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Matthew Pickering
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This commit introduces the `ExplicitLevelImports` and `ImplicitStagePersistence` language extensions as proposed in GHC Proposal #682. Key Features ------------ - `ExplicitLevelImports` adds two new import modifiers - `splice` and `quote` - allowing precise control over the level at which imported identifiers are available - `ImplicitStagePersistence` (enabled by default) preserves existing path-based cross-stage persistence behavior - `NoImplicitStagePersistence` disables implicit cross-stage persistence, requiring explicit level imports Benefits -------- - Improved compilation performance by reducing unnecessary code generation - Enhanced IDE experience with faster feedback in `-fno-code` mode - Better dependency tracking by distinguishing compile-time and runtime dependencies - Foundation for future cross-compilation improvements This implementation enables the separation of modules needed at compile-time from those needed at runtime, allowing for more efficient compilation pipelines and clearer code organization in projects using Template Haskell. Implementation Notes -------------------- The level which a name is availble at is stored in the 'GRE', in the normal GlobalRdrEnv. The function `greLevels` returns the levels which a specific GRE is imported at. The level information for a 'Name' is computed by `getCurrentAndBindLevel`. The level validity is checked by `checkCrossLevelLifting`. Instances are checked by `checkWellLevelledDFun`, which computes the level an instance by calling `checkWellLevelledInstanceWhat`, which sees what is available at by looking at the module graph. Modifications to downsweep -------------------------- Code generation is now only enabled for modules which are needed at compile time. See the Note [-fno-code mode] for more information. Uniform error messages for level errors --------------------------------------- All error messages to do with levels are now reported uniformly using the `TcRnBadlyStaged` constructor. Error messages are uniformly reported in terms of levels. 0 - top-level 1 - quote level -1 - splice level The only level hard-coded into the compiler is the top-level in GHC.Types.ThLevelIndex.topLevelIndex. Uniformly refer to levels and stages ------------------------------------ There was much confusion about levels vs stages in the compiler. A level is a semantic concept, used by the typechecker to ensure a program can be evaluated in a well-staged manner. A stage is an operational construct, program evaluation proceeds in stages. Deprecate -Wbadly-staged-types ------------------------------ `-Wbadly-staged-types` is deprecated in favour of `-Wbadly-levelled-types`. Lift derivation changed ----------------------- Derived lift instances will now not generate code with expression quotations. Before: ``` data A = A Int deriving Lift => lift (A x) = [| A $(lift x) |] ``` After: ``` lift (A x) = conE 'A `appE` (lift x) ``` This is because if you attempt to derive `Lift` in a module where `NoImplicitStagePersistence` is enabled, you would get an infinite loop where a constructor was attempted to be persisted using the instance you are currently defining. GHC API Changes --------------- The ModuleGraph now contains additional information about the type of the edges (normal, quote or splice) between modules. This is abstracted using the `ModuleGraphEdge` data type. Fixes #25828 ------------------------- Metric Increase: MultiLayerModulesTH_Make -------------------------
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