diff --git a/dub.sdl b/dub.sdl
index bfd34b7..5808c6a 100644
--- a/dub.sdl
+++ b/dub.sdl
@@ -8,6 +8,8 @@ dependency "taggedalgebraic" version="~>0.11.23"
 targetType "executable"
 buildRequirements "requireBoundsCheck" "requireContracts"
 
+versions "LoxConcatNonStrings" "LoxExtraNativeFuncs"
+
 configuration "jlox" {
 	sourcePaths "src/jlox"
 }
diff --git a/src/jlox/expr.d b/src/jlox/expr.d
index ddf4df5..e942d4d 100644
--- a/src/jlox/expr.d
+++ b/src/jlox/expr.d
@@ -14,6 +14,7 @@ abstract class Expr{
 	interface Visitor(R){
 		R visit(Assign expr);
 		R visit(Binary expr);
+		R visit(Call expr);
 		R visit(Grouping expr);
 		R visit(Literal expr);
 		R visit(Logical expr);
@@ -40,6 +41,12 @@ abstract class Expr{
 		Expr right;
 		mixin defCtorAndAccept;
 	}
+	static class Call : typeof(this){
+		Expr callee;
+		Token paren;
+		Expr[] arguments;
+		mixin defCtorAndAccept;
+	}
 	static class Grouping : typeof(this){
 		Expr expression;
 		mixin defCtorAndAccept;
diff --git a/src/jlox/interpreter.d b/src/jlox/interpreter.d
index 1fde9e2..fd61716 100644
--- a/src/jlox/interpreter.d
+++ b/src/jlox/interpreter.d
@@ -2,11 +2,11 @@ module jlox.interpreter;
 
 import std.conv;
 import std.stdio;
+import std.algorithm;
+import std.array;
 import std.format : format;
 import std.functional : ctEval;
 
-import taggedalgebraic;
-
 import jlox.expr;
 import jlox.stmt;
 import jlox.token;
@@ -14,6 +14,7 @@ import jlox.tokentype;
 import jlox.token : TValue;
 import jlox.main;
 import jlox.environment;
+import jlox.loxfunction;
 
 class RuntimeError : Exception{
 	const Token token;
@@ -22,9 +23,38 @@ class RuntimeError : Exception{
 		this.token = token;
 	}
 }
+class Return : Exception{
+	const TValue value;
+	this(TValue value){
+		super(null);
+		this.value = value;
+	}
+}
 
