diff --git a/.vscode/launch.json b/.vscode/launch.json
index 0597115..a8d2c36 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -9,7 +9,7 @@
             "type": "cppdbg",
             "request": "launch",
             "program": "${workspaceFolder}/src/lisp/lisp",
-            "args": ["test-closures.lisp"],
+            "args": ["test-args.lisp"],
             "stopAtEntry": false,
             "cwd": "${workspaceFolder}/src/lisp",
             "environment": [
diff --git a/lib/lisp/std/std.lisp b/lib/lisp/std/std.lisp
index 096b885..388637b 100644
--- a/lib/lisp/std/std.lisp
+++ b/lib/lisp/std/std.lisp
@@ -1,5 +1,7 @@
 ;;;; std.lisp -- Lisp standard library
 
+;; Boring utilities
+
 (defun caar (val)
   (car (car val)))
 
@@ -15,12 +17,32 @@
 (defun caddar (val)
   (car (cdr (cdr (car val)))))
 
+;; /Boring utilitites
+
+
+
 ;; Instead of a function this is a macro for a slight performance increase
 (defmacro not (val)
   (nilp val))
 
 ;; TODO: make tail recursive (for this `flet` would be nice)
 (defun length (list)
+  "Returns the length of `list`, or `nil` if it is not a list"
   (if (nilp list)
     0
     (+ 1 (length (cdr list)))))
+
+(defmacro when (cond & body)
+  "Evaluate `body` when `cond` is truthy.
+When `cond` is truthy, evaluates `body` in order, finally evaluating to
+the final item."
+  (list 'if cond
+    (cons 'progn body)))
+
+(defmacro unless (cond & body)
+  "Evaluate `body` unless `cond` is truthy.
+When `cond` is nil, evaluates `body` in order, finally evaluating to the
+final item."
+  (list 'if cond
+    nil
+    (cons 'progn body)))
diff --git a/src/lisp/Jmk b/src/lisp/Jmk
index 628a33d..3f50e85 100644
--- a/src/lisp/Jmk
+++ b/src/lisp/Jmk
@@ -13,7 +13,7 @@
 archetype(c)
 archetype(asm)
 
-CFLAGS += -Ivendor/luajit/dynasm
+CFLAGS += -Ivendor/luajit/dynasm -Werror
 ASMFLAGS += -felf -Fdwarf
 
 OBJECTS = main.o \
diff --git a/src/lisp/compiler.dasc b/src/lisp/compiler.dasc
index 2807967..2a60640 100644
--- a/src/lisp/compiler.dasc
+++ b/src/lisp/compiler.dasc
@@ -126,7 +126,8 @@
 
 struct dasm_State *compile_function(value_t args, enum namespace namespace,
                                     struct environment *env, struct local *local_out,
-                                    struct local *local_parent, int *nargs, char *name)
+                                    struct local *local_parent, struct args **args_out,
+									char *name)
 {
 	dasm_State *d;
 	dasm_State **Dst = &d;
@@ -160,17 +161,13 @@
 	value_t arglist = car(args);
 	value_t body = cdr(args);
 
-	local.num_args = length(arglist);
+	// This will add the arguments to local too.
+	struct args *ar = list_to_args(env, arglist, &local);
+	local.args = ar;
 
-	value_t a = arglist;
-	for (int i = 0; !nilp(a); a = cdr(a), i++)
+	if (!ar)
 	{
-		if (!symbolp(car(a)))
-		{
-			err("defun argument must be a symbol");
-		}
-
-		add_variable(&local, V_ARGUMENT, (char *)(car(a) ^ SYMBOL_TAG), i);
+		err("Malformed args list");
 	}
 
 	for (value_t body_ = body; !nilp(body_); body_ = cdr(body_))
@@ -193,8 +190,8 @@
 	if (local_out)
 		*local_out = local;
 
-	if (nargs)
-		*nargs = length(arglist);
+	if (args_out)
+		*args_out = ar;
 
 	return d;
 }
@@ -215,13 +212,13 @@
 			namespace = NS_MACRO;
 
 		struct local local;
-		int nargs;
+		struct args *a;
 		char *name = (char *)(car(args) ^ SYMBOL_TAG);
 
