrote:

...

Hello Vasili,

Friday, June 20, 2008, 11:51:11 PM, you wrote:

...

data Bonzo = A | B |C

...

How do I write the poke functions and call them?

instance Storable Bonzo poke A = poke 0 poke B = poke 1 poke C = poke 4

call as "poke x"

probably, you don't understand differences between OOP classes and type classes. look at http://haskell.org/haskellwiki/OOP_vs_type_classes and papers mentioned there

-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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On Sat, 2008-06-21 at 00:20 +0400, Bulat Ziganshin wrote:

probably, you don't understand differences between OOP classes and type classes. look at http://haskell.org/haskellwiki/OOP_vs_type_classes and papers mentioned there

I took the liberty to update some of the C++ code on that page so that the code will compile and work as expected. Reading the rest of the page I have a question regarding section 4.2, Type classes correspond to parameterized abstract classes (Gabriel Dos Reis). This is probably more related to C++ than Haskell, so not all readers might find it of interest. --- off topic --- Looking at 4.2, it's interesting to see how type classes are translated into C++. Admittedly C++ isn't the prettiest language to look at, but I feel one must be fair, and the code presented at the Wiki is too complex, using virtuals and templates where they're not needed. Normal overloading suffices. Operator definitions can already be placed outside of a class scope, e.g. the many overloads of << to print something to a stream. By default these definitions cannot access private members of their operands but the code at the Wiki has the same problem. Just a small example: // --- somewhere in a file --- struct Aap { int x; explicit Aap(int x) : x(x) {} Aap add(Aap const& other) const { return Aap(x + other.x); } }; // --- somewhere in another file --- Aap operator+(Aap const& lhs, Aap const& rhs) { return lhs.add(rhs); } Overloading functions is not different. I've omitted some other (more complex) ways you can achieve the same (aside from normal overloads it's also allowed to introduce new specializations later). At the time of writing it's not possible in C++ to restrict a type to allow a certain operation (concepts[1] will hopefully fill that gap some day, but I think it'll be a long time before tool support is ready for prime-time). Rather, it's common to just use the operation and let the compiler generate a (sometimes very cryptic) error message if it cannot find the right code. [1]: http://www.generic-programming.org/software/ConceptGCC/ A very poor man's solution to see if an operation is defined would be: template<typename T> struct has_add; template<> struct has_add<Aap> {}; int main() { (void)sizeof(has_add<Aap>); // okay  (void)sizeof(has_add<int>); // fails to compile, put useful comment here } It's not hard to add some primitive predicate logic (at compile time) to resemble type classes in some sense, but I think this is off-topic enough as it is. Regards, Niels