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This commit splits up the zonker into a few separate components, described in Note [The structure of the zonker] in `GHC.Tc.Zonk.Type`. 1. `GHC.Tc.Zonk.Monad` introduces a pared-down `TcM` monad, `ZonkM`, which has enough information for zonking types. This allows us to refactor `ErrCtxt` to use `ZonkM` instead of `TcM`, which guarantees we don't throw an error while reporting an error. 2. `GHC.Tc.Zonk.Env` is the new home of `ZonkEnv`, and also defines two zonking monad transformers, `ZonkT` and `ZonkBndrT`. `ZonkT` is a reader monad transformer over `ZonkEnv`. `ZonkBndrT m` is the codensity monad over `ZonkT m`. `ZonkBndrT` is used for computations that accumulate binders in the `ZonkEnv`. 3. `GHC.Tc.Zonk.TcType` contains the code for zonking types, for use in the typechecker. It uses the `ZonkM` monad. 4. `GHC.Tc.Zonk.Type` contains the code for final zonking to `Type`, which has been refactored to use `ZonkTcM = ZonkT TcM` and `ZonkBndrTcM = ZonkBndrT TcM`. Allocations slightly decrease on the whole due to using continuation-passing style instead of manual state passing of ZonkEnv in the final zonking to Type. ------------------------- Metric Decrease: T4029 T8095 T14766 T15304 hard_hole_fits RecordUpdPerf Metric Increase: T10421 -------------------------
