Add proposal for cabal extension to support bytecode files

Author: mpickering

proposals/bytecode-files.md

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+# Cabal Support for Bytecode Objects and Bytecode Libraries
+
+## Summary
+
+Extend Cabal to support building, installing, and managing persistent bytecode
+artifacts (`.gbc` files) and bytecode libraries alongside native code, enabling
+faster build times and better tooling integration for development and REPL use
+cases.
+
+## Motivation
+
+Currently, GHC's bytecode interpreter only generates transient, in-memory
+bytecode that must be regenerated every time a module is loaded in GHCi or for
+Template Haskell evaluation. This proposal extends Cabal and `cabal-install` to support persistent
+bytecode artifacts, which will:
+
+- Enable caching of bytecode for faster (`cabal repl`) startup and tooling
+- Reduce compilation time for development workflows
+- Provide better integration between Cabal and GHC's bytecode capabilities
+- Support the new `-fwrite-bytecode` flag in GHC for persistent bytecode generation
+
+This is not intended to replace native code for production executables, but
+rather to extend Cabal's capabilities where fast build times, portability, and
+tooling integration are more important than peak execution speed.
+
+## Proposed Change
+
+### Changes to `ghc-pkg`
+
+- Add a new field `bytecode-library-dirs` to the package database to store the locations of bytecode libraries.
+
+### Changes to Cabal Library (Setup Interface)
+
+The main changes are to add a new bytecode "library way" to Cabal.
+
+- Add `--enable-library-bytecode` build option to the Cabal library interface which when enabled will pass `-fwrite-bytecode` to GHC
+- If requesting an object way and the bytecode way, then `-fbyte-code-and-object-code` will be passed to GHC to enable compilation of all artifacts in a single pass.
+- Once a library is finished being compiled, cabal will create a bytecode library archive containing all the `.gbc` files for the library.
+- The bytecode library will be installed into the package database alongside the native library.
+- Add a new option to enable the `repl` command to write bytecode files. This will be enabled by default.
+
+### Changes to cabal-install (User Interface)
+
+- `--enable-library-bytecode` flag: When specified, enables bytecode generation for the build
+- This flag exposes the underlying `--enable-library-bytecode` build option from the Cabal library
+- This flag is like `--enable-shared` or `--enable-library-profiling`, it is a way specifier.
+
+## Alternatives Considered
+
+- The other option is to not add support to Cabal for bytecode libraries, and
+instead have the user manually create a bytecode library archive and install it
+into the package database. This approach would lead to a difficult adoption of
+the feature.
+
+## Backwards Compatibility / Migration
+
+This change maintains full backwards compatibility:
+
+- Default Behavior: Bytecode generation is disabled by default, so existing builds are unaffected
+- Optional Feature: The `--enable-library-bytecode` flag is opt-in, requiring explicit user action
+
+## Interested parties
+
+- Haskell Tooling Developers: Those building IDEs, language servers, and development tools that would benefit from faster bytecode loading
+- GHC Developers: The GHC team implementing the `-fwrite-bytecode` functionality
+- Cabal Maintainers: The Cabal team who will review and integrate these changes
+- Package Authors: Developers who want faster development cycles and better tooling integration
+- Haskell Community: Users who would benefit from improved development experience
+
+### Contact Status
+
+- GHC Team: I am collaborating with Cheng Shao, and Rodrigo Mesquita on the bytecode implementation
+- Cabal Team: Will engage with Cabal maintainers for review and integration
+- Tooling Community: I made a [post on discourse](https://discourse.haskell.org/t/rfc-introduce-a-serialisable-bytecode-format-and-corresponding-bytecode-way/12678) asking for feedback on the proposal.
+
+## Implementation Notes
+
+### Implementation Plan
+
+1. Phase 1: Extend Cabal library to support bytecode configuration and artifact generation
+   - Add `--enable-library-bytecode` build option to the Cabal library
+   - Implement bytecode generation in the build system
+   - Extend package registration for bytecode artifacts
+
+2. Phase 2: Implement cabal-install user interface changes
+   - Add `--enable-library-bytecode` command-line flag
+
+3. Phase 3: Integration testing and documentation
+   - Test across different platforms and configurations
+   - Document new features and usage patterns
+
+Either myself or a colleague can implement these changes.
+
+## Open Questions
+
+- `cabal-install` could also support the option to use `-fprefer-bytecode`, which would imply generating bytecode for dependencies rather than object code
+  in the same build way as the compiler.
+- There currently isn't support for building "bytecode executables", and hence no `--enable-executable-bytecode` flag.
+
+## References
+
+* [GHC Issue 26298](https://gitlab.haskell.org/ghc/ghc/-/issues/26298)
+* [Cabal Issue 11188](https://github.com/haskell/cabal/issues/11188)
+* Discourse post: [RFC: Introduce a serialisable bytecode format and corresponding bytecode way](https://discourse.haskell.org/t/rfc-introduce-a-serialisable-bytecode-format-and-corresponding-bytecode-way/12678)