One core or all of them

The problem: Code written for one core usually has to be rewritten, or at least re-checked, to use several.
An example: A web service starts on one core. When the load grows, it should use all of them without a rewrite.
What Nikaia checks: The choice is one line in nikaia.toml: user-parallelism = "no" (the default) or "yes". At no, two pieces of your code are never running at the same moment. At yes, a multi-threaded runtime spreads the work over the cores, and thread safety is checked at compile time. A library built at no is still checked against the rules parallel code needs, so it also works when a program built at yes uses it.
The limit: The setting allows parallelism; it does not set a thread count. How many threads serve yes is the runtime’s choice, because it depends on the machine. At no, only your own code is limited to one thread: waiting for I/O happens on the runtime’s own threads, and the standard library may still use every core.

The setting

[build]
user-parallelism = "no"    # the default; "yes" for the multi-threaded runtime

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