simplify EvalTree::next

src/func/instances/stackless.rs

@@ -48,18 +48,22 @@ enum EvalTree<'a> {
 type Oet<'a> = Option<EvalTree<'a>>;
 
 impl<'a> EvalTree<'a> {
+    fn _next_composite(self, other: Box<Self>) -> Self {
+        match self {
+            Self::Atom(f) => match f.next() {
+                Some(newleft) => Self::Composite(Box::new(newleft), other),
+                None => *other,
+            },
+            Self::Composite(etll, etlr) => {
+                Self::Composite(etll, Box::new(Self::Composite(etlr, other)))
+            }
+        }
+    }
+
     fn next(self) -> Oet<'a> {
         match self {
             Self::Atom(f) => f.next(),
-            Self::Composite(etl, etr) => match *etl {
+            Self::Composite(etl, etr) => Some(etl._next_composite(etr)),
-                Self::Atom(f) => match f.next() {
-                    Some(newleft) => Some(Self::Composite(Box::new(newleft), etr)),
-                    None => Some(*etr),
-                },
-                Self::Composite(etll, etlr) => {
-                    Some(Self::Composite(etll, Box::new(Self::Composite(etlr, etr))))
-                }
-            },
         }
     }
 }