Eff vs Effect: Trade‑offs for Diagnosing Intermittent Connection Pool Exhaustion

Answer

The Eff monad provides clearer, actionable type‑level information for detecting intermittent connection‑pool leaks, while the Effect monad trades that visibility for simpler signatures and better integration with the newer Aff async model.

Likely explanation

Because Eff tracks each individual effect (e.g., Acquire, Release) in an extensible row, a missing release appears as an uncovered effect in a function’s inferred type. This gives a compile‑time clue that a connection may not be returned to the pool under certain call paths. Effect collapses all effects into a single Effect type, so the type system cannot distinguish acquire from release; diagnosing a leak then requires runtime instrumentation such as logging or custom wrappers.

Confirmed facts

• Eff’s extensible‑row effect types expose each acquire/release operation at the type level (PureScript 0.14+).
• Effect’s single‑type representation hides which specific resource effect is being performed.
• Running runEff or runEffect adapters preserves the underlying effect rows, but any module that lowers Eff to Effect loses the fine‑grained information unless the adapter is retained.
• The PureScript team marks Eff as deprecated for new projects, favoring Effect.

Steps to diagnose with Eff

1. Identify the function that acquires a connection from the pool.
2. Inspect its inferred type; you should see a row containing Acquire | Release | ....
3. If a release is omitted, the compiler will report that the Release effect is missing from the row, pointing directly to the leaky call site.
4. Optionally add a temporary Log effect to the row to verify that the compiler flags its absence when a release is skipped.
5. Run a bursty load test and monitor pool size; if the type‑level check passes but exhaustion still occurs, the issue is runtime‑only (e.g., timing‑dependent) and requires additional logging or metrics.

When Effect may be preferable

If the codebase already uses Effect extensively, adding Eff solely for debugging introduces verbosity and maintenance overhead. In that case, wrap the pool‑acquisition/release functions with a small Eff‑based wrapper that exposes the Acquire and Release effects, or inject logging/metrics to trace pool usage at runtime.

Missing diagnostic detail

If you cannot add custom effects or wrappers (e.g., due to library constraints), please confirm whether you can enable runtime pool‑usage logging or metrics; that information determines whether a pure‑type‑level approach (Eff) or a runtime‑instrumentation approach (Effect) is feasible.