"Magic" behavior when cabal exec is used with with-compiler:

[sol]

Describe the magic behavior

When you set with-compiler: ghc-x.y.z in cabal.project then:

• cabal exec ghc refers to ghc-x.y.z
• However, cabal exec which ghc does not return the path to ghc-x.y.z

On the surface this seems like magic behavior.

1. Is this documented somewhere?
2. What is the exact mechanism here?
3. For implementing cabal-doctest, I need to determine which GHC executable is used by cabal. Any other ideas how I could achieve this?

To Reproduce

$ cat cabal.project
with-compiler: ghc-9.10.1

$ cabal exec -v0 -- ghc --numeric-version
9.10.1

$ $(cabal exec -v0 which ghc) --numeric-version
9.2.8

System information

$ cabal --version
cabal-install version 3.12.1.0
compiled using version 3.12.1.0 of the Cabal library

[hasufell]

Yes, it's documented:

cabal/cabal-install/src/Distribution/Client/CmdExec.hs

Line 136 in eacb6d0

++ " v2-exec ghc` will consult the configuration"

[hasufell]

For implementing cabal-doctest, I need to determine which GHC executable is used by cabal. Any other ideas how I could achieve this?

Yes, see https://github.com/haskell/hie-bios/blob/6a06ed72706a533f20a06823650f165203aeb221/src/HIE/Bios/Cradle.hs#L768

[geekosaur]

Isn't the principled way to do this a cabal hc command? (Possibly with support for something similar to --repl-multi-file.)

[sol]

Thanks @hasufell.

1. I think from a user perspective it's a disaster that e.g. cabal exec ghc ... and cabal exec env ghc ... don't do the same thing.

2. I think I agree with @geekosaur that a dedicated sub command would have offered a more principled way to invoke the configured ghc (e.g. cabal exec-compiler). However, changing this now would be a breaking change.

3. For cabal-doctest I'm using the output of cabal path, which requires cabal-install-3.12.*. However, as sub commands are only supported since 3.12.* that's acceptable for cabal-doctest.