.ghc.environment files should probably support thinning and renaming

[23Skidoo]

Let's say that I use the following common stanza to replace the standard Prelude with ClassyPrelude:

common classy-prelude
  build-depends:       base >=4.11 && <4.13
                     , classy-prelude
  mixins:              base hiding (Prelude)
                     , classy-prelude (ClassyPrelude as Prelude)

This results in approximately the following flags being passed to ghc:

-package-id 'base-4.12.0.0 (Control.Applicative, Control.Arrow, [... lots of module names omitted])' -package-id 'classy-prelude-1.5.0-ad629aa84... (ClassyPrelude as Prelude)'

The problem now is that the generated .ghc.environment file looks like this:

package-id base-4.12.0.0
[...]
package-id classy-prelude-1.5.0-ad629aa84...

That is, information about thinning and renaming has been lost, which, unfortunately, breaks tools that rely purely on the environment file, such as ghcid. It looks like this should be also fixed on the GHC side, since the package environment files don't seem to support the full thinning and renaming syntax that GHC's -package-id option supports.

Related: #5962.

/cc @hvr

[23Skidoo]

I sort of managed to work around this with :set -hide-package base -package "classy-prelude (ClassyPrelude as Prelude)" in .ghci, but then ghcid itself starts producing weird errors:

<interactive>:1:1: error:
    Ambiguous module name ‘Prelude’:
      it is bound as ClassyPrelude by a package flag
      it is bound as base-4.12.0.0:Prelude by package base-4.12.0.0

<interactive>:9:1: error:
    Not in scope: ‘INTERNAL_GHCID.putStrLn’
    No module named ‘INTERNAL_GHCID’ is imported.

<interactive>:10:1: error:
    Not in scope: ‘INTERNAL_GHCID.hPutStrLn’
    No module named ‘INTERNAL_GHCID’ is imported.

<interactive>:10:26: error:
    Not in scope: ‘INTERNAL_GHCID.stderr’
    No module named ‘INTERNAL_GHCID’ is imported.

Anyway, on second thought, this kind of thing should probably be handled by cabal new-repl rather than environment files, though that currently doesn't work that well for me either.

[phadej]

ghcid -c 'cabal new-repl'

[23Skidoo]

Yeah, I tried that, but it doesn't work that great either. I guess I should close this ticket and open new ones about improving new-repl.