Add train and further methods for extending thunk actions

src/Nix/Thunk.hs

@@ -38,6 +38,9 @@ class MonadThunkId m => MonadThunk t m a | t -> m, t -> a where
     queryM :: t -> m r -> (a -> m r) -> m r
     force :: t -> (a -> m r) -> m r
     forceEff :: t -> (a -> m r) -> m r
+    -- | Modify the action to be performed by the thunk. For some implicits
+    --   this modifies the thunk, for others it may create a new thunk.
+    further :: t -> (m a -> m a) -> m t
 
 newtype ThunkLoop = ThunkLoop String -- contains rendering of ThunkId
     deriving Typeable

src/Nix/Value/Monad.hs

@@ -6,3 +6,7 @@ module Nix.Value.Monad where
 class MonadValue v m where
   defer :: m v -> m v
   demand :: v -> (v -> m r) -> m r
+  -- | If 'v' is a thunk, 'train' allows us to modify the action to be
+  --   peformed by the thunk, perhaps by enriching it with scpoe info, for
+  --   example.
+  train :: v -> (m v -> m v) -> m v