- returns: NULL if we couldn't evaluate this THUNK_SELECTOR, or
- a closure pointer if we evaluated it and this is the result. Note
- that "evaluating" the THUNK_SELECTOR doesn't necessarily mean
- reducing it to HNF, just that we have eliminated the selection.
- The result might be another thunk, or even another THUNK_SELECTOR.
-
- If the return value is non-NULL, the original selector thunk has
- 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)