diff --git a/make.sh b/make.sh
index 80f8179..f273625 100644
--- a/make.sh
+++ b/make.sh
@@ -5,4 +5,4 @@ gcc `find src/ -name '*.c'` \
 	submodules/utilitiec/build/lib/liballocator-interface.a \
 	submodules/utilitiec/build/lib/libStringView.a \
 	submodules/utilitiec/build/lib/libScratchpad.a \
-	-lm -ggdb -Wall -o bin/flup
+	-lm -ggdb -Wall -Wextra -pedantic -o bin/flup
diff --git a/src/interpreter.c b/src/interpreter.c
index 38832bd..53d218e 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -2,18 +2,31 @@
 
 #include <stdalign.h>
 
-static int CallFrame_Create(CallFrame* self, FlupFunctionAlternative* f)
+static int CallFrame_Create(CallFrame* self, FlupFunction* self_function)
 {
 	if (DynamicArray_Create(&self->stack, sizeof(Value), 8, NULL)) {
 		return ENOMEM;
 	}
 	self->memory_pad.memory = NULL;
-	self->function = f;
-	self->instruction_pointer = f->body_token_start;
+	self->self_function = self_function;
+	self->alternative = self->self_function->alternatives;
+	self->instruction_pointer = self->alternative->condition_token_start;
+	
+	self->variables = NULL;
+	self->functions = NULL;
 
 	return EXIT_SUCCESS;
 }
 
+static void CallFrame_Destroy(CallFrame* self)
+{
+	if (self->memory_pad.memory != NULL) {
+		Scratchpad_Destroy(&self->memory_pad);
+	}
+
+	DynamicArray_Destroy(&self->stack);
+}
+
 static void* CallFrame_Reserve(CallFrame* self, size_t amount, size_t alignment)
 {
 	if (self->memory_pad.memory == NULL) {
@@ -66,6 +79,7 @@ int Interpreter_ParseFunction(Interpreter* self, CallFrame* top_frame, size_t st
 
 	Token* function_name = Interpreter_GetToken(self, top_frame->instruction_pointer);
 
+	memset(f, 0, sizeof(*f));
 	f->name = function_name->get.identifier;
 	top_frame->instruction_pointer++;
 
@@ -122,7 +136,7 @@ int Interpreter_ParseFunction(Interpreter* self, CallFrame* top_frame, size_t st
 		return EXIT_FAILURE;
 	}
 
-	FlupFunctionAlternative** fdef = &f->alternative;
+	FlupFunctionAlternative** fdef = &f->alternatives;
 	while (alternative_start->type == TOKENTYPE_PIPE) {
 		top_frame->instruction_pointer++;
 		size_t condition_start = top_frame->instruction_pointer;
@@ -134,17 +148,19 @@ int Interpreter_ParseFunction(Interpreter* self, CallFrame* top_frame, size_t st
 			if (current == NULL) {
 				return EXIT_FAILURE;
 			}
+			// TODO: Count inner colon and semicolon to allow for nested functions
 		} while (current->type != TOKENTYPE_ARROW);
-		size_t condition_end = top_frame->instruction_pointer - 1;
+		size_t condition_end = top_frame->instruction_pointer - 2;
 
-		size_t body_start = top_frame->instruction_pointer + 1;
+		size_t body_start = top_frame->instruction_pointer;
 		do {
 			current = Interpreter_ExpectToken(self, top_frame, stop_index);
 			top_frame->instruction_pointer++;
 			if (current == NULL) {
 				return EXIT_FAILURE;
 			}
-		} while (current->type != TOKENTYPE_PIPE && current->type != TOKENTYPE_COLON);
+			// TODO: Count inner colon and semicolon to allow for nested functions
+		} while (current->type != TOKENTYPE_PIPE && current->type != TOKENTYPE_SEMICOLON);
 		top_frame->instruction_pointer--;
 		size_t body_end = top_frame->instruction_pointer;
 
@@ -168,9 +184,239 @@ int Interpreter_ParseFunction(Interpreter* self, CallFrame* top_frame, size_t st
 	// one token colon
 	top_frame->instruction_pointer++;
 
