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4.14.6 Promises

Promises represent the result of a computation that may or may not complete at some later point in time. They are the primitive upon which asynchronous concurrency is based in JavaScript, whether using promises directly or through the async/await syntax.

Hoot fibers use these host promises under the hood and the (fibers promises) module provides the API for doing so. This is mostly transparent to the programmer, except at program startup. Programs that use fibers must be called in an async context in which they receive special values used to resolve or reject the promise representing the result of the entire program. See Fibers example for a walkthrough of this process.

Procedure: await-promise-operation promise ¶

Make an operation that will complete when promise is resolved. Performing the operation produces one value: a thunk which when called will either return the value or throw an exception.

Procedure: await promise ¶

Suspend the current fiber until promise is resolved. If the promise resolves successfully, one value is returned. Otherwise, an exception is thrown.

Procedure: call-with-async-result resolved rejected thunk ¶

Call thunk and resolve the promise with resolved and the returned values. If an exception is thrown, the promise is rejected with rejected and the exception.

resolved and rejected are external host values that are obtained by calling a Scheme procedure asynchronously from the host. See the Procedure.call_async method in JavaScript API reference and the hoot-apply-async procedure in Reflection for more information on asynchronous procedure calls.