structured-concurrency

Structured Concurrency Examples

Demonstrates parallel I/O without threads or async/await. parallel and race are built-in step kinds that require no effect imports.

Examples

File	Description
structured-concurrency.cov	Parallel fetches, race patterns, and timeout handling

Key Concepts

Parallel Execution

Execute branches concurrently and wait for all to complete:

step id="s1" kind="parallel"
  branch id="b1"
    step id="b1.1" kind="call"
      fn="http.get"
      arg name="url" lit="https://api.example.com/users"
      as="users"
    end
  end
  branch id="b2"
    step id="b2.1" kind="call"
      fn="http.get"
      arg name="url" lit="https://api.example.com/products"
      as="products"
    end
  end
  as="results"
end

Race Pattern

Execute branches and return the first to complete:

step id="s1" kind="race"
  branch id="b1"
    step id="b1.1" kind="call"
      fn="redis.get"
      arg name="key" from="key"
      as="cached"
    end
  end
  branch id="b2"
    step id="b2.1" kind="call"
      fn="postgres.query"
      arg name="sql" lit="SELECT * FROM entries WHERE key = $1"
      arg name="params" from="key"
      as="db_result"
    end
  end
  as="first_result"
end

Error Handling Strategies

• on_error="fail_fast" (default) — Cancel other branches on first error
• on_error="collect_all" — Wait for all branches, collect successes and failures
• on_error="ignore_errors" — Continue with successful results only

Timeout Handling

step id="s1" kind="parallel"
  timeout=5s
  on_timeout="cancel"  // or "return_partial"
  branch id="b1"
    // ...
  end
  as="results"
end

Key Properties

1. No threads — Concurrency without thread management
2. No async/await — No function coloring
3. Scoped — Always wait for results before proceeding
4. Deterministic — Results collected in declaration order
5. Isolated — Branches share no mutable state
6. Built-in — No effect import required for parallel or race