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System.Mem.StableName

Portability | non-portable
Stability | experimental
Maintainer | libraries@haskell.org

Contents

Stable Names

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

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.

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.

Instances

Eq (StableName a)
Typeable a => Typeable (StableName a)

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 `StableName`s 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).
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