+	f->next = top_frame->functions;
+	top_frame->functions = f;
+
 	return EXIT_SUCCESS;
 }
 
+FlupFunction* Interpreter_FindFunction(Interpreter* self, StringView name)
+{
+	DYNAMICARRAY_REVERSEITERATE(self->call_frames, CallFrame, frame) {
+		for (FlupFunction* function = frame->functions; function != NULL; function = function->next) {
+			if (StringView_Equal(function->name, name)) {
+				return function;
+			}
+		}
+	}
+
+	return NULL;
+}
+
+bool _are_types_compatible(enum ValueType type, StringView type_name)
+{
+	const enum ValueType type_mapping[] = {VALUETYPE_INT64, VALUETYPE_DOUBLE};
+	const char* name_mapping[] = {"int", "double"};
+
+	for (size_t i = 0; i < sizeof(name_mapping) / sizeof(name_mapping[0]); i++) {
+		if (1==1
+			&& type == type_mapping[i] 
+			&& StringView_Equal(type_name, StringView_FromString(name_mapping[i]))
+			) {
+			return true;
+		}
+	}
+
+	return false;
+}
+
+int CallFrame_DefineVariable(CallFrame* self, StringView name, Value value)
+{
+	FlupVariable* v = CallFrame_Reserve(self, sizeof(FlupVariable), alignof(FlupVariable));
+	if (v == NULL) {
+		return ENOMEM;
+	}
+
+	v->name = name;
+	v->value = value;
+
+	v->next = self->variables;
+	self->variables = v;
+
+	return EXIT_SUCCESS;
+}
+
+int Interpreter_CallFunction(Interpreter* self, CallFrame* parent_frame, Token* name)
+{
+	FlupFunction* function = Interpreter_FindFunction(self, name->get.identifier);
+	if (function == NULL) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+
+	CallFrame* new_frame;
+	DynamicArray_AppendEmpty(&self->call_frames, (void**) &new_frame);
+	parent_frame = DynamicArray_GetPointer(&self->call_frames, DynamicArray_GetLength(&self->call_frames) - 2);
+	if (new_frame == NULL) {
+		// TODO: Error message
+		return ENOMEM;
+	}
+	CallFrame_Create(new_frame, function);
+
+	size_t argument_count = 0;
+	size_t stack_size = DynamicArray_GetLength(&parent_frame->stack);
+	for (
+			ParameterDefinition* d = function->parameters;
+			d != NULL;
+			d = d->next,
+			argument_count++);
+	if (stack_size < argument_count) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+
+	ParameterDefinition* parameter_def = function->parameters;
+	for (size_t stack_index = stack_size - argument_count; stack_index != stack_size; stack_index++, parameter_def = parameter_def->next) {
+		Value* parameter_value = DynamicArray_GetPointer(&parent_frame->stack, stack_index);
+		if (! _are_types_compatible(parameter_value->type, parameter_def->type)) {
+			// TODO: Error message
+			return EXIT_FAILURE;
+		}
+
+		int define_code = CallFrame_DefineVariable(new_frame, parameter_def->name, *parameter_value);
+		if (define_code != EXIT_SUCCESS) {
+			return define_code;
+		}
+	}
+
+	for (size_t i = 0; i < argument_count; i++) {
+		DynamicArray_Remove(&parent_frame->stack, DynamicArray_GetLength(&parent_frame->stack) - 1);
+	}
+
+	return EXIT_SUCCESS;
+}
+
+FlupVariable* CallFrame_FindVariable(CallFrame* self, StringView name)
+{
+	for (FlupVariable* variable = self->variables; variable != NULL; variable = variable->next) {
+		if (StringView_Equal(variable->name, name)) {
+			return variable;
+		}
+	}
+
+	return NULL;
+}
+
+int CallFrame_StackPop(CallFrame* self, Value* dest)
+{
+	size_t stack_size = DynamicArray_GetLength(&self->stack);
+	Value* popped = DynamicArray_GetPointer(&self->stack, stack_size - 1);
+	if (popped == NULL) {
+		return ENOTFOUND;
+	}
+
+	*dest = *popped;
+	DynamicArray_Remove(&self->stack, stack_size - 1);
+
+	return EXIT_SUCCESS;
+}
+
+int CallFrame_StackPush(CallFrame* self, Value* value)
+{
+	return DynamicArray_Append(&self->stack, value);
+}
+
+bool Value_Equal(Value* v1, Value* v2)
+{
+	if (v1->type != v2->type) return false;
+	switch (v1->type) {
+		case VALUETYPE_INT64:
+			return v1->get.i64 == v2->get.i64;
+		case VALUETYPE_DOUBLE:
+			return v1->get.f64 == v2->get.f64;
+		case VALUETYPE_BOOLEAN:
+			return v1->get.boolean = v2->get.boolean;
+	}
+
+	return false;
+}
+
+int BuiltinFunction_Equality(CallFrame* top_frame)
+{
+	Value v1;
+	Value v2;
+
+	CallFrame_StackPop(top_frame, &v1);
+	if (CallFrame_StackPop(top_frame, &v2) != EXIT_SUCCESS) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+
+	Value result;
+	result.type = VALUETYPE_BOOLEAN;
+	result.get.boolean = Value_Equal(&v1, &v2);
+
