cabal 3.8.1.0 compiles temporary build path for datadir into alex, happy, other tools

[essandess]

Describe the bug

I’m migrating several MacPorts Haskell builds from stack to cabal to get native arm64 binaries linked to MacPorts libraries.

I observe that cabal version 3.8.1.0 compiles a temporary build path for datadir from store-dir into alex, happy, other binaries it compiles. This breaks projects for which datadir is required.

I’ve tried many different approaches to setting datadir To the correct, permanent location, but none of these methods work:

• datadir: … in the config file: https://github.com/macports/macports-ports/blob/69b89680a5f48245784b8376241f948bbd40a1c6/_resources/port1.0/group/haskell_cabal-1.0.tcl#L105
• --datadir=… in the command line
• Paths_alex.hs that specifies datadir = …
• Use the --enable-relocatable flag

Because these tools are necessary to bootstrap and use Haskell world, I’ve thrown in the towel and just used an ugly hack of replacing this incorrect path in the binary with the correct path padded with extraneous /‘s. See: macports/macports-ports#16154

How can cabal compile a project like alex from source and set the correct datadir?

Likely related: #3586

To Reproduce

Compile cabal build/install of alex, happy, and so forth.

System information

• macOS 12.6 21G115 x86_64
• Xcode 14.0 14A309
• cabal 3.8.1.0, ghc 9.4.2

Additional context
MacPorts build commands:

• https://github.com/macports/macports-ports/blob/master/devel/alex/Portfile
• https://github.com/macports/macports-ports/blob/master/_resources/port1.0/group/haskell_cabal-1.0.tcl

[gbaz]

The linked ticket cross-links to a large other number of related issues. I really do think we should tackle this for the next cabal release. The problem is I'm not sure what the right design to implement is! If someone wants to propose the exact semantics (perhaps with more flags) that cabal implement, this seems tractable and worthwhile.

[essandess]

The linked ticket cross-links to a large other number of related issues. I really do think we should tackle this for the next cabal release. The problem is I'm not sure what the right design to implement is! If someone wants to propose the exact semantics (perhaps with more flags) that cabal implement, this seems tractable and worthwhile.

Thanks you—that would be much appreciated. I’ve discovered that the slash hack doesn’t work on arm architectures, and even this hack doesn’t provide a viable alternative on that platform. See macports/macports-ports#16228.

[gbaz]

Based on the discussion there, could you just override this by setting $alex_datadir etc in the environment?

[gbaz]

I believe this is resolved by #8556

[Mikolaj]

Could anybody kindly verify using master branch (or cabal 3.10 when it gets released)? Let me close, but please open a new ticket if problems still occur.