Basic evaluator functionality

Interpreter now works for simple arithmetic expressions

src/evaluator.rs

@@ -4,27 +4,77 @@ struct Evaluator {
     ast: AST
 }
 
-impl Evaluator {
-    pub fn run(&mut self) -> String {
-        while self.ast.can_reduce() {
-            self.ast.reduce();
-        }
+pub type EvaluatorResult<T> = Result<T, EvaluatorError>;
 
-        format!("{}", self.ast)
+struct EvaluatorError {
+    err: String
+}
+
+impl Evaluator {
+    pub fn run(self) -> String {
+        match reduce_full(self.ast) {
+            Err(e) => format!("{}", e.err),
+            Ok(ast) => format!("{}", ast)
+        }
     }
 }
 
 impl AST {
     fn can_reduce(&self) -> bool {
-        false
+        use parser::AST::*;
+        match self {
+            &Number(_) => false,
+            &Name(_) => false,
+            _ => true
+        }
+    }
+}
+
+fn reduce_full(ast: AST) -> EvaluatorResult<AST> {
+    let mut ast = ast;
+    while ast.can_reduce() {
+        ast = try!(reduce_step(ast));
     }
 
-    fn reduce(&mut self) {
+    Ok(ast)
+}
+
+fn reduce_step(ast: AST) -> EvaluatorResult<AST> {
+    use parser::AST::*;
+    match ast {
+        BinOp(left, op, right) => {
+            let left = try!(reduce_full(*left));
+            let op = try!(reduce_full(*op));
+            let right = try!(reduce_full(*right));
+            match (left, op, right) {
+                (Number(l), Name(op), Number(r)) => {
+                    match &op[..] {
+                        "+" => Ok(Number(l + r)),
+                        "-" => Ok(Number(l - r)),
+                        "*" => Ok(Number(l * r)),
+                        "/" => {
+                            if r == 0.0 {
+                                Err(EvaluatorError { err: format!("Divide by zero") })
+                            } else {
+                                Ok(Number(l / r))
+                            }
+                        },
+                        _ => Err(EvaluatorError { err: format!("Bad BinOp operator") })
+                    }
+                },
+                _ => Err(EvaluatorError { 
+                    err: format!("Bad arguments for BinOp")
+                })
+            }
+        },
+        Number(_) => Ok(ast),
+        Name(_) => Ok(ast),
 
     }
 }
 
+
 pub fn evaluate(ast: AST) -> String {
-    let mut ev = Evaluator { ast: ast };
+    let ev = Evaluator { ast: ast };
     ev.run()
 }

src/parser.rs

@@ -13,7 +13,11 @@ pub enum AST {
 
 impl fmt::Display for AST {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", "GENERIC DISPLAY")
+        match self {
+            &AST::Number(ref n) => write!(f, "{}", n),
+            &AST::Name(ref s) => write!(f, "{}", s),
+            astnode => write!(f, "UNEXPANDED AST NODE: {:?}", astnode)
+        }
     }
 }