A perilous implementation of thread-local storage for Haskell. This module uses a fair amount of GHC internals to enable performing lookups of context for any threads that are alive. Caution should be taken for consumers of this module to not retain ThreadId references indefinitely, as that could delay cleanup of thread-local state.

Thread-local contexts have the following semantics:

• A value attached to a ThreadId will remain alive at least as long as the ThreadId.
• A value may be detached from a ThreadId via detach by the library consumer without detriment.
• No guarantees are made about when a value will be garbage-collected once all references to ThreadId have been dropped. However, this simply means in practice that any unused contexts will cleaned up upon the next garbage collection and may not be actively freed when the program exits.

Note: This library assumes that ThreadIds aren't reused before a finalizer runs to clean up the ThreadStorageMap, so quick thread churn of this nature.

Also note: This implementation of context sharing is mildly expensive (~40ns to attach a value) relative to using pure code / idiomatic things like MonadReader, hard to reason about without deep knowledge of threading in the code you are using, and has limited guarantees of behavior across GHC versions due to internals usage.