GHC as a Stateless Black Box

[zlonast]

Proposal

GHC as a Stateless Black Box: Ideally, GHC should have no built-in knowledge of the user's environment.

Build Tools as Context Providers: The build tool (Cabal) acts as the sole source of truth, fully defining the dependency graph via explicit CLI arguments, effectively overriding any implicit GHC defaults.

Isolation as a Build Guarantee: Any attempt by GHC to resolve dependencies outside of the provided flags is technically a violation of the build's isolation.

Motivation

Current GHC architecture relies on "implicit discovery" of the system toolchain (C compiler, linker, etc.). When GHC compiles to machine code, it often relies on hardcoded paths from the settings file or looks up the toolchain in the host's $PATH. This makes GHC a "host-dependent" tool rather than a truly "stateless" one. This leads to non-deterministic behavior where the same build can produce different binaries depending on which version of gcc or ld is present on the host system, completely bypassing the isolation provided by build tools.

Plan (.cabal, cabal.project and cabal ***** --[flags])

• Pass explicit -*-options flags
• Pass explicit -pgm* flags
• Pass explicit -opt* flags
• Pass explicit -O[n] and -g[n] flags

The list will be expanded

[zlonast]

Current attempts to get closer

Pass *-options to GHC when compiling ordinary Haskell sources

cc-options, cxx-options, jspp-options, asm-options,
cmm-options, ld-options, cpp-options should be always passed
when invoking GHC, similarly as ghc-options should be always used when
invoking ghc - regardless of what is the intention of a particular GHC-call.
GHC might use or not use the options, Cabal cannot know and should not guess.

Adding explicit optimization flags for interaction with GHC

Cabal now adds explicit -optc-O2, -optcxx-O2 and -opta-O2 flags when
invoking GHC for C, C++ and assembler source files that are built with
optimisation.
Also explicitly add flags optimization -O[n] and debug -g[n] for ghc

Adding an explicit pgmc flag for interaction with GHC

Added an explicit pgmc flag to Cabal's GHC invocation logic. This allows for
reliable and consistent passing of cc-options, ld-options, and cpp-options
to GHC by removing conditional filtering of these arguments.

Adding an explicit pgmcxx flag for interaction with GHC

Add an explicit pgmcxx flag to Cabal's GHC invocation logic.
This change allows users to remove extra-libraries: stdc++ from their
configuration files, as g++ automatically handles C++ standard library
linking, unlike the legacy approach of invoking C++ via gcc.

Adding an explicit pgma flag for interaction with GHC

Add an explicit pgma flag to Cabal's GHC invocation logic. Pass the assembler
program explicitly to GHC. This change allows users to specify a custom as
program to compile assembly manually.

More tests for asm-options, ld-options, cpp-options

[ulysses4ever]

this looks like a good candidate for our lightweight proposal process https://github.com/haskell/cabal-proposals/

also, it's always nicer to the maintainers to discuss (even if just declare) before actually starting to code

[zlonast]

Hmm, sounds logical, I'll try

[zlonast]

[RFC] Separate linking #9256

[zlonast]

Per-project linker choice is not honoured #10769

[TeofilC]

Perhaps https://gitlab.haskell.org/ghc/ghc/-/issues/27249 is also relevant to this

[zlonast]

Too many duplicates in hsc command line arguments, causing build failures due to GCC argument limits #10158