diff --git a/AlgebraicDataflowArchitectureModel/src/models/dataConstraintModel/DataConstraintModel.java b/AlgebraicDataflowArchitectureModel/src/models/dataConstraintModel/DataConstraintModel.java
index 2510f5a..3bdb1dd 100644
--- a/AlgebraicDataflowArchitectureModel/src/models/dataConstraintModel/DataConstraintModel.java
+++ b/AlgebraicDataflowArchitectureModel/src/models/dataConstraintModel/DataConstraintModel.java
@@ -4,6 +4,7 @@
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Stack;
@@ -472,7 +473,54 @@
 	public static final Symbol remove = new Symbol("remove", 2, Symbol.Type.PREFIX, "remove", Symbol.Type.METHOD_WITH_SIDE_EFFECT);		// Don't calculate here (Calculated in simulator).
 	public static final Symbol head = new Symbol("head", 1, Symbol.Type.PREFIX, "($x)->$x.get(0)", Symbol.Type.LAMBDA);
 	public static final Symbol tail = new Symbol("tail", 1, Symbol.Type.PREFIX, "($x)->$x.subList(1, $x.size())", Symbol.Type.LAMBDA);
-	public static final Symbol length = new Symbol("length", 1, Symbol.Type.PREFIX, "($x)->$x.size()", Symbol.Type.LAMBDA);
+	public static final Symbol length = new Symbol("length", 1, Symbol.Type.PREFIX, "($x)->$x.size()", Symbol.Type.LAMBDA, new Symbol.ICalculator() {
+		@Override
+		public Expression calculate(List<Expression> args) {
+			if (args.get(0).getClass() == Constant.class && ((Constant) args.get(0)).getSymbol().equals(nil)) {
+				return new Constant("0", typeInt);
+			}
+			if (args.get(0) instanceof Term) {
+				Term term = (Term) args.get(0);
+				Type type = term.getType();
+				if (typeList.isAncestorOf(type)) {
+					int len = 0;
+					while (term.getSymbol().equals(DataConstraintModel.append)) {
+						len++;
+						term = (Term) term.getChild(0);
+					}
+					if (term instanceof ListTerm) {
+						len += ((ListTerm) term).getChildren().size();
+						return new Constant(Integer.toString(len), typeInt);
+					}
+				} else if (typeMap.isAncestorOf(type)) {
+					HashSet<String> keySet = new HashSet<>();
+					while (term.getSymbol().equals(DataConstraintModel.insert)) {
+						if (term.getChild(1).getClass() == Constant.class) {
+							keySet.add((String) ((Constant) term.getChild(1)).getValue());
+						}
+						term = (Term) term.getChild(0);
+					}
+					if (term instanceof MapTerm) {
+						keySet.addAll(((MapTerm) term).keySet());
+						return new Constant(Integer.toString(keySet.size()), typeInt);
+					}
+				} else if (typeJson.isAncestorOf(type)) {
+					HashSet<String> keySet = new HashSet<>();
+					while (term.getSymbol().equals(DataConstraintModel.addMember)) {
+						if (term.getChild(1).getClass() == Constant.class) {
+							keySet.add((String) ((Constant) term.getChild(1)).getValue());
+						}
+						term = (Term) term.getChild(0);
+					}
+					if (term instanceof JsonTerm) {
+						keySet.addAll(((JsonTerm) term).keySet());
+						return new Constant(Integer.toString(keySet.size()), typeInt);
+					}
+				}
+			}
+			return null;
+		}
+	});
 	public static final Symbol get = new Symbol("get", 2, Symbol.Type.PREFIX, "get", Symbol.Type.METHOD, new Symbol.ICalculator() {
 		@Override
 		public Expression calculate(List<Expression> args) {