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
|data StableName a |
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 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
- 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
|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).
|Produced by Haddock version 2.3.0|