| Portability | non-portable | 
|---|---|
| Stability | experimental | 
| Maintainer | libraries@haskell.org | 
| Safe Haskell | Trustworthy | 
System.Mem.StableName
Contents
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.
- data StableName a
- makeStableName :: a -> IO (StableName a)
- hashStableName :: StableName a -> Int
Stable Names
data StableName a
An abstract name for an object, that supports equality and hashing.
Stable names have the following property:
-  If sn1 :: StableNameandsn2 :: StableNameandsn1 == sn2thensn1andsn2were created by calls tomakeStableNameon 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 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.
Instances
makeStableName :: a -> IO (StableName a)
Makes a StableName for an arbitrary object.  The object passed as
 the first argument is not evaluated by makeStableName.
hashStableName :: StableName a -> Int
Convert a StableName to an Int.  The Int returned is not
 necessarily unique; several StableNames may map to the same Int
 (in practice however, the chances of this are small, so the result
 of hashStableName makes a good hash key).