-		dasm_State *d = compile_function(cdr(args), namespace, env, &local, NULL, &nargs, name);
+		dasm_State *d = compile_function(cdr(args), namespace, env, &local, NULL, &a, name);
 
 		add_function(env, name, link_program(&d),
-		             nargs, namespace);
+		             a, namespace);
 
 		dasm_free(&d);
 		del_local(&local);
@@ -483,7 +480,7 @@
 			}
 
 			struct function *f = find_function(env, (char *)(car(args) ^ SYMBOL_TAG));
-			value_t closure = create_closure(f->code_ptr, f->nargs, 0);
+			value_t closure = create_closure(f->code_ptr, f->args, 0);
 
 			| mov eax, (closure);
 		}
@@ -563,12 +560,12 @@
 			struct function *func = find_function(env, name);
 			
 			bool is_recursive = false;
-			int nargs_needed = 0;
+			struct args *nargs_needed = NULL;
 
 			if (symstreq(fsym, local->current_function_name))
 			{
 				is_recursive = true;
-				nargs_needed = local->num_args;
+				nargs_needed = local->args;
 			}
 			else
 			{
@@ -578,24 +575,38 @@
 					err("Function undefined");
 				}
 
-				nargs_needed = func->nargs;
+				nargs_needed = func->args;
 			}
 
-			if (nargs != nargs_needed)
+			if (!are_args_acceptable(nargs_needed, nargs))
 			{
 				fprintf(stderr, "Function call: %s at %s:%d, want %d args but given %d\n",
-					name, cons_file(val), cons_line(val), nargs_needed, nargs);
+					name, cons_file(val), cons_line(val), nargs_needed->num_required, nargs);
 				err("wrong number of args");
 			}
 
 			if (is_recursive || func->namespace == NS_FUNCTION)
 			{
-				for (int i = length(args) - 1; i >= 0; i--)
+				int nargs = length(args);
+
+				if (nargs <= nargs_needed->num_required)
+				{
+					// Push the variadic list (nil)
+					| push (nil);
+				}
+
+				for (int i = nargs_needed->num_optional - 1; i >= nargs - nargs_needed->num_required; i--)
+				{
+					// Push the default optional values
+					| push (nargs_needed->optional_arguments[i].value);
+				}
+
+				for (int i = nargs - 1; i >= 0; i--)
 				{
 					compile_expression(env, local, elt(args, i), Dst);
 					| push eax;
 				}
-				
+
 				if (is_recursive)
 				{
 					| call ->function_start;
@@ -687,7 +698,8 @@
 
 			if (v)
 			{
-				// We found this in a parent scope, add it as a V_FREE variable to skip the search.
+				// We found this in a parent scope, add it as a V_FREE variable
+				// to skip the search.
 				v = add_variable(local, V_FREE, name, local->num_closure_slots++);
 			}
 		}
@@ -697,12 +709,188 @@
 
 extern value_t _call_list(void *addr, value_t list, value_t *edi);
 
-value_t call_list(struct function *func, value_t list)
+value_t call_list_args(void *code_ptr, struct args *args, value_t list, void *data)
 {
-	return _call_list(func->code_ptr, list, NULL);
+	list = deep_copy(list);
+	int nargs = length(list);
+
+	value_t *val = NULL;
+
+	for (value_t i = list; !nilp(i); i = cdr(i))
+	{
+		val = cdrref(i);
+	}
+
+	int total_required = args->num_required + args->num_optional;
+
+	if (nargs > total_required)
+	{
+		// Take the remainder of the list and put it as the last item in the
+		// list.
+		value_t trailing = cxdr(list, total_required);
+		value_t last_item = cons(trailing, nil);
+
+		*cxdrref(&list, total_required) = last_item;
+	}
+	else if (nargs < total_required)
+	{
+		for (int i = nargs - args->num_required; i < args->num_optional; i++)
+		{
+			// Append the i-th defualt argument
+			value_t appended = cons(args->optional_arguments[i].value, nil);
+			*val = appended;
+			val = cdrref(appended);
+		}
+	}
+
+	// We want to call this if we pass the correct # of arguments or less, just
+	// not if we have already passed varargs. Appends a nil argument.
+	if (nargs <= total_required)
+	{
+		// Enough real arguments but no variadic arguments. Pass a nil list.
+		*val = cons(nil, nil);
+	}
+
+	return _call_list(code_ptr, list, data);
+}
+
+value_t call_list(struct function *fun, value_t list)
+{
+	return call_list_args(fun->code_ptr, fun->args, list, NULL);
 }
 
