Allow $topdir and ${pkgroot} as --prefix values.

[angerman]

That lifts the need to rewrite the prefix in the registration hook, when one wants to install packages $topdir or ${pkgroot} relative.

[angerman]

This would allow to register a package relocatable, without resorting to the following hack

registerPackage :: Context -> Action ()
registerPackage context@Context {..} = do
    top     <- topDirectory
    ctxPath <- (top -/-) <$> Context.contextPath context
    Just (Cabal _ _ _ gpd _ _) <- readCabalFile context
    let userHooks = Hooks.autoconfUserHooks
        regHooks = userHooks
        hooks = regHooks {
          Hooks.regHook = \pd lbi us flags ->
              let lbi' = lbi { C.installDirTemplates = updateInstallDirTemplates (C.installDirTemplates lbi) }
              in (Hooks.regHook regHooks) pd lbi' us flags
          }

    liftIO $ defaultMainWithHooksNoReadArgs hooks gpd ["register", "--builddir", ctxPath]

  -- XXX: allow configure to set a prefix with a known variable. $topdir or $pkgroot
  --      that would elivate the need for this hack.
  where updateInstallDirTemplates :: LBI.InstallDirTemplates -> LBI.InstallDirTemplates
        updateInstallDirTemplates idts = idts { LBI.prefix = LBI.toPathTemplate "${pkgroot}/.." }

[23Skidoo]

IIRC @dcoutts was opposed to implementing relocatable packages via $topdir/$pkgroot, don't remember why...

[angerman]

Hmm... the logic for that is still in ghc-cabal and ghc-pkg though. And it's used on the ghc windows builds by default I believe.

[angerman]

After discussing with @hvr on a related issue yesterday. I believe the reason @dcoutts is against it might be that cabal hardcodes absolute paths (w.r.t. the $prefix) in the Paths_* file.

As such I plan to allow only ${pkgroot} and make it a non-documented/advertised option. With a note in the sourced, that this needs to be used with care. To get the Paths_* issue right, we might try to evaluate querying gkc-pkg for the path's instead. This however would likely work for global and user databases out of the box, but would need some further exploration for other package databases and how to pass their path's correctly.