Haskell Objective-C FFI: Messaging

Here is an example of how we import selectors:

foreign import objc "@selector locationInView:" s'locationInView
   :: Ptr CGPoint -> C'UITouch -> C'UIView -> IO ()

The first argument is the receiver object. All following arguments are message arguments (one per colon, as usual). A foreign import is just about issuing Objective-C messages. We are not trying to model overloading at this point or anything similar. So, if we, for example, want s'locationInView for a subclass, we need to define another function with a different name.

We might use the imported function as follows:

-- ObjC: touchedPoint = [myTouchObject locationInView:currentView];
s'locationInView touchedPoint myTouchObject currentView

This will translate to something which essentially has the same effect as

objc_msgSend_stret (touchedPoint, myTouchObject, sel_getUid("locationInView"), currentView);

Implementation note: The foreign import should arrange for a call to sel_getUid at program start or when an imported selector is used for the first time. The result should be cached to speed up subsequent uses of the same selector in the same program.

Usage note: To wrap imported functions handling C structs in a nice way, we define a Foreign.Storable instance and a convenience wrapper as usual in the plain C FFI.

Distinguishing the three messaging types

The ObjC runtime has three kinds of messaging functions: objc_msgSend (when the message returns an object), objc_msgSend_stret (when the message returns a structure), and objc_msgSend_fpret (when the structure returns a floating-point value). During code generation for foreign import, we select the appropriate messaging function as follows:

• If the returned type is (), we use objc_msgSend_stret; the first argument to the function must be a (Ptr t) and the second must be the receiver object.
• If the returned type is CFloat, CDouble, or CLDouble, we use objc_msgSend_fpret (with the function pointer cast to the appropriate return type first); the first argument must be the receiver object.
• Otherwise, we use objc_msgSend; the first argument must be the receiver object. We need to cast obc_msgSend's function pointer first.

The decision which function to use is unfortunately a bit more complicated - see http://www.sealiesoftware.com/blog/archive/2008/10/30/objc_explain_objc_msgSend_stret.html and http://www.sealiesoftware.com/blog/archive/2008/11/16/objc_explain_objc_msgSend_fpret.html.

Return values

Like in the C FFI, return values can be pure (no IO), but it is the programmer responsibility to ensure that these foreign imports are safe.

Messaging super

The ObjC runtime has two extra functions, objc_msgSendSuper and objc_msgSendSuper_stret, to message the super object.

An obvious approach to providing calls to super might be:

• foreign import the class' super
• Superclass instances would be accommodated as part of the type of a method that is foreign exported.

These functions need to get an objc_super struct as the first parameter, which the compiler is supposed to compute from the enclosing class definition.