 value_t call_list_closure(struct closure *c, value_t list)
 {
-	return _call_list(c->function, list, c->data);
+	return call_list_args(c->function, c->args, list, c->data);
+}
+
+struct args *new_args()
+{
+	struct args *a = malloc(sizeof(struct args));
+	a->num_optional = 0;
+	a->num_required = 0;
+	a->variadic = false;
+
+	return a;
+}
+
+struct args *add_optional_arg(struct args *args, value_t name,
+                              value_t value)
+{
+	int i = args->num_optional++;
+	args = realloc(args, sizeof(struct args) +
+		sizeof(struct optional_argument) * args->num_optional);
+
+	args->optional_arguments[i] = (struct optional_argument)
+	{
+		.value = value ,
+		.name = name,
+	};
+
+	return args;
+}
+
+bool are_args_acceptable(struct args *args, int number)
+{
+	if (args->variadic)
+	{
+		return number >= args->num_required;
+	}
+	else
+	{
+		return number >= args->num_required &&
+			number <= args->num_required + args->num_optional;
+	}
+}
+
+struct args *list_to_args(struct environment *env, value_t list, struct local *local)
+{
+	struct args *args = new_args();
+
+	bool in_optional = false;
+
+	for (value_t i = list; !nilp(i); i = cdr(i))
+	{
+		value_t val = car(i);
+		if (symbolp(val))
+		{
+			if (!args->variadic && symstreq(val, "&"))
+			{
+				i = cdr(i);
+				value_t name = car(i);
+
+				if (!symbolp(name))
+				{
+					err("You must provide a symbol after & in an argument list to bind the\n"
+						"variadic arguments to.");
+				}
+
+				args->variadic = true;
+
+				add_variable(local, V_ARGUMENT, (char *)(name ^ SYMBOL_TAG),
+					args->num_optional + args->num_required);
+
+				continue;
+			}
+
+			if (!in_optional)
+			{
+				add_variable(local, V_ARGUMENT, (char *)(val ^ SYMBOL_TAG), args->num_required++);
+			}
+			else
+			{
+				char *name = (char *)(val ^ SYMBOL_TAG);
+				if (name[0] == '&')
+				{
+					err("Non-optional argument following optional arguments starts with a &\n"
+						"did you mean to declare a variadic argument? If so leave a space\n"
+						"between the & and name.");
+				}
+				else
+				{
+					err("Cannot define a non-optional argument after an optional one.");
+				}
+			}
+		}
+		else if (listp(val))
+		{
+			in_optional = true;
+			int len = length(val);
+
+			if (len != 2)
+			{
+				err("A list defining an optional value must be structured like (name expr)\n"
+					"with exactly two arguments.");
+			}
+
+			value_t name = car(val);
+			value_t expr = car(cdr(val));
+
+			value_t function = cons(nil, cons(expr, nil));
+
+			dasm_State *d = compile_function(function, NS_ANONYMOUS, env, NULL, NULL, NULL, NULL);
+
+			// TODO: GC stack top!
+			value_t (*compiled)() = link_program(&d);
+
+			value_t value = compiled();
+			args = add_optional_arg(args, name, value);
+
+			add_variable(local, V_ARGUMENT, (char *)(name ^ SYMBOL_TAG), args->num_required + args->num_optional - 1);
+		}
+	}
+
+	return args;
+}
+
+void display_args(struct args *args)
+{
+	printf("Args object taking %d require arguments and %d optionals:\n",
+		args->num_required, args->num_optional);
+
+	for (int i = 0; i < args->num_optional; i++)
+	{
+		printf("   %d\t%s\n", i, (char *)(args->optional_arguments[i].name ^ SYMBOL_TAG));
+		printval(args->optional_arguments[i].value, 2);
+	}
 }
diff --git a/src/lisp/compiler.h b/src/lisp/compiler.h
index 62b43c0..e03d93a 100644
--- a/src/lisp/compiler.h
+++ b/src/lisp/compiler.h
@@ -15,10 +15,20 @@
 	NS_ANONYMOUS,
 };
 
