On Wed, 2008-12-10 at 08:33 -0200, Mauricio wrote:
Hi,
When I do:
foreign import "nameOfFunction" nameOfFunction :: IO MyType
I can get a function that return MyType only if it's a pointer or some of the C* type family. Is it possible to write a new MyType and make it allowed as a return type from foreign functions? Is changing the compiler the only way to do that?
Of course you're not really returning a MyType but a pointer to one. So use: foreign import "nameOfFunction" nameOfFunction :: IO (Ptr MyType) Using raw Ptrs is not very nice, so we would usually either wrap that in a ForeignPtr or use a Storable instance to get a MyType directly. Which approach to use depends on if you want MyType to have value or reference semantics. If the C type is abstract and you only access it via a pointer then the ForeignPtr approach is sensible. If you directly access all the fields of the C type then using an equivalent Haskell MyType and converting using Storable is the sensible approach. If do recommend reading the FFI spec. It's quite readable and explains a lot of the issues. Getting familiar with the Foreign libraries will help too. The most important bits are understanding ForeignPtr and the Storable class. Duncan