How does Haskell’s garbage collector handle cyclic references?

Haskell garbage collector is based on a tracing algorithm that identifies live objects by performing a reachability analysis, starting from the roots (global variables, thread stacks, and registers). Any object not reachable from a root is considered "garbage" and can be deallocated. In the case of Haskell, this is typically implemented as a generational garbage collector, in order to improve performance by exploiting the weak generational hypothesis.

Cyclic references can become unreachable if there is no root path to them, causing memory leaks in reference-counting garbage collection systems. For example, consider two objects A and B, where A points to B and B points to A. When reference counting is in place, each object has a reference count equal to 1 at this point. If there are no other references to A and B, the objects would still not be collected, as their counts are nonzero, creating a memory leak.

However, Haskell’s garbage collector is not susceptible to memory leaks due to cyclic references because it is based on reachability rather than reference counting. In the case of unreachable cyclic references, the garbage collector would still be able to reclaim the memory used by these objects, as they are not reachable from any of the roots. To illustrate this, let’s consider the same example as before, but in Haskell this time:

data Foo = Foo Int (IORef Foo)

We can create two ‘Foo‘ objects with cyclic references as follows:

makeCyclic :: IO (IORef Foo, IORef Foo)
makeCyclic = do
  refA <- newIORef $ Foo 1 undefined
  refB <- newIORef $ Foo 2 refA
  writeIORef refA $ Foo 1 refB
  return (refA, refB)

In this example, if there are no references to ‘refA‘ and ‘refB‘ except those inside the ‘Foo‘ values themselves forming the cycle, the garbage collector will still be able to reclaim the memory used by these objects, as they are not reachable from any root.

Here is an overview of the garbage collection process in Haskell, particularly for cyclic references:

1. Mark phase: The garbage collector identifies all live objects by traversing the object graph starting from the roots. Any object not visited during this traversal is considered garbage. Cyclic references are still recognized by the tracing algorithm as long as they are reachable from a root.

2. Sweep phase: The garbage collector deallocates all objects that are not marked as live. In the case of unreachable cyclic references, these objects will be deallocated without causing memory leaks.

In conclusion, Haskell’s garbage collector handles cyclic references by identifying live (reachable) objects via the tracing algorithm. As long as cyclic references are not reachable from any root, they will be successfully deallocated, without causing any memory leaks, unlike reference-counting garbage collection systems.