+
+struct args *new_args();
+struct args *add_optional_arg(struct args *args, value_t name,
+                              value_t expression);
+
+/**
+ * @returns if `number` is an acceptable number of arguments for `args`.
+ */
+bool are_args_acceptable(struct args *args, int number);
+
 struct function
 {
 	char *name;
-	int nargs; // number of arguments
+	struct args *args;
 	enum namespace namespace;
 
 	union
@@ -72,7 +82,8 @@
 	/// for a lambda.
 	char *current_function_name;
 
-	int num_vars, num_args;
+	int num_vars;
+	struct args *args;
 	/// Most recently defined variable
 	struct variable *first;
 	int npc;
@@ -86,6 +97,19 @@
 	int num_closure_slots;
 };
 
+/**
+ * Parse a list of arguments to an args object and add them as V_ARGUMENT
+ * variables to `local`. The list should be in the same format as is accepted by
+ * `defun`, `defmacro`, `lambda`, etc.
+ * @returns NULL if the list is malformed.
+ */
+struct args *list_to_args(struct environment *env, value_t list, struct local *local);
+
+/**
+ * Print out `args` to stdout. Useful for debugging.
+ */
+void display_args(struct args *args);
+
 void compile_expression(struct environment *env, struct local *local,
                         value_t val, dasm_State **Dst);
 
@@ -99,14 +123,14 @@
  * should since you need to free the stack slot allocation map).
  * @param local_parent Parent local environment, only needed for closures. NULL
  * if no parent.
- * @param nargs The number of arguments for this function will be returned here.
- * NULL if you don't care about it.
+ * @param args An object representing the arguments this function accepts will
+ * be returned here. Set this to NULL if you don't care.
  * @returns The compiled function state. You should probably give this to
  * `add_function` or something similar.
  */
 struct dasm_State *compile_function(value_t args, enum namespace namespace,
                                     struct environment *env, struct local *local_out,
-                                    struct local *local_parent, int *nargs, char *name);
+                                    struct local *local_parent, struct args **ar, char *name);
 
 void compile_variable(struct variable *v, dasm_State *Dst);
 
diff --git a/src/lisp/istream.c b/src/lisp/istream.c
index 957b2b5..8ac275a 100644
--- a/src/lisp/istream.c
+++ b/src/lisp/istream.c
@@ -130,6 +130,7 @@
 	int next;
 	bool has_next;
 	int line;
+	char *path;
 };
 
 int fistream_peek(struct istream *is)
@@ -192,7 +193,7 @@
 	struct fistream_private *p = is->data;
 
 	*line = p->line;
-	*name = "<FILE *>";
+	*name = p->path;
 }
 
 struct istream *new_fistream(char *path, bool binary)
@@ -213,6 +214,7 @@
 	p->has_next = false;
 	p->file = fp;
 	p->line = 1;
+	p->path = path;
 
 	is->data = p;
 	is->get = fistream_get;
diff --git a/src/lisp/lib/std.c b/src/lisp/lib/std.c
index c7efca0..3681e47 100644
--- a/src/lisp/lib/std.c
+++ b/src/lisp/lib/std.c
@@ -52,34 +52,57 @@
 	return call_list_closure((struct closure *)(func ^ CLOSURE_TAG), args);
 }
 
