base-4.20.2.0: Core data structures and operations

Copyright | (c) The University of Glasgow 2001
License | BSD-style (see the file libraries/base/LICENSE)
Maintainer | libraries@haskell.org
Stability | stable
Portability | non-portable
Safe Haskell | Safe
Language | Haskell2010

System.Mem.StableName

Contents

- Stable Names

Description

Stable names are a way of performing fast ( \(\mathcal{O}(1)\) ), 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.

See Stretching the storage manager: weak pointers and stable names in Haskell by Simon Peyton Jones, Simon Marlow and Conal Elliott for detailed discussion of stable names. An implementation of a memo table with stable names can be found in `stable-memo` package.

Synopsis

- data StableName a
- makeStableName :: a -> IO (StableName a)
- hashStableName :: StableName a -> Int
- eqStableName :: StableName a -> StableName b -> Bool

# 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 `makeStableName` 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 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

Instances details

Eq (StableName a) | Since: base-2.1
Instance details Defined in GHC.Internal.StableName Methods (==) :: StableName a -> StableName a -> Bool Source # (/=) :: StableName a -> StableName a -> Bool Source #

makeStableName :: a -> IO (StableName a) Source #

Makes a `StableName` for an arbitrary object. The object passed as the first argument is not evaluated by `makeStableName`.

hashStableName :: StableName a -> Int Source #

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).

eqStableName :: StableName a -> StableName b -> Bool Source #

Equality on `StableName` that does not require that the types of the arguments match.

Since: base-4.7.0.0