Stable names are a way of performing fast (O(1)), not-quite-exact comparison between objects.
Stable names solve the following problem: suppose you want to build a hash table with Haskell objects as keys, but you want to use pointer equality for comparison; maybe because the keys are large and hashing would be slow, or perhaps because the keys are infinite in size. We can't build a hash table using the address of the object as the key, because objects get moved around by the garbage collector, meaning a re-hash would be necessary after every garbage collection.
An abstract name for an object, that supports equality and hashing.
Stable names have the following property:
sn1 :: StableNameand
sn2 :: StableNameand
sn1 == sn2then
sn2were created by calls to
makeStableNameon the same object.
The reverse is not necessarily true: if two stable names are not
equal, then the objects they name may still be equal. Note in particular
mkStableName may return a different
StableName after an
object is evaluated.
Stable Names are similar to Stable Pointers (Foreign.StablePtr), but differ in the following ways:
- There is no
freeStableNameoperation, unlike Foreign.StablePtrs. Stable names are reclaimed by the runtime system when they are no longer needed.
- There is no
deRefStableNameoperation. You can't get back from a stable name to the original Haskell object. The reason for this is that the existence of a stable name for an object does not guarantee the existence of the object itself; it can still be garbage collected.