been BLACKHOLE'd, and should be updated with an indirection or a
forwarding pointer. If the return value is NULL, then the selector
thunk is unchanged.
+
+ ***
+ ToDo: the treatment of THUNK_SELECTORS could be improved in the
+ following way (from a suggestion by Ian Lynagh):
+
+ We can have a chain like this:
+
+ sel_0 --> (a,b)
+ |
+ |-----> sel_0 --> (a,b)
+ |
+ |-----> sel_0 --> ...
+
+ and the depth limit means we don't go all the way to the end of the
+ chain, which results in a space leak. This affects the recursive
+ call to evacuate() in the THUNK_SELECTOR case in evacuate(): *not*
+ the recursive call to eval_thunk_selector() in
+ eval_thunk_selector().
+
+ We could eliminate the depth bound in this case, in the following
+ way:
+
+ - traverse the chain once to discover the *value* of the
+ THUNK_SELECTOR. Mark all THUNK_SELECTORS that we
+ visit on the way as having been visited already (somehow).
+
+ - in a second pass, traverse the chain again updating all
+ THUNK_SEELCTORS that we find on the way with indirections to
+ the value.
+
+ - if we encounter a "marked" THUNK_SELECTOR in a normal
+ evacuate(), we konw it can't be updated so just evac it.
+
+ Program that illustrates the problem:
+
+ foo [] = ([], [])
+ foo (x:xs) = let (ys, zs) = foo xs
+ in if x >= 0 then (x:ys, zs) else (ys, x:zs)
+
+ main = bar [1..(100000000::Int)]
+ bar xs = (\(ys, zs) -> print ys >> print zs) (foo xs)
+
-------------------------------------------------------------------------- */
static inline rtsBool