The “default” implementation of Freya uses F# async functions for most operations, and the internals are built as async. However, there is an alternative implementation available using the Hopac concurrent programming library available.
The Hopac version of Freya is effectively identical in functionality and usage, but uses a different underlying concurrency model – see Functions. This can make Freya easier to integrate with existing code where Hopac is in use, and also potentially gives some performance/resource usage gains (the Freya Benchmarks project is beginning to measure these reliably).
Packages which use the Hopac concurrency model are suffixed with .Hopac. If you are using the Freya meta-package for example, rather than taking a dependency on Freya, you would take a dependency on Freya.Hopac. This convention applies to all packages available in both variants.
Not all packages are dependent on a concurrency abstraction, so not all packages come in both “default” and Hopac variants. Packages which do cannot be mixed and matched – attempting to use one package built for async and one built for Hopac together will result in errors!