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+ <title>The GHC Commentary - why we have <tt>ForeignPtr</tt></title>
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+
+ <h1>On why we have <tt>ForeignPtr</tt></h1>
+
+ <p>Unfortunately it isn't possible to add a finalizer to a normal
+ <tt>Ptr a</tt>. We already have a generic finalization mechanism:
+ see the Weak module in package lang. But the only reliable way to
+ use finalizers is to attach one to an atomic heap object - that
+ way the compiler's optimiser can't interfere with the lifetime of
+ the object.
+
+ <p>The <tt>Ptr</tt> type is really just a boxed address - it's
+ defined like
+
+ <pre>
+data Ptr a = Ptr Addr#
+</pre>
+
+ <p>where <tt>Addr#</tt> is an unboxed native address (just a 32-
+ or 64- bit word). Putting a finalizer on a <tt>Ptr</tt> is
+ dangerous, because the compiler's optimiser might remove the box
+ altogether.
+
+ <p><tt>ForeignPtr</tt> is defined like this
+
+ <pre>
+data ForeignPtr a = ForeignPtr ForeignObj#
+</pre>
+
+ <p>where <tt>ForeignObj#</tt> is a *boxed* address, it corresponds
+ to a real heap object. The heap object is primitive from the
+ point of view of the compiler - it can't be optimised away. So it
+ works to attach a finalizer to the <tt>ForeignObj#</tt> (but not
+ to the <tt>ForeignPtr</tt>!).
+
+ <p>There are several primitive objects to which we can attach
+ finalizers: <tt>MVar#</tt>, <tt>MutVar#</tt>, <tt>ByteArray#</tt>,
+ etc. We have special functions for some of these: eg.
+ <tt>MVar.addMVarFinalizer</tt>.
+
+ <p>So a nicer interface might be something like
+
+<pre>
+class Finalizable a where
+ addFinalizer :: a -> IO () -> IO ()
+
+instance Finalizable (ForeignPtr a) where ...
+instance Finalizable (MVar a) where ...
+</pre>
+
+ <p>So you might ask why we don't just get rid of <tt>Ptr</tt> and
+ rename <tt>ForeignPtr</tt> to <tt>Ptr</tt>. The reason for that
+ is just efficiency, I think.
+
+ <p><small>
+<!-- hhmts start -->
+Last modified: Wed Sep 26 09:49:37 BST 2001
+<!-- hhmts end -->
+ </small>
+ </body>
+</html>
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