Build.hs

-- | Pure item Selector
type Mask a = a -> Bool

-- | Build record, containing an action to build an item, and the dependencies. 
-- 
-- The return value of the action computed in its monad is a set of created resources, 
-- along with their compiler.
-- 
-- This derived resources depends on the up to date state of the builded one for their existence. 
-- 
-- This is a different dependency from a uptodate to uptodate dependency. 
--
-- The u2udeps are the normal uptodate to uptodate dependency, for the compiler. They functionally don't  
-- on the item actually compiled.
--
data Build m a = Build {
  buildAction :: a -> m [(a,Build m a)],
  u2udeps :: Mask a
  }

-- | Base set of builds, selectable by mask. 
--
-- Notice how this is different from the structure returned from a Build action. 
-- Here the actions are valid for a subspace of the items. 
--
-- Any item matched by the 'Mask' must be compiled with the associated 'Build' value.
--
type Builder m a = [(Mask a, Build m a)]