Refactor Target

[snowleopard]

We currently have the following Target datatype:

-- | Parameters relevant to the current build target.
data Target = Target
    { stage   :: Stage      -- ^ Stage being built
    , package :: Package    -- ^ Package being built
    , builder :: Builder    -- ^ Builder to be invoked
    , way     :: Way        -- ^ Way to build (set to vanilla for most targets)
    , inputs  :: [FilePath] -- ^ Source files passed to the builder
    , outputs :: [FilePath] -- ^ Files to be produced
    } deriving (Show, Eq, Generic)

It makes sense to factor out Stage, Package and Way into a separate datatype called Context:

• At the moment we mix different notions in one datatype: target-specific ones (builder, inputs, outputs) and context-specific ones (what is the current stage, package and way?).

• By factoring out GHC-specific bits into Context we make it easier to reuse our build abstractions in other build systems. Arguably, builder, inputs and outputs are common for all non-trivial build systems, however, not all build systems need way, package or, especially, stage.

• We already have a datatype called PartialTarget in the build system:

  -- | A partially constructed Target with fields 'Stage' and 'Package' only.
  -- 'PartialTarget's are used for generating build rules.
  data PartialTarget = PartialTarget Stage Package deriving (Eq, Show)

  This is almost exactly what we want to have as a Context.

/cc @simonpj @simonmar @ndmitchell

[ndmitchell]

Seems reasonable to me. Will we be able to kill PartialTarget afterwards?

[snowleopard]

@ndmitchell Yes, I think so. It will be consumed by Context.