Implement listen/watch edges

scone/head/dag.py

@@ -137,6 +137,9 @@ class RecipeDag:
 
         self.resource_time: Dict[Resource, int] = dict()
 
+        # true for 'only when changed', false for '@when changed'
+        self.watching: Dict[Recipe, Dict[Vertex, bool]] = defaultdict(dict)
+
     def add(self, vertex: Vertex):
         self.vertices.add(vertex)
         if isinstance(vertex, Recipe):
@@ -211,3 +214,10 @@ class RecipeDag:
             after_meta.incoming_uncompleted += 1
             # TODO if after_meta.state ==
         # TODO else ...
+
+    def watches(self, recipe: "Recipe", watching: Vertex, only_when: bool) -> None:
+        if watching not in self.edges or recipe not in self.edges[watching]:
+            raise ValueError(
+                f"{recipe} needs to be after {watching} before it can watch it."
+            )
+        self.watching[recipe][watching] = only_when

scone/head/grammar/scoml.tx

@@ -80,8 +80,8 @@ ListenEdgeDirectiveKind:
 
 ListenEdgeDirective[ws=' \t']:
   kind=ListenEdgeDirectiveKind
-  (recipe_id=IdString | resource=Resource)
-  'changes'
+  ((':' recipe_id=IdString) | resource=Resource)
+  'changes' /\n/+
 ;
 
 

scone/head/kitchen.py

@@ -208,9 +208,32 @@ class Kitchen:
     ) -> Tuple[Optional[DependencyBook], bool]:
         """
         :param recipe: recipe to inquire about
-        :return: dep book, or None if there wasn't one
+        :return: dep book, or None if there wasn't one needed to compute this
             and true if the recipe should be skipped, false otherwise.
         """
+
+        only_when_flag_set = False
+
+        if recipe in self.head.dag.watching:
+            for watching, only_when in self.head.dag.watching[recipe].items():
+                if isinstance(watching, Resource):
+                    # only recipe watches are accepted here.
+                    # resource watches are handled by adding them to the watchlist
+                    # in the dependency book
+                    continue
+                assert isinstance(watching, Recipe)
+                only_when_flag_set |= only_when
+                watch_rmeta = self.head.dag.recipe_meta[watching]
+                if watch_rmeta.state == RecipeState.COOKED:
+                    # underlying recipe changed. Ideally want a new changed state.
+                    # TODO(design): have a 'changed' state for recipes?
+                    return None, False
+
+        if only_when_flag_set:
+            # TODO(design) is it sensible to skip this here? What if we need to
+            #  provide something? I suppose it's not guaranteed to be provided.
+            return None, True
+
         inquiry = await self._dependency_store.inquire(recipe)
         if inquiry is None:
             return None, False
@@ -288,6 +311,16 @@ class Kitchen:
                         meta.state = RecipeState.FAILED
                         raise RuntimeError(f"Recipe {next_job} failed!") from e
                     eprint(f"cooked {next_job}")
+
+                    if next_job in self.head.dag.watching:
+                        for watching, only_when in self.head.dag.watching[
+                            next_job
+                        ].items():
+                            if isinstance(watching, Resource):
+                                # recipe watches are handled when loading the
+                                # dependency book.
+                                tracker.watch(watching)
+
                     await self._store_dependency(next_job)
                     meta.state = RecipeState.COOKED
             elif isinstance(next_job, Resource):

scone/head/menu_reader.py

@@ -74,6 +74,14 @@ class ResourceEdgeDirective:
     resource: Resource
 
 
+@attr.s(auto_attribs=True)
+class ListenEdgeDirective:
+    # "when" or "only when"
+    kind: str
+
+    recipe_or_resource: Union[str, Resource]
+
+
 @attr.s(auto_attribs=True, eq=False)
 class MenuBlock:
     id: Optional[None]
@@ -109,6 +117,7 @@ class MenuRecipe:
     for_directives: List[ForDirective] = attr.ib(factory=list)
     recipe_edges: List[RecipeEdgeDirective] = attr.ib(factory=list)
     resource_edges: List[ResourceEdgeDirective] = attr.ib(factory=list)
+    listen_edges: List[ListenEdgeDirective] = attr.ib(factory=list)
 
 
 def convert_textx_value(txvalue) -> Any:
@@ -165,6 +174,14 @@ def convert_textx_recipe(txrecipe_or_subblock, parent: Optional[MenuBlock]):
                 recipe.recipe_edges.append(
                     RecipeEdgeDirective(directive.kind[1:], directive.id)
                 )
+            elif isinstance(directive, scoml_classes["ListenEdgeDirective"]):
+                recipe.listen_edges.append(
+                    ListenEdgeDirective(
+                        directive.kind[1:],
+                        directive.recipe_id
+                        or convert_textx_resource(directive.resource),
+                    )
+                )
             else:
                 raise ValueError(f"Unknown directive {directive}")
 
@@ -185,7 +202,9 @@ def convert_textx_resource(txresource) -> Resource:
         else:
             sous = txresource.sous
 
-    return Resource(txresource.type, txresource.primary, sous, extra_params)
+    return Resource(
+        txresource.type, convert_textx_value(txresource.primary), sous, extra_params
+    )
 
 
 def convert_textx_block(txblock, parent: Optional[MenuBlock]) -> MenuBlock:
@@ -450,7 +469,7 @@ class MenuLoader:
         fors: Tuple[ForDirective, ...],
         applicable_souss: Iterable[str],
         sous_mask: Optional[Set[str]],
-    ):
+    ) -> None:
         # TODO(feature): add edges
 
         # add fors
@@ -497,7 +516,36 @@ class MenuLoader:
                     elif resource_edge.kind == "provides":
                         self._dag.provides(instance, resource)
 
-                # XXX apply specific edges here including those from parent
+                for listen_edge in recipe.listen_edges:
+                    if isinstance(listen_edge.recipe_or_resource, Resource):
+                        # TODO(design): is it right for this to NEED it rather
+                        #   than WANT it?
+                        resource = listen_edge.recipe_or_resource
+                        if resource.sous == "(self)":
+                            resource = attr.evolve(resource, sous=sous)
+                        self._dag.needs(instance, resource)
+                        self._dag.watches(
+                            instance, resource, listen_edge.kind == "only when",
+                        )
+                    elif isinstance(listen_edge.recipe_or_resource, str):
+                        target = self.resolve_ref(
+                            recipe, listen_edge.recipe_or_resource
+                        )
+
+                        if isinstance(target, MenuRecipe):
+                            for target_instance in self.get_related_instances(
+                                sous, for_indices, recipe, target
+                            ):
+                                self._dag.add_ordering(target_instance, instance)
+                                self._dag.watches(
+                                    instance,
+                                    target_instance,
+                                    listen_edge.kind == "only when",
+                                )
+                        else:
+                            raise RuntimeError(f"not supported on target: {target!r}")
+
+                # XXX apply edges from parent
 
     def postdagify_block(
         self,