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GHC 9.10.3 · lts/ghc-9.10.x · 248f8f0 · 2026-10-05

Modulebase-4.20.2.0Haskell2010

GHC.StableName

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.

  • 1 type
  • 3 values
  • Packagebase-4.20.2.0
  • Exports4
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceStableName.hs

Stable Names

4 declarations
datadata 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 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.

Constructors

Instances1Eq
  • Eq (StableName a)Defined in ghc-internal-9.1003.0 · GHC.Internal.StableName