+	top_frame->instruction_pointer++;
+	return CallFrame_StackPush(top_frame, &result);
+
+}
+
+int BuiltinFunction_Plus(CallFrame* top_frame)
+{
+	Value v1;
+	Value v2;
+
+	CallFrame_StackPop(top_frame, &v1);
+	if (CallFrame_StackPop(top_frame, &v2) != EXIT_SUCCESS) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+	if (v1.type != v2.type) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+
+	Value result;
+	result.type = v1.type;
+	switch (v1.type) {
+		case VALUETYPE_INT64:
+			result.get.i64 = v2.get.i64 + v1.get.i64;
+			break;
+		case VALUETYPE_DOUBLE:
+			result.get.f64 = v2.get.f64 + v1.get.f64;
+			break;
+		case VALUETYPE_BOOLEAN:
+			result.get.boolean = v2.get.boolean + v1.get.boolean;
+			break;
+        }
+
+	top_frame->instruction_pointer++;
+	return CallFrame_StackPush(top_frame, &result);
+}
+
+int BuiltinFunction_Minus(CallFrame* top_frame)
+{
+	Value v1;
+	Value v2;
+
+	CallFrame_StackPop(top_frame, &v1);
+	if (CallFrame_StackPop(top_frame, &v2) != EXIT_SUCCESS) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+	if (v1.type != v2.type) {
+		// TODO: Error message
+		return EXIT_FAILURE;
+	}
+
+	Value result;
+	result.type = v1.type;
+	switch (v1.type) {
+		case VALUETYPE_INT64:
+			result.get.i64 = v2.get.i64 - v1.get.i64;
+			break;
+		case VALUETYPE_DOUBLE:
+			result.get.f64 = v2.get.f64 - v1.get.f64;
+			break;
+		case VALUETYPE_BOOLEAN:
+			result.get.boolean = v2.get.boolean - v1.get.boolean;
+			break;
+        }
+
+	top_frame->instruction_pointer++;
+	return CallFrame_StackPush(top_frame, &result);
+}
+
 int Interpreter_ExecuteNext(Interpreter* self, size_t stop_token)
 {
 	size_t frame_count = DynamicArray_GetLength(&self->call_frames);
@@ -200,24 +446,82 @@ int Interpreter_ExecuteNext(Interpreter* self, size_t stop_token)
 		case TOKENTYPE_IDENTIFIER:
 		{
 			Token* next = Interpreter_GetToken(self, top_frame->instruction_pointer + 1);
+			FlupVariable* var;
 			if (next != NULL && next->type == TOKENTYPE_COLON) {
 				// function definition
 				return Interpreter_ParseFunction(self, top_frame, stop_token);
+			} else if ((var = CallFrame_FindVariable(top_frame, t->get.identifier)) != NULL) {
+				Value* stack_value;
+				DynamicArray_AppendEmpty(&top_frame->stack, (void**) &stack_value);
+				if (stack_value == NULL) {
+					return ENOMEM;
+				}
+				*stack_value = var->value;
+				top_frame->instruction_pointer++;
 			} else {
+				top_frame->instruction_pointer++;
+				return Interpreter_CallFunction(self, top_frame, t);
 				// regular call
 			}
+
+			break;
+		}
+
+		case TOKENTYPE_EQUALITY:
+		{
+			return BuiltinFunction_Equality(top_frame);
+		}
+
+		case TOKENTYPE_PIPE:
+		case TOKENTYPE_SEMICOLON:
+		{
+			// return from function body
+			if (StringView_Equal(top_frame->self_function->return_type, StringView_FromString("void"))) {
+				DynamicArray_Remove(&self->call_frames, frame_count - 1);
+				return EXIT_SUCCESS;
+			}
+
+			CallFrame* parent_frame = DynamicArray_GetPointer(&self->call_frames, frame_count - 2);
+			Value return_value;
+			if (CallFrame_StackPop(top_frame, &return_value)) {
+				// TODO: Error message
+				return EXIT_FAILURE;
+			}
+
+			char function_name[top_frame->self_function->name.length + 1];
+			memset(function_name, 0, sizeof(function_name));
+			StringView_Paste(function_name, top_frame->self_function->name);
+
+			if (! _are_types_compatible(return_value.type, top_frame->self_function->return_type)) {
+				// TODO: Error message
+				return EXIT_FAILURE;
+			}
+
+			CallFrame_Destroy(top_frame);
+			DynamicArray_Remove(&self->call_frames, frame_count - 1);
+			if (CallFrame_StackPush(parent_frame, &return_value)) {
+				return ENOMEM;
+			}
+
+			break;
 		}
 		case TOKENTYPE_LEFT_BRACE:
 		case TOKENTYPE_RIGHT_BRACE:
 		case TOKENTYPE_AMPERSAND:
 		case TOKENTYPE_PLUS:
+		{
+			BuiltinFunction_Plus(top_frame);
+			break;
+		}
 		case TOKENTYPE_MINUS:
+		{
+			BuiltinFunction_Minus(top_frame);
+			break;
+		}
 		case TOKENTYPE_MULTIPLY:
 		case TOKENTYPE_DIVIDE:
-		case TOKENTYPE_PIPE:
 		case TOKENTYPE_ARROW:
 		case TOKENTYPE_COLON:
-		case TOKENTYPE_EQUALITY:
 		case TOKENTYPE_INEQUALITY:
 		case TOKENTYPE_LESSTHAN:
 		case TOKENTYPE_LESSEQUAL:
@@ -228,9 +532,24 @@ int Interpreter_ExecuteNext(Interpreter* self, size_t stop_token)
 
