asyncdispatch thread-pool scheduler replaces async single-threaded loop in Nim 1.0

Confirmed facts

• Nim 1.0 makes asyncdispatch the default asynchronous scheduler.
• The legacy async module exists as a thin wrapper that forwards all calls to asyncdispatch and is marked deprecated.
• Both modules ultimately use the same scheduler instance; importing both in the same binary does not create separate schedulers.
• Scheduler initialization is performed lazily on first use via asyncdispatch.initScheduler; no explicit call is required.

Recommended import strategy for new code

For code that targets Nim 1.0 and later, import asyncdispatch directly. To keep the same source compilable with pre‑1.0 Nim releases, wrap the import in a version check:

when defined(nimVersion) and nimVersion >= 100:
  import asyncdispatch
else:
  import async

This yields a single scheduler instance regardless of which branch is taken, because the async wrapper in older Nim simply delegates to the original single‑threaded loop, while the asyncdispatch branch uses the thread‑pool scheduler.

Mixing the two imports

If a project accidentally imports both async and asyncdispatch (e.g., via a dependency), the Nim linker merges them to the same underlying scheduler; no duplicate initialization occurs and the behavior remains consistent with the scheduler selected by the active import.

Deprecation timeline

The release notes for Nim 1.0 label the async module as deprecated but do not publish a specific removal date or version. The wrapper is expected to be dropped in a future major release after the community has had time to migrate, but no formal timeline is currently documented. Monitor the Nim changelog for any announcement.