-void add_function(struct environment *env, char *name, void *func, int nargs, enum namespace ns)
+value_t l_nilp(value_t val)
+{
+	return nilp(val) ? t : nil;
+}
+
+void add_function(struct environment *env, char *name, void *func, struct args *args, enum namespace ns)
 {
 	struct function *last, *new = malloc(sizeof(struct function));
 
 	last = env->first;
 	new->prev = last;
 	new->name = name;
-	new->nargs = nargs;
+	new->args = args;
 	new->code_ptr = func;
 	new->namespace = ns;
 
 	env->first = new;
 }
 
+void add_c_function(struct environment *env, char *name, void *func, int nargs)
+{
+	struct args *args = new_args();
+	args->num_required = nargs;
+
+	add_function(env, name, func, args, NS_FUNCTION);
+}
+
+value_t l_elt(value_t seq, value_t i)
+{
+	if (!listp(seq) || !integerp(i))
+		return nil;
+	
+	return elt(seq, i >> 2);
+}
+
 void load_std(struct environment *env)
 {
-	add_function(env, "+", l_plus, 2, NS_FUNCTION);
-	add_function(env, "-", l_minus, 2, NS_FUNCTION);
-	add_function(env, "*", l_times, 2, NS_FUNCTION);
-	add_function(env, "/", l_divide, 2, NS_FUNCTION);
+	add_c_function(env, "+", l_plus, 2);
+	add_c_function(env, "-", l_minus, 2);
+	add_c_function(env, "*", l_times, 2);
+	add_c_function(env, "/", l_divide, 2);
 
-	add_function(env, "car", car, 1, NS_FUNCTION);
-	add_function(env, "cdr", cdr, 1, NS_FUNCTION);
-	add_function(env, "cons", cons, 2, NS_FUNCTION);
+	add_c_function(env, "car", car, 1);
+	add_c_function(env, "cdr", cdr, 1);
+	add_c_function(env, "cons", cons, 2);
 
-	add_function(env, "print", l_printval, 1, NS_FUNCTION);
+	add_c_function(env, "print", l_printval, 1);
+	add_c_function(env, "apply", l_apply, 2);
 
-	add_function(env, "apply", l_apply, 2, NS_FUNCTION);
+	add_c_function(env, "nilp", l_nilp, 1);
+	add_c_function(env, "elt", l_elt, 2);
 
 	if (!load_library(env, "std"))
 	{
diff --git a/src/lisp/lib/std.h b/src/lisp/lib/std.h
index a117b5d..dae0ac3 100644
--- a/src/lisp/lib/std.h
+++ b/src/lisp/lib/std.h
@@ -6,6 +6,7 @@
 value_t l_plus(value_t a, value_t b);
 value_t l_printval(value_t val);
 
-void add_function(struct environment *env, char *name, void *func, int nargs, enum namespace ns);
+void add_function(struct environment *env, char *name, void *func, struct args *args, enum namespace ns);
+void add_c_function(struct environment *env, char *name, void *func, int nargs);
 void load_std(struct environment *env);
 bool load_library(struct environment *env, char *name);
diff --git a/src/lisp/lisp.c b/src/lisp/lisp.c
index c606e29..e151670 100644
--- a/src/lisp/lisp.c
+++ b/src/lisp/lisp.c
@@ -14,7 +14,7 @@
 
 unsigned char max_pool = 0, current_pool = 0;
 
-void err(const char *msg)
+__attribute__((noreturn)) void err(const char *msg)
 {
 	fprintf(stderr, "ERROR: %s\n", msg);
 	exit(1);
@@ -239,7 +239,7 @@
 	{
 		struct closure *c = (void *)(v ^ CLOSURE_TAG);
 		printf("closure %p taking %d argument(s) and capturing %d value(s)\n",
-		       c->function, c->num_args, c->num_captured);
+		       c->function, c->args->num_required, c->num_captured);
 	}
 	else
 	{
@@ -598,13 +598,13 @@
 	return c->name;
 }
 
