Support backpack builds

[shlevy]

Fixes #244

[andreabedini]

I am not sure I follow here. By deleting exactDep you effectively disable it? if the --dependency flag is invalid, can that be fixed instead?

[shlevy]

No, you don't disable it, and you already have the needed flag that you would fix it to. The --dependency flag from the uninstantiated variant is the one we want to pass to Setup.hs, and the instantiated variant has the uninstantiated one in its propagated build inputs. The instantiated one is enabled because it's in the package db and Setup knows which package db entry because it's a function of the uninstantiated unit ID (passed via --dependency) and the unit IDs of the hole fillings specified in the cabal file.

Relevant cabal file subset:

library sig
  build-depends: base
  signatures: Module

library impl
  build-depends: base
  exposed-modules: Mod

library consumer
  build-depends: base, backpack:sig
  signatures: Module

executable backpack
  build-depends: base, backpack:consumer, backpack:impl
  mixins: backpack:consumer requires (Module as Mod)

Relevant buildInputs of ig12a19g4mdgrcxcf7f7jwk155xya42a-backpack-exe-backpack-0.1.0.0.drv:

• /nix/store/caas5rc8ahs5mbq4p38vgr9wjznkjkgb-backpack-lib-consumer-0.1.0.0

caas5rc8ahs5mbq4p38vgr9wjznkjkgb-backpack-lib-consumer-0.1.0.0 is a backpack instantiation. Relevant propagatedBuildInputs:

• /nix/store/hmiprpqxp8mzyvl6mqz24gqrkzlq7wy7-backpack-lib-consumer-0.1.0.0

Which is the version of consumer without any of its holes filled.

So for caa*consumer*, we get these configure flags:

• --dependency=backpack:consumer=backpack-0.1.0.0-F4eEDqjvUdHGGjwvlSTTdF-consumer # comes from hmipr*consumer* being in buildInputs
• --instantiate-with=Module=backpack-0.1.0.0-A0nxY63swED6kmHNYgsA4d-impl:Mod # comes from the fact that we're instantiating one of hmipr*consumer*'s holes with some concrete module

And we do not have exactDeps in $out.

And for ig12*exe-backpack*, we have these configure flags:

• --dependency=backpack:consumer=backpack-0.1.0.0-F4eEDqjvUdHGGjwvlSTTdF-consumer # comes from hmipr*consumer* being /propagated/ by caa*consumer*

But Setup.hs looks at the cabal file and sees we don't want the uninstantiated version of consumer, we want a particular instantiation, and it calculates the expected hash, and it finds the relevant dependency in the package db because caa*consumer* is in buildInputs.