Find the ghc binary path from libdir

[Daniel Gröber]

Hi Fendor,

On Mon, Feb 08, 2021 at 08:08:45PM +0100, Fendor wrote:
> Basically, we want to know which `ghc` version `cabal-install` is going to
> pick for compiling the project in a platform independent way without
> resorting to linking to `Cabal`.

For v2-build this can be found out by looking at plan.json for which you
only need to link to cabal-plan (or a json lib of your choice).

You do need to have logic run `cabal --dry-run` (IIRC) to generate
plan.json if it's not there. I have some code for that in cabal-helper,
it's not very tricky as long as you're assuming the cabal project isn't in
a broken state anyway.

> You can execute `cabal exec -- ghc` which launches the correct `ghc`
> version, but as far as we found out, there is no programmatic way of finding
> the path to the `ghc` executable itself (`cabal exec -- where ghc` or `cabal
> exec -- which ghc` fails on windows). 

Seems to me it would be relatively straightforward to add a "which" mode to whatever executable you're shipping (we're talking about hls, right?) so you can do something like:

    cabal exec -- $path_to_hls which

which then just prints the path to GHC on PATH found using the appropriate
library function without running into the problem of not having `which` on
windows.

--Daniel