PureScript Aff & JavaScript AbortController: Propagating Cancellation

Integrating Aff Cancellation with AbortController

To propagate PureScript Aff cancellation to a JavaScript AbortController, you must bind the lifecycle of the controller to the execution scope of the Aff. Because Aff cancellation is cooperative, the bridge must be established using a finalizer that triggers the abort() method when the PureScript runtime cancels the fiber.

Implementation Pattern

The most reliable way to ensure the abort event fires is to use bracket or finally. This ensures that regardless of whether the Aff completes successfully, fails, or is cancelled, the underlying JavaScript resources are released.

-- Conceptual bridge pattern
withAbortController :: Aff a -> Aff a
withAbortController action = 
  bracket
    (liftIO createAbortController) -- Acquire
    (\controller -> liftIO (abort controller)) -- Release/Cancel
    (\controller -> do
        signal <- liftIO (getSignal controller)
        actionWithSignal signal) -- Use

Addressing Key Constraints

• Nested Propagation: PureScript's Aff runtime natively propagates cancellation from parent to child provided the child is awaited (e.g., via join or race). If a parent Aff is cancelled, all currently pending child Aff computations are also cancelled, which in turn triggers their respective bracket finalizers.
• Timer Leak Prevention: To prevent dangling timers, avoid manual setTimeout calls. Instead, use race between your primary task and a delay Aff. When the primary task wins, the delay Aff is cancelled; when the timeout wins, the primary task is cancelled. This ensures the runtime cleans up the underlying timer.
• Type-Safe Bridge: To avoid race conditions, the AbortController should be instantiated immediately before the Aff starts and passed as a signal to the FFI call. This ensures the signal is active for the entire duration of the asynchronous operation.

Verification Steps

To verify the integration is working as expected:

1. Network Inspection: Wrap a fetch call in the withAbortController pattern. Cancel the Aff from the PureScript side and verify that the browser's Network tab marks the request as (canceled).
2. Finalizer Logging: Place a console.log inside the bracket release function to confirm it executes immediately upon Aff cancellation.
3. Timeout Validation: Use a race between a long-running fetch and a short delay. Verify that the fetch is aborted when the timeout triggers.

Assumptions and Uncertainty

This approach assumes the use of a modern browser environment supporting AbortController and a standard Aff implementation (such as purescript-aff). It is assumed that the FFI wrappers for AbortController are correctly typed to handle the JavaScript Signal object. We assume the Aff is not performing long-running synchronous blocking operations, as these would prevent the runtime from reaching the cancellation point.

Diagnostic Detail Needed: Are you using a specific Aff library version or a custom effect wrapper that modifies the standard fiber cancellation behavior?