Поддержка операций сложения и вычитания

src/com/annimon/everlastingsummer/Parser.java

@@ -4,6 +4,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import com.annimon.everlastingsummer.ast.*;
+import com.annimon.everlastingsummer.ast.BinaryExpression.Operator;
 import android.text.TextUtils;
 
 /**
@@ -411,7 +412,25 @@ public final class Parser {
     }
     
     private Expression expression() {
-        return unary();
+        return additive();
+    }
+    
+    private Expression additive() {
+        Expression expression = unary();
+        
+        while (true) {
+            if (match(TokenType.PLUS)) {
+                expression = new BinaryExpression(Operator.ADD, expression, unary());
+                continue;
+            }
+            if (match(TokenType.MINUS)) {
+                expression = new BinaryExpression(Operator.SUBTRACT, expression, unary());
+                continue;
+            }
+            break;
+        }
+        
+        return expression;
     }
     
     private Expression unary() {

src/com/annimon/everlastingsummer/ast/BinaryExpression.java

@@ -0,0 +1,32 @@
+package com.annimon.everlastingsummer.ast;
+
+/**
+ * @author aNNiMON
+ */
+public class BinaryExpression implements Expression {
+    
+    public static enum Operator {
+        ADD, SUBTRACT
+    }
+
+    public final Operator operator;
+    private final Expression left, right;
+
+    public BinaryExpression(Operator op, Expression left, Expression right) {
+        this.operator = op;
+        this.left = left;
+        this.right = right;
+    }
+
+    @Override
+    public double eval() {
+        final double leftVal = left.eval();
+        final double rightVal = right.eval();
+        switch (operator) {
+            case ADD: return leftVal + rightVal;
+            case SUBTRACT: return leftVal - rightVal;
+            default:
+                throw new RuntimeException("Неизвестный оператор " + operator.name());
+        }
+    }
+}