 class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
-	private Environment environment = new Environment();
+	Environment globals = new Environment();
+	private Environment environment;
+	this(){
+		environment = globals;
+
+		import std.datetime.stopwatch;
+		auto sw = StopWatch(AutoStart.yes);
+		globals.define("clock", TValue.cal(new class LoxCallable{
+			int arity() => 0;
+			TValue call(Interpreter interpreter, TValue[] arguments) => TValue.dbl(sw.peek.total!"usecs" / (1000.0 * 1000.0));
+		}));
+
+		version(LoxExtraNativeFuncs){
+			globals.define("sleep", TValue.cal(new class LoxCallable{
+				int arity() => 1;
+				import core.thread.osthread;
+				TValue call(Interpreter interpreter, TValue[] arguments){
+					Thread.sleep(dur!"usecs"(cast(long)(arguments[0].dblValue * 1000 * 1000)));
+					return TValue.nil(tvalueNil);
+				}
+			}));
+		}
+	}
 	void interpret(Stmt[] statements){ 
 		try {
 			foreach(statement; statements)
@@ -33,10 +63,10 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 			Lox.runtimeError(error);
 		}
 	}
-	private void execute(Stmt stmt){
+	package void execute(Stmt stmt){
 		stmt.accept(this);
 	}
-	private void executeBlock(Stmt[] statements, Environment environment){
+	package void executeBlock(Stmt[] statements, Environment environment){
 		Environment previous = this.environment;
 		try {
 			this.environment = environment;
@@ -75,6 +105,10 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 	void visit(Stmt.Expression stmt){
 		evaluate(stmt.expression);
 	}
+	void visit(Stmt.Function stmt){
+		LoxFunction func = new LoxFunction(stmt);
+		environment.define(stmt.name.lexeme, TValue.cal(func));
+	}
 	void visit(Stmt.If stmt){
 		if(isTruthy(evaluate(stmt.condition)))
 			execute(stmt.thenBranch);
@@ -85,8 +119,12 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 		TValue value = evaluate(stmt.expression);
 		writeln(tvalueToString(value));
 	}
+	void visit(Stmt.Return stmt){
+		TValue value = stmt.value !is null ? evaluate(stmt.value) : TValue.nil(tvalueNil);
+		throw new Return(value);
+	}
 	void visit(Stmt.Var stmt){
-		environment.define(stmt.name.lexeme, stmt.initialiser is null ? TValue.nil(0) : evaluate(stmt.initialiser));
+		environment.define(stmt.name.lexeme, stmt.initialiser is null ? TValue.nil(tvalueNil) : evaluate(stmt.initialiser));
 	}
 	void visit(Stmt.While stmt){
 		while(isTruthy(evaluate(stmt.condition)))
@@ -126,28 +164,30 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 		TValue left = evaluate(expr.left);
 		TValue right = evaluate(expr.right); 
 		static string m(TokenType t, string op, string v, string vv){
-			return q{case %s: return TValue.%s( left.%s %s right.%s );}.format(t, v, vv, op, vv);
-		}
-		with(TokenType) switch(expr.operator.type){
-			static foreach(t, op; [ MINUS: "-", SLASH: "/", STAR: "*" ]){
+			return q{case %s:
 				checkNumberOperand(expr.operator, left);
 				checkNumberOperand(expr.operator, right);
+				return TValue.%s( left.%s %s right.%s );
+			}.format(t, v, vv, op, vv);
+		}
+		with(TokenType) switch(expr.operator.type){
+			static foreach(t, op; [ MINUS: "-", SLASH: "/", STAR: "*" ])
 				mixin(ctEval!(m(t, op, "dbl", "dblValue")));
-			}
+
 			case PLUS:
 				if(left.isDbl && right.isDbl)
 					return TValue.dbl(left.dblValue + right.dblValue);
 				else if(left.isStr && right.isStr)
 					return TValue.str(left.strValue ~ right.strValue);
+				version(LoxConcatNonStrings){
+					if(left.isStr || right.isStr)
+						return TValue.str(tvalueToString(left) ~ tvalueToString(right));
+				}
 				checkNumberOperand(expr.operator, left);
 				checkNumberOperand(expr.operator, right);
-				assert(0);
 
-			static foreach(t, op; [ GREATER: ">", GREATER_EQUAL: ">=", LESS: "<", LESS_EQUAL: "<=" ]){
-				checkNumberOperand(expr.operator, left);
-				checkNumberOperand(expr.operator, right);
+			static foreach(t, op; [ GREATER: ">", GREATER_EQUAL: ">=", LESS: "<", LESS_EQUAL: "<=" ])
 				mixin(ctEval!(m(t, op, "bln", "dblValue")));
-			}
 
 			case BANG_EQUAL:
 				return TValue.bln(!isEqual(left, right));
@@ -157,6 +197,16 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 				assert(0);
 		}
 	}
+	TValue visit(Expr.Call expr){
+		TValue callee = evaluate(expr.callee);
+		if(!callee.isCal)
+			throw new RuntimeError(expr.paren, "Can only call functions and classes.");
+		auto arguments = expr.arguments.map!(a => evaluate(a));
+		LoxCallable func = callee.calValue;
+		if(arguments.length != func.arity())
+			throw new RuntimeError(expr.paren, "Expected " ~ func.arity().to!string ~ " arguments but got " ~ arguments.length.to!string ~ ".");
+		return func.call(this, arguments.array);
+	}
 	TValue visit(Expr.Variable expr){
 		return environment.get(expr.name);
 	}
diff --git a/src/jlox/loxfunction.d b/src/jlox/loxfunction.d
new file mode 100644
index 0000000..39a91ff
--- /dev/null
+++ b/src/jlox/loxfunction.d
@@ -0,0 +1,28 @@
+module jlox.loxfunction;
+
+import std.conv;
+
+import jlox.token;
+import jlox.stmt;
+import jlox.interpreter;
+import jlox.environment;
+import jlox.util;
+
+class LoxFunction : LoxCallable{
+	private Stmt.Function declaration;
+	mixin defaultCtor;
+	
+	int arity() => declaration.params.length.to!int;
+	TValue call(Interpreter interpreter, TValue[] arguments){
+		Environment environment = new Environment(interpreter.globals);
+		foreach(i; 0 .. declaration.params.length)
+			environment.define(declaration.params[i].lexeme, arguments[i]);
+		try{
+			interpreter.executeBlock(declaration.body, environment);
+		} catch(Return returnValue){
+			return returnValue.value;
+		}
+		return TValue.nil(tvalueNil);
+	}
+}
+
diff --git a/src/jlox/parser.d b/src/jlox/parser.d
index 5cc5972..4f88207 100644
--- a/src/jlox/parser.d
+++ b/src/jlox/parser.d
@@ -8,6 +8,7 @@ import jlox.util;
 import jlox.expr;
 import jlox.main;
 import jlox.stmt;
+import jlox.loxfunction;
 