 		case TOKENTYPE_NONE:
 		case TOKENTYPE_ERROR:
-		default:
 			return EXIT_FAILURE;
         }
+
+	return EXIT_SUCCESS;
+}
+
+bool Value_IsTruthy(Value* v)
+{
+	switch (v->type) {
+		case VALUETYPE_INT64:
+			return v->get.i64 != 0;
+		case VALUETYPE_DOUBLE:
+			return v->get.f64 != 0.0;
+		case VALUETYPE_BOOLEAN:
+			return v->get.boolean;
+	}
+
+	return false;
 }
 
 #define RPN(a,b,c) a c b
@@ -239,22 +558,50 @@ int Interpreter_RunFrame(Interpreter* self)
 	size_t frame_count = DynamicArray_GetLength(&self->call_frames);
 	CallFrame* top_frame = DynamicArray_GetPointer(&self->call_frames, frame_count - 1);
 	if (
-			   top_frame->instruction_pointer >= top_frame->function->condition_token_start 
-			&& top_frame->instruction_pointer <= top_frame->function->condition_token_end
+			   top_frame->instruction_pointer >= top_frame->alternative->condition_token_start 
+			&& top_frame->instruction_pointer <= top_frame->alternative->condition_token_end
 			) {
 		// alternative executing
-		while (top_frame->instruction_pointer <= top_frame->function->condition_token_end) {
-			Interpreter_ExecuteNext(self, top_frame->function->condition_token_end);
+		while (top_frame->instruction_pointer <= top_frame->alternative->condition_token_end) {
+			int execute_code = Interpreter_ExecuteNext(self, top_frame->alternative->condition_token_end);
+			if (execute_code != EXIT_SUCCESS) {
+				return execute_code;
+			}
 
 			if (RPN(frame_count, DynamicArray_GetLength(&self->call_frames), !=)) {
 				return EXIT_SUCCESS;
 			}
 		}
+	} else if (top_frame->instruction_pointer == top_frame->alternative->condition_token_end + 1) {
+		// alternative evaluation done
+		Value alternative_success;
+		if (CallFrame_StackPop(top_frame, &alternative_success) != EXIT_SUCCESS) {
+			// TODO: Error message
+			return EXIT_FAILURE;
+		}
 
