In Standard ML, all recursive value definitions must have function literals as their right-hand sides. Thus, at runtime, recursion is guaranteed to go through f

In Standard ML, all recursive value definitions must have function literals as their right-hand sides. Thus, at runtime, recursion is guaranteed to go through functions (which normally aren't allocated in the heap) and/or mutable data structures (which can be handled in a special way by the garbage collector). Immutable data structures are generally guaranteed to be cycle-free. That being said, reference counting is a really bad fit for functional languages for several other reasons.