diff --git a/flamingo/flamingo.h b/flamingo/flamingo.h
index 383436b..4568d7c 100644
--- a/flamingo/flamingo.h
+++ b/flamingo/flamingo.h
@@ -12,7 +12,7 @@ typedef struct flamingo_val_t flamingo_val_t;
 typedef struct flamingo_var_t flamingo_var_t;
 typedef struct flamingo_scope_t flamingo_scope_t;
 
-typedef int (*flamingo_external_fn_cb_t)(flamingo_t* flamingo, size_t name_size, char* name, void* data);
+typedef int (*flamingo_external_fn_cb_t)(flamingo_t* flamingo, size_t name_size, char* name, void* data, size_t arg_count, flamingo_val_t** args, flamingo_val_t** rv);
 
 typedef enum {
 	FLAMINGO_VAL_KIND_NONE,
@@ -138,3 +138,9 @@ int flamingo_inherit_scope_stack(flamingo_t* flamingo, size_t stack_size, flamin
 int flamingo_run(flamingo_t* flamingo);
 
 flamingo_var_t* flamingo_scope_find_var(flamingo_t* flamingo, char const* key, size_t key_size);
+
+flamingo_val_t* flamingo_val_make_none(void);
+flamingo_val_t* flamingo_val_make_int(int64_t integer);
+flamingo_val_t* flamingo_val_make_str(size_t size, char* str);
+flamingo_val_t* flamingo_val_make_cstr(char* str);
+flamingo_val_t* flamingo_val_make_bool(bool boolean);
diff --git a/flamingo/grammar/call.h b/flamingo/grammar/call.h
index 7496f76..7449bb1 100644
--- a/flamingo/grammar/call.h
+++ b/flamingo/grammar/call.h
@@ -183,9 +183,11 @@ static int parse_call(flamingo_t* flamingo, TSNode node, flamingo_val_t** val) {
 			return error(flamingo, "cannot call external function without a external function callback being set");
 		}
 
-		// TODO Arguments, return value.
+		size_t arg_count = 0;
+		flamingo_val_t** args = NULL;
+		assert(flamingo->cur_fn_rv == NULL);
 
-		if (flamingo->external_fn_cb(flamingo, callable->name_size, callable->name, flamingo->external_fn_cb_data) < 0) {
+		if (flamingo->external_fn_cb(flamingo, callable->name_size, callable->name, flamingo->external_fn_cb_data, arg_count, args, &flamingo->cur_fn_rv) < 0) {
 			return -1;
 		}
 	}
diff --git a/flamingo/val.c b/flamingo/val.c
index 6c03063..bb98974 100644
--- a/flamingo/val.c
+++ b/flamingo/val.c
@@ -7,6 +7,7 @@
 
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 
 static flamingo_val_t* val_incref(flamingo_val_t* val) {
 	assert(val->ref_count > 0); // value has already been freed
@@ -16,9 +17,13 @@ static flamingo_val_t* val_incref(flamingo_val_t* val) {
 }
 
 static flamingo_val_t* val_init(flamingo_val_t* val) {
+	// By default, values are anonymous.
+
 	val->name = NULL;
 	val->name_size = 0;
 
+	// By default, values are nones.
+
 	val->kind = FLAMINGO_VAL_KIND_NONE;
 	val->ref_count = 1;
 
@@ -70,6 +75,8 @@ static char const* val_role_str(flamingo_val_t* val) {
 
 static flamingo_val_t* val_alloc(void) {
 	flamingo_val_t* const val = calloc(1, sizeof *val);
+	assert(val != NULL);
+
 	return val_init(val);
 }
 
@@ -111,3 +118,42 @@ static flamingo_val_t* val_decref(flamingo_val_t* val) {
 	val_free(val);
 	return NULL;
 }
+
+flamingo_val_t* flamingo_val_make_none(void) {
+	return val_alloc();
+}
+
+flamingo_val_t* flamingo_val_make_int(int64_t integer) {
+	flamingo_val_t* const val = val_alloc();
+
+	val->kind = FLAMINGO_VAL_KIND_INT;
+	val->integer.integer = integer;
+
+	return val;
+}
+
+flamingo_val_t* flamingo_val_make_str(size_t size, char* str) {
+	flamingo_val_t* const val = val_alloc();
+
+	val->kind = FLAMINGO_VAL_KIND_STR;
+
+	val->str.size = size;
+	val->str.str = malloc(size);
+	assert(val->str.str != NULL);
+	memcpy(val->str.str, str, size);
+
+	return val;
+}
+
+flamingo_val_t* flamingo_val_make_cstr(char* str) {
+	return flamingo_val_make_str(strlen(str), str);
+}
+
+flamingo_val_t* flamingo_val_make_bool(bool boolean) {
+	flamingo_val_t* const val = val_alloc();
+
+	val->kind = FLAMINGO_VAL_KIND_BOOL;
+	val->boolean.boolean = boolean;
+
+	return val;
+}
diff --git a/main.c b/main.c
index 8a4278f..05d323d 100644
--- a/main.c
+++ b/main.c
@@ -36,8 +36,31 @@ static void usage(void) {
 	exit(EXIT_FAILURE);
 }
 
-static int external_fn_cb(flamingo_t* flamingo, size_t name_size, char* name, void* data) {
-	printf("function call: %.*s\n", (int) name_size, name);
+static int external_fn_cb(flamingo_t* flamingo, size_t name_size, char* name, void* data, size_t arg_count, flamingo_val_t** args, flamingo_val_t** rv) {
+	if (memcmp(name, "test_return_number", name_size) == 0) {
+		*rv = flamingo_val_make_int(420);
+	}
+
+	else if (memcmp(name, "test_return_bool", name_size) == 0) {
+		*rv = flamingo_val_make_bool(true);
+	}
+
+	else if (memcmp(name, "test_return_str", name_size) == 0) {
+		*rv = flamingo_val_make_cstr("zonnebloemgranen");
+	}
+
+	else if (memcmp(name, "test_return_none", name_size) == 0) {
+		*rv = flamingo_val_make_none();
+	}
+
+	else if (memcmp(name, "test_do_literally_nothing", name_size) == 0) {
+	}
+
+	else {
+		printf("Runtime does not support the '%.*s' external function call (%zu args)\n", (int) name_size, name, arg_count);
+		return -1;
+	}
+
 	return 0;
 }
 
diff --git a/tests/proto.fl b/tests/proto.fl
index 2e98e3f..77b687e 100644
--- a/tests/proto.fl
+++ b/tests/proto.fl
@@ -1,3 +1,14 @@
-proto i_am_a_prototype
+proto test_do_literally_nothing
+assert test_do_literally_nothing() == none
 
-i_am_a_prototype()
+proto test_return_number
+assert test_return_number() == 420
+
+proto test_return_bool
+assert test_return_bool() == true
+
+proto test_return_str
+assert test_return_str() == "zonnebloemgranen"
+
+proto test_return_none
+assert test_return_none() == none