+		if (Value_IsTruthy(&alternative_success)) {
+			// into body
+			top_frame->instruction_pointer = top_frame->alternative->body_token_start;
+		} else  {
+			if (top_frame->alternative->next == NULL) {
+				// TODO: Error message
+				return EXIT_FAILURE;
+			}
+
+			top_frame->alternative = top_frame->alternative->next;
+			top_frame->instruction_pointer = top_frame->alternative->condition_token_start;
+			// next alternative
+		}
+
+		return EXIT_SUCCESS;
 	} else {
 		// function executing
-		while (top_frame->instruction_pointer <= top_frame->function->body_token_end) {
-			Interpreter_ExecuteNext(self, top_frame->function->body_token_end);
+		while (top_frame->instruction_pointer <= top_frame->alternative->body_token_end) {
+			int execute_code = Interpreter_ExecuteNext(self, top_frame->alternative->body_token_end);
+			if (execute_code != EXIT_SUCCESS) {
+				return execute_code;
+			}
 
 			if (RPN(frame_count, DynamicArray_GetLength(&self->call_frames), !=)) {
 				return EXIT_SUCCESS;
@@ -268,15 +615,14 @@ int Interpreter_RunFrame(Interpreter* self)
 
 int Interpreter_Run(Interpreter* self)
 {
-	size_t frame_count;
+	size_t frame_count = 1;
 	do  {
-		frame_count = DynamicArray_GetLength(&self->call_frames);
-
 		if (Interpreter_RunFrame(self)) {
 			// TODO: Return the error code
 			return EXIT_FAILURE;
 		}
 
+		frame_count = DynamicArray_GetLength(&self->call_frames);
 	} while (frame_count != 0);
 	return EXIT_SUCCESS;
 }
@@ -284,18 +630,28 @@ int Interpreter_Run(Interpreter* self)
 int Interpreter_Interpret(Interpreter* self)
 {
 	CallFrame* first_frame;
-	FlupFunctionAlternative* file_function = FlupFunctionAlternative_Malloc(SIZE_MAX, SIZE_MAX, 0, DynamicArray_GetLength(self->tokens));
-	if (file_function == NULL) {
+	FlupFunctionAlternative* main_alternative = FlupFunctionAlternative_Malloc(SIZE_MAX-1, SIZE_MAX-1, 0, DynamicArray_GetLength(self->tokens));
+	FlupFunction main_function =  {
+		.name = StringView_FromString("<main>"),
+		.alternatives = main_alternative,
+		.return_type = StringView_FromString("void"),
+		.next = NULL,
+		.parameters = NULL,
+	};
+	if (main_alternative == NULL) {
 		return ENOMEM;
 	}
 
 	DynamicArray_AppendEmpty(&self->call_frames, (void**) &first_frame);
-	if (CallFrame_Create(first_frame, file_function)) {
+	if (CallFrame_Create(first_frame, &main_function)) {
 		return ENOMEM;
 	}
+	first_frame->instruction_pointer = 0;
 
 	Interpreter_Run(self);
 
+	free(main_alternative);
+
 	return EXIT_SUCCESS;
 }
 
diff --git a/src/interpreter.h b/src/interpreter.h
index 47a44da..0ec4454 100644
--- a/src/interpreter.h
+++ b/src/interpreter.h
@@ -25,17 +25,20 @@ typedef struct FlupFunction_s {
 	StringView name;
 	ParameterDefinition* parameters;
 	StringView return_type;
-	FlupFunctionAlternative* alternative;
+	FlupFunctionAlternative* alternatives;
+	struct FlupFunction_s* next;
 } FlupFunction;
 
 enum ValueType {
 	VALUETYPE_INT64,
-	VALUETYPE_DOUBLE
+	VALUETYPE_DOUBLE,
+	VALUETYPE_BOOLEAN,
 };
 
 union ValueContent {
 	int64_t i64;
 	double  f64;
+	bool    boolean;
 };
 
 typedef struct Value_s {
@@ -43,13 +46,22 @@ typedef struct Value_s {
 	union ValueContent get;
 } Value;
 
