X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Fprelude%2Fprimops.txt.pp;h=adb2e110ff8ced4fc4348f767daa93185a37c4bf;hb=a385f0af5ea320a18d580f6a36c59c55b3516efd;hp=61a696463edec2ab9efa2fd5c923e7b95af10dca;hpb=4f92da533cd1c7b5f41ef8794ee6a284f1680413;p=ghc-hetmet.git

diff --git a/compiler/prelude/primops.txt.pp b/compiler/prelude/primops.txt.pp
index 61a6964..adb2e11 100644
--- a/compiler/prelude/primops.txt.pp
+++ b/compiler/prelude/primops.txt.pp
@@ -1808,7 +1808,14 @@ pseudoop   "unsafeCoerce#"
            known to be empty.  Also, a {\tt newtype} of a type {\tt T} has the same representation
            at runtime as {\tt T}.
 
-        Other uses of {\tt unsafeCoerce\#} are undefined.
+        Other uses of {\tt unsafeCoerce\#} are undefined.  In particular, you should not use
+	{\tt unsafeCoerce\#} to cast a T to an algebraic data type D, unless T is also
+	an algebraic data type.  For example, do not cast {\tt Int->Int} to {\tt Bool}, even if
+        you later cast that {\tt Bool} back to {\tt Int->Int} before applying it.  The reasons
+        have to do with GHC's internal representation details (for the congnoscenti, data values
+	can be entered but function closures cannot).  If you want a safe type to cast things
+	to, use {\tt Any}, which is not an algebraic data type.
+	
         }
 
 -- NB. It is tempting to think that casting a value to a type that it doesn't have is safe