Parallelism

How to run independent prerequisites at the same time. Part of the guides; full syntax in the language guide.

Concept

By default a task’s dependencies run sequentially. Adding the [parallel] attribute lets its independent dependencies run concurrently, bounded by the available cores. The task itself still runs only after all its dependencies succeed.

Syntax

[parallel]
checks: lint test typecheck
    @echo "all checks passed"

lint:
    @echo "lint"
test:
    @echo "test"
typecheck:
    @echo "typecheck"

lint, test, and typecheck run together; checks runs once they all finish.

Runnable example

See examples/parallel.

Pitfalls

• Order between parallel tasks is not guaranteed. Don’t rely on lint finishing before test — if there’s a real ordering need, make one depend on the other.
• Interleaved output. Concurrent tasks’ output can interleave; use task-level logging if you need to attribute lines.
• Shared mutable state. Parallel tasks that write the same file will race — keep them independent, or serialize them with a dependency.

Next steps

• Dependencies & post-hooks — the ordering model parallelism builds on.
• Caching — combine with [cache] to skip unchanged parallel steps.