structural change: separate crates out

I put functionalities that are not strictly core to separate crates,
so that the whole package becomes more modular, and makes it easier to
try other parsing algorithms in the future.

Also I have to figure the forests out before finishing the core
chain-rule algorithm, as the part about forests affects the labels of
the grammars directly.  From my experiences in writing the previous
version, it is asking for trouble to change the labels type
dramatically at a later point: too many places need to be changed.
Thus I decide to figure the rough part of forests out.

Actually I only have to figure out how to attach forests fragments to
edges of the underlying atomic languages, and the more complex parts
of putting forests together can be left to the recorders, which is my
vision of assembling semi-ring values during the chain-rule machine.
It should be relatively easy to produce forests fragments from
grammars since we are just trying to extract some information from the
grammar, not to manipulate those information in some complicated way.
We have to do some manipulations in the process, though, in order to
make sure that the nulling and epsilon-removal processes do not
invalidate these fragments.

1 files changed, 7 insertions, 7 deletions

diff --git a/graph/src/lib.rs b/graph/src/lib.rs
index 2d23af3..d4f6d7c 100644
--- a/graph/src/lib.rs
+++ b/graph/src/lib.rs
@@ -233,6 +233,12 @@ pub trait LabelGraph<T: GraphLabel>: Graph {
         Self: 'a,
         T: 'a;
 
+    /// A type that iterates over labels of an edge.
+    type EdgeLabelIter<'a>: Iterator<Item = T> + 'a
+    where
+        Self: 'a,
+        T: 'a;
+
     #[inline]
     /// Return the label of a vertex or an error if the node is
     /// invalid.
@@ -247,15 +253,9 @@ pub trait LabelGraph<T: GraphLabel>: Graph {
         }
     }
 
-    #[inline]
     /// Return the label of an edge or an error if some node is
     /// invalid.
-    ///
-    /// The default implementation always returns an empty vector for
-    /// valid nodes.
-    fn edge_label(&self, source: usize, target: usize) -> Result<Vec<T>, Error> {
-        self.has_edge(source, target).map(|_| Vec::new())
-    }
+    fn edge_label(&self, source: usize, target: usize) -> Result<Self::EdgeLabelIter<'_>, Error>;
 
     /// Return an iterator of edges out of a node, whose associated
     /// label is as given.