OCaml 5.0.0 introduces a completely new runtime environment with support for shared memory parallelism and effect handlers.

As a language, OCaml 5 is fully compatible with OCaml 4 down to the performance characteristics of your programs. In other words, any code that works with OCaml 4 should work the same with OCaml 5.

The currently known exceptions to this rule are:

• the removal of many long-deprecated functions and modules
• changes to the internal runtime API
• the performance of ephemerons is currently (and temporarily) strongly degraded.