System.Mem.StableName (original) (raw)

Description

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.

Synopsis

• data StableName a
• makeStableName :: a -> IO (StableName a)
• hashStableName :: StableName a -> Int

Stable Names

data StableName a Source

An abstract name for an object, that supports equality and hashing.

Stable names have the following property:

• If sn1 :: StableName and sn2 :: StableName and sn1 == sn2 then sn1 and sn2 were created by calls to makeStableName on 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 that mkStableName may return a different [StableName](System-Mem-StableName.html#t:StableName) after an object is evaluated.

Stable Names are similar to Stable Pointers (Foreign.StablePtr), but differ in the following ways:

• There is no freeStableName operation, unlike Foreign.StablePtrs. Stable names are reclaimed by the runtime system when they are no longer needed.
• There is no deRefStableName operation. 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.