+typedef struct FlupVariable_s {
+	StringView name;
+	Value value;
+	struct FlupVariable_s* next;
+} FlupVariable;
+
 typedef struct CallFrame_s {
 	size_t instruction_pointer;
 	/* ip = condition start : start
 	 * ip = condition_end + 1 : done
 	 *
 	 */
-	FlupFunctionAlternative* function;
+	FlupFunction* self_function;
+	FlupFunctionAlternative* alternative;
+	FlupFunction* functions; // functions defined in this callframe
+	FlupVariable* variables; // variables defined in this callframe
 	DynamicArray stack; // Value
 	Scratchpad memory_pad;
 } CallFrame;
diff --git a/src/main.c b/src/main.c
index e9ebfd3..f90263d 100644
--- a/src/main.c
+++ b/src/main.c
@@ -64,9 +64,9 @@ char* load_file_string(StringView path)
 		return NULL;
 	}
 
-	size_t bytes_read = fread(buffer, 1, length + 1,  stream);
-	if (bytes_read != length) {
-		fprintf(stderr, "Fatal Error: Failed read %li bytes from script file, got only %li\n", length, bytes_read);
+	size_t objects_read = fread(buffer, length, 1, stream);
+	if (objects_read != 1) {
+		fprintf(stderr, "Fatal Error: Failed read %li bytes from script file, got only %li\n", length, objects_read);
 		free(buffer);
 		return NULL;
 	}
diff --git a/src/tokenizer.c b/src/tokenizer.c
index 1677205..4f23327 100644
--- a/src/tokenizer.c
+++ b/src/tokenizer.c
@@ -96,7 +96,7 @@ static Token _Tokenizer_IdentifierToken(StringView* source)
 
 static Token _Tokenizer_SimpleToken(StringView* source)
 {
-	const char* literal_table[] = { "{", "}", "&", ":", "+", "->", "-", "*", "/", "|", "==", "!=", "<", "<=", ">", ">=", "," };
+	const char* literal_table[] = { "{", "}", "&", ":", "+", "->", "-", "*", "/", "|", "==", "!=", "<", "<=", ">", ">=", ",", ";" };
 	const enum TokenType type_table[] = { 
 		TOKENTYPE_LEFT_BRACE,
 		TOKENTYPE_RIGHT_BRACE,
@@ -115,6 +115,7 @@ static Token _Tokenizer_SimpleToken(StringView* source)
 		TOKENTYPE_GREATERTHAN,
 		TOKENTYPE_GREATEREQUAL,
 		TOKENTYPE_COMMA,
+		TOKENTYPE_SEMICOLON,
 	};
 
 	for (size_t i = 0; i < sizeof(literal_table) / sizeof(literal_table[0]); i++) {
@@ -205,5 +206,10 @@ const char* TokenType_ToString(enum TokenType type)
 			return "TOKENTYPE_GREATEREQUAL";
 		case TOKENTYPE_COMMA:
 			return "TOKENTYPE_COMMA";
+		case TOKENTYPE_SEMICOLON:
+			return "TOKENTYPE_SEMICOLON";
+
 	}
+
+	return "INVALID";
 }
diff --git a/src/tokenizer.h b/src/tokenizer.h
index 35ae203..be20d2f 100644
--- a/src/tokenizer.h
+++ b/src/tokenizer.h
@@ -19,6 +19,7 @@ enum TokenType {
 	TOKENTYPE_PIPE,
 	TOKENTYPE_ARROW,
 	TOKENTYPE_COLON,
+	TOKENTYPE_SEMICOLON,
 	TOKENTYPE_EQUALITY,
 	TOKENTYPE_INEQUALITY,
 	TOKENTYPE_LESSTHAN,