Attempt to write tree printing for the parse tree

I have _no_ idea if this will work or not.

src/parser/nodes/mod.rs

@@ -9,16 +9,32 @@ pub use self::constant::Constant;
 mod constant;
 mod program;
 
+use self::constant::ConstantValue;
+
+trait Expression {}
+impl fmt::Debug for Expression {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Expression")
+    }
+}
+impl TreeDebug for Expression {}
+impl<T: constant::ConstantValue> Expression for Constant<T> {}
+
+/// Conveniently print out a node in the tree
 trait TreeDebug: fmt::Debug {
-    fn tree_indent(indent: u8) -> String {
-        (0..10).fold(String::new(), |mut acc, i| {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.tree_fmt(f, 0)
+    }
+
+    fn tree_indent(&self, indent: u8) -> String {
+        (0..10).fold(String::new(), |mut acc, _| {
             acc.push(' ');
             acc
         })
     }
 
     fn tree_fmt(&self, f: &mut fmt::Formatter, indent: u8) -> fmt::Result {
-        write!(f, "{}{:?}", self.tree_indent(indent), self)
+        let spaces: String = self.tree_indent(indent);
+        write!(f, "{}{:?}", spaces, self)
     }
 }
-

src/parser/nodes/program.rs

@@ -5,17 +5,23 @@
 use std::fmt;
 
 use super::TreeDebug;
-use super::Constant;
+use super::Expression;
 
 pub struct Program {
-    forms: Vec<Constant>,
+    forms: Vec<Box<Expression>>,
+}
+
+impl fmt::Debug for Program {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.tree_fmt(f, 0)
+    }
 }
 
 impl TreeDebug for Program {
-    fn tree_fmt(&self, f: fmt::Formatter, indent: u8) -> fmt::Result {
+    fn tree_fmt(&self, f: &mut fmt::Formatter, indent: u8) -> fmt::Result {
         let spaces = self.tree_indent(indent);
         let mut result = write!(f, "{}Program", spaces);
-        for form in self.forms {
+        for form in self.forms.iter() {
             if result.is_err() {
                 break;
             }