Keep language extensions with their owner

Keep established project-wide extensions in the Cabal configuration. Synchronize those shared defaults across the components that use the same dialect. Preserve intentional component-specific settings. Synchronizing defaults does not authorize adding a novel extension globally.

1. Inventory the repository's Cabal files, imported common stanzas, component overrides, conditionals, source pragmas, and effective compiler flags. Include maintained starter and testbed packages. Record the effective settings for each affected module and build configuration before editing.
2. Identify which settings are established shared defaults and which serve a particular component or compiler contract. Correct drift in shared Cabal defaults. Check actual uses before moving or removing a component-specific extension. Do not make every extension list identical by unioning them.
3. Remove a redundant source pragma only after confirming that every supported build path for that module supplies the same setting. Retain a source pragma for a justified module-specific setting, a deliberate opt-out, or a standalone compiler fixture's required tool contract. Record that reason in verification. A lint workaround alone does not justify repeating a Cabal default in source.
4. Compare compiler settings with the parser settings used by HLint and other affected tools. Correct mismatches at the owning tool configuration or invocation. Use static-check verification to prove valid code passes and a meaningful invalid control still fails. Never enable Strict, remove a necessary lazy pattern, or weaken a lint gate merely to silence a parser mismatch.
5. Compile affected components with the maintained toolchain and run behavior checks for demand, exceptions, or generated code when those contracts can change. Record the old and new effective settings and any unavailable build configurations. Use consumer verification when starter inputs or generated app contracts change.

In an illustrative library, a shared Cabal stanza enables Strict for its library and tests. Adding {-# LANGUAGE Strict #-} to every module duplicates that policy. A standalone rejection fixture compiled outside Cabal can instead need an explicit extension setting to reproduce its intended diagnostic. Check that fixture's invocation before removing the setting.