How should bios cradles run ghc?

[lukel97]

This a further piece of work from #207:

Cabal and stack cradles can run GHC to get the libdir from cabal exec ghc -- --print-libdir and stack exec ghc -- --print-libdir respectively.
Direct cradles just use the ghc on the path.
But what about bios programs? What ghc are they using exactly? There should be a way for the bios program to report this, either via the program or in a hie.yaml file

[pepeiborra]

I was coming around to request this, well spotted @bubba !

A bios cradle currently has the following shape:

  bios:
    program: "program to run"
    dependency-program: "optional program to run"
    shell: build-tool flags $HIE_BIOS_ARG
    dependency-shell: build-tool dependencies

where each component has the type

program :: FilePath -> IO ([Flag], [Target])
shell :: String -> IO ([Flag], [Target])
dependency_program :: FilePath
dependency_shell :: [FilePath]

The path to GHC is not necessarily known statically, so we need a program to compute it, and it should be the same for the entire session - we don't want a different GHC per component or per file!

With that in mind, I propose adding a new optional field to the bios cradle section, which if present is used to compute the ghc path:

 with-ghc :: IO String
 with-ghc: "program to run"

[fendor]

@pepeiborra The important bit is to document what kind of command line options this ghc must accept to be functional. E.g. currently we need it to accept --numeric-version and -print-libdir. What else might be helpful?

[pepeiborra]

@fendor that too. But I came here because I want my bios scripts to be able to set the path to ghc, that's the important feature!