Creating a mocked variant of MonadEffects

[jwiegley]

I'd like to build a mocking platform for hnix. This would have the following benefits:

• It would work in all its various modes, you'd just need to specify --mock and pass it a Nix file declaring the contents of the store.

• It would give us a set of "known store contents" to run tests against.

• The mocked store could reside in memory, isolating the system's store from accumulating cruft during testing (right now, a lot of bogus stuff gets added due to calls to addPath).

• The mocked store, being in memory, could run at a much higher speed, which is needed for testing options like --find.

I see this work consisting of a few parts:

• [ ] Create a variant of MonadEffects and Lazy to implement the Mock store and ensure there are no other effects allowed other than a new MonadStore interface (i.e., no MonadIO in this mode).

• [ ] Write the code that takes a Nix file describing the contents of the store, and which can instantiate this store given a MonadStore instance.

• [ ] Evolve MonadStore into what we'd use for talking to the real store via its wire protocol.

• [ ] Add a new --mock FILE option to enable this machinery.

I then imagine we would always run the tests against a mocked store, meaning that a build + test cycle of hnix should never have any impact on the system outside of the Cabal dist directory.

[shlevy]

Greg and I have already talked about both a readonly and true mock version of hnix-store, so once that's ready the only effects necessary to mock on the hnix end will be file import and downloading I think.

[mightybyte]

Mocking downloading seems like it will be really useful in the future for making a --offline option that doesn't need the internet.

[mightybyte]

I believe @ryantrinkle is working on this?