 class Parser{
 	private Token[] tokens;
@@ -84,11 +85,35 @@ class Parser{
 		consume(TokenType.SEMICOLON, "Expect ';' after value.");
 		return new Stmt.Print(value);
 	}
+	private Stmt returnStatement(){
+		Token keyword = previous();
+		Expr value;
+		if(!check(TokenType.SEMICOLON))
+			value = expression();
+		consume(TokenType.SEMICOLON, "Expect ';' after return value.");
+		return new Stmt.Return(keyword, value);
+	}
 	private Stmt expressionStatement(){
 		Expr expr = expression();
 		consume(TokenType.SEMICOLON, "Expect ';' after expression.");
 		return new Stmt.Expression(expr);
 	}
+	private Stmt.Function fun(string kind){
+		Token name = consume(TokenType.IDENTIFIER, "Expect " ~ kind ~ " name.");
+		consume(TokenType.LEFT_PAREN, "Expect '(' after " ~ kind ~ " name.");
+		Token[] parameters;
+		if(!check(TokenType.RIGHT_PAREN)){
+			do{
+				if(parameters.length >= 255)
+					error(peek(), "Can't have more than 255 parameters.");
+				parameters ~= consume(TokenType.IDENTIFIER, "Expect parameter name.");
+			} while(match(TokenType.COMMA));
+		}
+		consume(TokenType.RIGHT_PAREN, "Expect ')' after parameters.");
+		consume(TokenType.LEFT_BRACE, "Expect '{' before " ~ kind ~ " body.");
+		Stmt[] body = block();
+		return new Stmt.Function(name, parameters, body);
+	}
 	private Stmt ifStatement(){
 		consume(TokenType.LEFT_PAREN, "Expect '(' after 'if'.");
 		Expr condition = expression();
@@ -142,6 +167,8 @@ class Parser{
 			return whileStatement();
 		if(match(TokenType.PRINT))
 			return printStatement();
+		if(match(TokenType.RETURN))
+			return returnStatement();
 		if(match(TokenType.LEFT_BRACE))
 			return new Stmt.Block(block());
 		return expressionStatement();
@@ -227,7 +254,29 @@ class Parser{
 			Expr right = unary();
 			return new Expr.Unary(operator, right);
 		}
-		return primary();
+		return call();
+	}
+	private Expr call(){
+		Expr finishCall(Expr callee){
+			Expr[] arguments;
+			if(!check(TokenType.RIGHT_PAREN)){
+				do {
+					if(arguments.length >= 255)
+						error(peek(), "Can't have more than 255 arguments.");
+					arguments ~= expression();
+				} while(match(TokenType.COMMA));
+			}
+			Token paren = consume(TokenType.RIGHT_PAREN, "Expect ')' after arguments.");
+			return new Expr.Call(callee, paren, arguments);
+		}
+		Expr expr = primary();
+		while(true){ 
+			if(match(TokenType.LEFT_PAREN))
+				expr = finishCall(expr);
+			else
+				break;
+		}
+		return expr;
 	}
 	private Expr primary(){
 		if(match(TokenType.IDENTIFIER))
@@ -237,7 +286,7 @@ class Parser{
 		if(match(TokenType.TRUE))
 			return new Expr.Literal(TValue.bln(true));
 		if(match(TokenType.NIL))
-			return new Expr.Literal(TValue.nil(0));
+			return new Expr.Literal(tvalueNil);
 		if(match(TokenType.NUMBER, TokenType.STRING))
 			return new Expr.Literal(previous().literal);
 		if(match(TokenType.LEFT_PAREN)){
@@ -257,6 +306,8 @@ class Parser{
 	}
 	private Stmt declaration(){
 		try {
+			if(match(TokenType.FUN))
+				return fun("function");
 			if(match(TokenType.VAR))
 				return varDeclaration();
 			return statement();
diff --git a/src/jlox/scanner.d b/src/jlox/scanner.d
index bcb7740..5d0a412 100644
--- a/src/jlox/scanner.d
+++ b/src/jlox/scanner.d
@@ -30,7 +30,7 @@ class Scanner {
 		current++;
 		return true;
 	}
-	private void addToken(TokenType type, TValue literal = TValue.nil(0)){
+	private void addToken(TokenType type, TValue literal = TValue.nil(tvalueNil)){
 		string text = source[start .. current];
 		tokens ~= new Token(type, text, literal, line);
 	}
diff --git a/src/jlox/stmt.d b/src/jlox/stmt.d
index f088a41..8310bd6 100644
--- a/src/jlox/stmt.d
+++ b/src/jlox/stmt.d
@@ -13,8 +13,10 @@ abstract class Stmt{
 	interface Visitor(R){
 		R visit(Block expr);
 		R visit(Expression expr);
+		R visit(Function expr);
 		R visit(If expr);
 		R visit(Print expr);
+		R visit(Return expr);
 		R visit(Var expr);
 		R visit(While expr);
 	}
@@ -35,6 +37,12 @@ abstract class Stmt{
 		Expr expression;
 		mixin defCtorAndAccept;
 	}
+	static class Function : typeof(this){
+		Token name;
+		Token[] params;
+		Stmt[] body;
+		mixin defCtorAndAccept;
+	}
 	static class If : typeof(this){
 		Expr condition;
 		Stmt thenBranch;
@@ -45,6 +53,11 @@ abstract class Stmt{
 		Expr expression;
 		mixin defCtorAndAccept;
 	}
+	static class Return : typeof(this){
+		Token keyword;
+		Expr value;
+		mixin defCtorAndAccept;
+	}
 	static class Var : typeof(this){
 		Token name;
 		Expr initialiser;
diff --git a/src/jlox/token.d b/src/jlox/token.d
index 83418a8..fb2c117 100644
--- a/src/jlox/token.d
+++ b/src/jlox/token.d
@@ -5,14 +5,22 @@ import std.conv;
 import taggedalgebraic;
 
