On Thu, 2010-10-14 at 17:45 +0000, Johannes Waldmann wrote:
Hi. I wonder how to do the following properly.
I have one (large) C type, let's call it T, and I want to sell it as an abstract type in Haskell.
I want to use C functions as if they were of type T -> T (pure function, returns a modified copy of the input) and the question is, how to do the memory allocation for that, in particular, how to avoid IO showing up in the (visible) types on the Haskell side:
I don't want IO because I don't want to declare some artificial order of execution - instead I want lazy evaluation. E.g., I might have some Haskell record with a T component which may or may not be evaluated (accessed) at all.
It is exactly for this purpose that the Haskell FFI library includes unsafePerformIO. This is basically *the* legitimate use case for it, so you don't need to feel bad about it. The FFI spec says: Sometimes an external entity is a pure function, except that it passes arguments and/or results via pointers. To permit the packaging of such entities as pure functions, Foreign provides the following primitive: unsafePerformIO :: IO a -> a http://www.cse.unsw.edu.au/~chak/haskell/ffi/ffi/ffise5.html#x8-240005.1 Duncan