Efficient hash-consing for arbitrary data types

Version on this page:0.9.2
LTS Haskell 22.25:0.9.5@rev:1
Stackage Nightly 2024-06-15:0.9.5@rev:1
Latest on Hackage:0.9.5@rev:1

See all snapshots intern appears in

BSD-3-Clause licensed by Edward A. Kmett
Maintained by Edward A. Kmett
This version can be pinned in stack with:intern-0.9.2@sha256:7724b9cbc6ebd7ad2521ef1548a93b723621fafcc5c28f789fca81bb9d14602b,1552


Hackage Build Status

Efficient hash-consing for arbitrary data types.

Contact Information

Contributions and bug reports are welcome!

Please feel free to contact me through github or on the #haskell IRC channel on

-Edward Kmett



  • Add a Semigroup IntSet instance.


  • Removed identity from the Interned class, to support applications where the identity is obtained by other means (e.g. a unique Ptr value)


  • Disabled cache removal as it was causing problems on large data sets. There is no good way to ensure that both references remain alive long enough to finish comparisons.
  • Switched to IORef from MVar


  • Fixed problem where comparisons could happen between data structures while one was still a thunk, leading to equal structures comparing as inequal in limited circumstances, by appropriately using strictness annotations.


  • Widened the caches so they don’t go through a single MVar per type. This has made a dramatic impact on performance. However, this broke the previous invariant that newer entries always had higher Ids than older entries.


  • Added Data.Interned.IntSet