add context analysis and custom visitors

src/main/java/de/hsrm/compiler/Klang/visitors/EvalVisitor.java

@@ -0,0 +1,66 @@
+package de.hsrm.compiler.Klang.visitors;
+
+import de.hsrm.compiler.Klang.Value;
+import de.hsrm.compiler.Klang.nodes.expressions.*;
+import de.hsrm.compiler.Klang.nodes.statements.*;
+
+public class EvalVisitor implements Visitor<Value> {
+
+    @Override
+    public Value visit(IntegerExpression e) {
+        return new Value(e.value);
+    }
+
+    @Override
+    public Value visit(MultiplicativeExpression e) {
+        Value a = e.lhs.welcome(this);
+        Value b = e.rhs.welcome(this);
+        return new Value(a.asInteger() * b.asInteger());
+    }
+
+    @Override
+    public Value visit(AdditiveExpression e) {
+        Value a = e.lhs.welcome(this);
+        Value b = e.rhs.welcome(this);
+        return new Value(a.asInteger() + b.asInteger());
+    }
+
+    @Override
+    public Value visit(ModuloExpression e) {
+        Value a = e.lhs.welcome(this);
+        Value b = e.rhs.welcome(this);
+        return new Value(a.asInteger() % b.asInteger());
+    }
+
+    @Override
+    public Value visit(NegateExpression e) {
+        Value a = e.lhs.welcome(this);
+        return new Value(-a.asInteger());
+    }
+
+    @Override
+    public Value visit(IfStatement e) {
+        // In the future we have to make sure that the
+        // value is actually a type that we can use as boolean
+        Value condition = e.cond.welcome(this);
+
+        if (condition.asInteger() != 0) {
+            e.then.welcome(this);
+        } else if(e.alt != null) {
+            e.alt.welcome(this);
+        }
+
+        return null;
+    }
+
+    @Override
+    public Value visit(PrintStatement e) {
+        Value value = e.expression.welcome(this);
+
+        // In the future we have to determine of which type the value is
+        // before calling an "asX()" method
+        System.out.println(value.asInteger());
+        return null;
+    }
+
+}

src/main/java/de/hsrm/compiler/Klang/visitors/Visitor.java

@@ -0,0 +1,14 @@
+package de.hsrm.compiler.Klang.visitors;
+
+import de.hsrm.compiler.Klang.nodes.expressions.*;
+import de.hsrm.compiler.Klang.nodes.statements.*;
+
+public interface Visitor<R> {
+    R visit(IntegerExpression e);
+    R visit(MultiplicativeExpression e);
+    R visit(AdditiveExpression e);
+    R visit(NegateExpression e);
+    R visit(ModuloExpression e);
+    R visit(IfStatement e);
+    R visit(PrintStatement e);
+}