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