-value_t create_closure(void *code, int nargs, int ncaptures)
+value_t create_closure(void *code, struct args *args, int ncaptures)
 {
 	struct closure_alloc *ca = malloc_aligned(sizeof(struct closure_alloc) +
 	                                          ncaptures * sizeof(value_t));
 
 	ca->closure.function = code;
-	ca->closure.num_args = nargs;
+	ca->closure.args = args;
 	ca->closure.num_captured = ncaptures;
 
 	add_this_alloc(&ca->alloc, CLOSURE_TAG);
@@ -621,3 +621,68 @@
 
 	c->data[index] = value;
 }
+
+value_t cxdr(value_t v, int index)
+{
+	if (!listp(v) || index >= length(v))
+		return nil;
+
+	for (int i = 0; i < index; i++)
+	{
+		v = cdr(v);
+	}
+
+	return v;
+}
+
+value_t *cxdrref(value_t *v, int index)
+{
+	if (!listp(*v) || index >= length(*v))
+		return NULL;
+
+	value_t *p = v;
+
+	for (int i = 0; i < index; i++)
+	{
+		p = cdrref(*p);
+	}
+
+	return p;
+}
+
+value_t deep_copy(value_t val)
+{
+	if (integerp(val) || val == nil || val == t)
+	{
+		return val;
+	}
+	else if (symbolp(val))
+	{
+		return symval((char *)(val ^ SYMBOL_TAG));
+	}
+	else if (stringp(val))
+	{
+		return strval((char *)(val ^ STRING_TAG));
+	}
+	else if (consp(val))
+	{
+		return cons(deep_copy(car(val)), deep_copy(cdr(val)));
+	}
+	else if (closurep(val))
+	{
+		struct closure *c = (void *)(val ^ CLOSURE_TAG);
+		value_t new = create_closure(c->function, c->args, c->num_captured);
+		struct closure *new_c = (void *)(new ^ CLOSURE_TAG);
+
+		for (int i = 0; i < c->num_captured; i++)
+		{
+			new_c->data[i] = deep_copy(c->data[i]);
+		}
+
+		return new;
+	}
+	else
+	{
+		err("Don't know how to deep copy this, sorry... please report this bug :)");
+	}
+}
diff --git a/src/lisp/lisp.h b/src/lisp/lisp.h
index 7c0e571..a8bea1c 100644
--- a/src/lisp/lisp.h
+++ b/src/lisp/lisp.h
@@ -39,10 +39,39 @@
 	char *name;
 };
 
+/**
+ * Represents how many arguments a function takes.
+ */
+struct args
+{
+	/// The minimum valid number of arguments
+	int num_required;
+
+	/// The number of optional values
+	int num_optional;
+
+	/// Does this function accept variadic arguments? If `true`, any arguments
+	/// after the required and optional arguments will be `cons`-ed to a list
+	/// and passed as a final argument.
+	bool variadic;
+
+	/// The default values for the optional arguments, as expressions. These
+	/// should be evaluated at the call site. They are known not to reference
+	/// anything that could clash with scope at the call site.
+	struct optional_argument
+	{
+		/// The default value of this argument
+		value_t value;
+
+		/// The name of this argument as a symbol
+		value_t name;
+	} optional_arguments[];
+};
+
 struct closure
 {
 	/// How many arguments does this closure take
-	int num_args;
+	struct args *args;
 	/// How many free variables does it capture (i.e. length of `data`)
 	int num_captured;
 	/// The function pointer itself
@@ -146,6 +175,12 @@
 value_t cdr(value_t v);
 value_t *carref(value_t v);
 value_t *cdrref(value_t v);
+/// @returns the `index`-th `cdr`
+value_t cxdr(value_t v, int index);
+/// @returns a reference to the `index`-th `cdr`
+value_t *cxdrref(value_t *v, int index);
+
+value_t deep_copy(value_t val);
 
 int cons_line(value_t val);
 char *cons_file(value_t val);
@@ -167,7 +202,7 @@
 
 bool symstreq(value_t sym, char *str);
 
-value_t create_closure(void *code, int nargs, int ncaptures);
+value_t create_closure(void *code, struct args *args, int ncaptures);
 
 /**
  * Set the `index`th capture variable of `closure`. This should really only be
diff --git a/src/lisp/test-args.lisp b/src/lisp/test-args.lisp
new file mode 100644
index 0000000..14364a8
--- /dev/null
+++ b/src/lisp/test-args.lisp
@@ -0,0 +1,7 @@
+(defun with-optional (required (optional 3))
+  (+ required optional))
+
+(defun main ()
+  (when t
+    (print (with-optional 2))
+    (print (with-optional 2 4))))