Course of Raku / Functional, concurrent, reactive, and web programming / Concurrent programming / Promises

start and await

The start keyword runs a block in the background and immediately returns a Promise:

my $p = start { 2 + 2 };

The work happens on Raku’s thread pool while your program carries on. To get the result, use await, which waits for the promise to finish and gives you its value:

my $p = start { 2 + 2 };
say await $p; # 4

await blocks just long enough for the promise to complete, then returns whatever the block produced — here, 4. If the work is already done by the time you await, you get the result at once.

This pair — start to launch, await to collect — is the core of promise-based concurrency. You can launch several pieces of work, let them run at the same time, and await their results when you are ready, which is exactly what the next topic builds on.

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