 import jlox.tokentype;
+import jlox.interpreter;
 
+interface LoxCallable{
+	int arity();
+	TValue call(Interpreter interpreter, TValue[] arguments);
+}
 private struct Value{
 	string str;
 	double dbl;
 	bool bln;
-	bool nil = false;
+	LoxCallable cal;
+	struct Nil{}
+	Nil nil;
 }
 alias TValue = TaggedUnion!Value;
+immutable tvalueNil = Value.Nil();
 string tvalueToString(TValue val){
 	final switch(val.kind){
 		case TValue.Kind.str:
@@ -21,6 +29,8 @@ string tvalueToString(TValue val){
 			return val.dblValue.to!string;
 		case TValue.Kind.bln:
 			return val.blnValue ? "true" : "false";
+		case TValue.Kind.cal:
+			return "<function>";
 		case TValue.Kind.nil:
 			return "nil";
 	}
diff --git a/test/all.d b/test/all.d
index 14ca0b7..d22369c 100755
--- a/test/all.d
+++ b/test/all.d
@@ -4,17 +4,18 @@ import std.process;
 import std.conv;
 
 void main(){
-	string fib21(){
+	string fib(uint n){
 		string r = "";
 		double a = 0;
 		double temp;
-		for(double b = 1; a < 10000; b = temp + b){
+		for(double b = 1; a <= n; b = temp + b){
 			r ~= a.to!string ~ "\n";
 			temp = a;
 			a = b;
 		}
 		return r;
 	}
-	assert([ "./lox", "test/fib21.lox" ].execute.output == fib21());
+	assert([ "./lox", "test/fib21.lox" ].execute.output == fib(6765));
+	assert([ "./lox", "test/fib10.lox" ].execute.output == fib(34));
 }
 
diff --git a/test/fib10.lox b/test/fib10.lox
new file mode 100644
index 0000000..a991ddd
--- /dev/null
+++ b/test/fib10.lox
@@ -0,0 +1,11 @@
+
+fun fib(n){
+	if(n <= 1)
+		return n;
+	return fib(n - 2) + fib(n - 1);
+}
+
+for(var i = 0; i < 10; i = i + 1){
+	print fib(i);
+}
+