Ambiguous type variable error during Stackage LTS migration

Answer

Switching to the latest LTS snapshot does not automatically fix ambiguous type variable errors; you must examine the constraints that GHC cannot infer.

Likely explanation

The error occurs when a type variable appears in a function’s result type without enough class constraints or type family applications to determine a unique instance. Moving to a newer LTS may bring stricter ambiguity checks (GHC 9.8) or change instance resolution, exposing previously hidden ambiguity.

Confirmed facts

• GHC 9.8 enables the ambiguity check by default (‑Wambiguous‑polys).
• Stackage LTS snapshots pin exact package versions; upgrading only changes those versions, not the language extensions.
• Atomic binary replacement on POSIX filesystems can be achieved by writing the new binary to a temporary path and renaming it over the old one; rename() is atomic.
• Health‑check patterns that combine a startup probe (ensuring the process has bound its listen socket) with a readiness probe (responding to a lightweight endpoint) give zero‑downtime guarantees when used with a load balancer that drains connections.

Steps for this case

1. Reproduce the error with cabal v2-build --snapshot=lts-22 --ghc-options=-Werror and note the exact message.
2. Add an explicit type annotation or a needed constraint (e.g., :: (SomeClass a) => ...) to the ambiguous variable.
3. If the ambiguity stems from a type family, ensure the family is fully applied or add a TypeApplications annotation.
4. Run cabal v2-test to confirm the fix does not break existing tests.
5. Build the production binary with cabal v2-build --snapshot=lts-22 --enable-executable-stripping.
6. Write the binary to /opt/myapp/bin/myapp.new, then execute mv /opt/myapp/bin/myapp.new /opt/myapp/bin/myapp (atomic rename).
7. Signal the running instance to finish current requests (e.g., SIGTERM) and start the new binary via your process manager.
8. Configure the load balancer to perform a health check on /ready that returns 200 only after the application has completed initialization and bound its listening port.

Missing diagnostic detail

To give a precise fix, we need to know which module or function GHC reports as having the ambiguous type variable. Please provide the exact error message (including the variable name and location).