diff --git a/flamingo/flamingo.c b/flamingo/flamingo.c
index cdcd209..65559f3 100644
--- a/flamingo/flamingo.c
+++ b/flamingo/flamingo.c
@@ -146,8 +146,9 @@ char* flamingo_err(flamingo_t* flamingo) {
 	return flamingo->err;
 }
 
-void flamingo_register_cb_call(flamingo_t* flamingo, flamingo_cb_call_t cb, void* data) {
-	// fprintf(stderr, "%s: not implemented\n", __func__);
+void flamingo_register_external_fn_cb(flamingo_t* flamingo, flamingo_external_fn_cb_t cb, void* data) { // TODO Better name for this.
+	flamingo->external_fn_cb = cb;
+	flamingo->external_fn_cb_data = data;
 }
 
 static int parse(flamingo_t* flamingo, TSNode node) {
diff --git a/flamingo/flamingo.h b/flamingo/flamingo.h
index ac324d6..383436b 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_cb_call_t)(flamingo_t* flamingo, char* name, void* data);
+typedef int (*flamingo_external_fn_cb_t)(flamingo_t* flamingo, size_t name_size, char* name, void* data);
 
 typedef enum {
 	FLAMINGO_VAL_KIND_NONE,
@@ -26,7 +26,7 @@ typedef enum {
 typedef enum {
 	FLAMINGO_FN_KIND_FUNCTION,
 	FLAMINGO_FN_KIND_CLASS,
-	FLAMINGO_FN_KIND_PROTO,
+	FLAMINGO_FN_KIND_PROTO, // XXX Maybe a better name for this is '*_EXTERN'?
 } flamingo_fn_kind_t;
 
 typedef void* flamingo_ts_node_t; // Opaque type, because user shouldn't have to include Tree-sitter stuff in their namespace (or concern themselves with Tree-sitter at all for that matter).
@@ -104,7 +104,8 @@ struct flamingo_t {
 	char err[256];
 	bool errors_outstanding;
 
-	flamingo_cb_call_t cb_call;
+	flamingo_external_fn_cb_t external_fn_cb;
+	void* external_fn_cb_data;
 
 	// Runtime stuff.
 
@@ -132,7 +133,7 @@ int flamingo_create(flamingo_t* flamingo, char const* progname, char* src, size_
 void flamingo_destroy(flamingo_t* flamingo);
 
 char* flamingo_err(flamingo_t* flamingo);
-void flamingo_register_cb_call(flamingo_t* flamingo, flamingo_cb_call_t cb, void* data);
+void flamingo_register_external_fn_cb(flamingo_t* flamingo, flamingo_external_fn_cb_t cb, void* data);
 int flamingo_inherit_scope_stack(flamingo_t* flamingo, size_t stack_size, flamingo_scope_t** stack);
 int flamingo_run(flamingo_t* flamingo);
 
diff --git a/flamingo/grammar/call.h b/flamingo/grammar/call.h
index 6c9e1a8..58ad06c 100644
--- a/flamingo/grammar/call.h
+++ b/flamingo/grammar/call.h
@@ -172,12 +172,23 @@ static int parse_call(flamingo_t* flamingo, TSNode node, flamingo_val_t** val) {
 		}
 	}
 
-	// Actually parse the function's body.
+	// If external function: call the external function callback.
+	// If function or class: actually parse the function's body.
 
 	TSNode* const body = callable->fn.body;
 	flamingo_scope_t* inner_scope;
 
-	if (parse_block(flamingo, *body, is_class ? &inner_scope : NULL) < 0) {
+	if (callable->fn.kind == FLAMINGO_FN_KIND_PROTO) {
+		if (flamingo->external_fn_cb == NULL) {
+			return error(flamingo, "cannot call external function without a external function callback being set");
+		}
+
+		flamingo->external_fn_cb(flamingo, callable->name_size, callable->name, flamingo->external_fn_cb_data);
+
+		// TODO Rest.
+	}
+
+	else if (parse_block(flamingo, *body, is_class ? &inner_scope : NULL) < 0) {
 		return -1;
 	}
 
diff --git a/flamingo/grammar/import.h b/flamingo/grammar/import.h
index fe31e3b..cb97c6c 100644
--- a/flamingo/grammar/import.h
+++ b/flamingo/grammar/import.h
@@ -62,7 +62,7 @@ static int import(flamingo_t* flamingo, char* path) {
 		goto err_flamingo_create;
 	}
 
-	flamingo_register_cb_call(imported_flamingo, flamingo->cb_call, NULL);
+	flamingo_register_external_fn_cb(imported_flamingo, flamingo->external_fn_cb, flamingo->external_fn_cb_data);
 
 	// Set the scope stack for the imported flamingo instance to be the same as ours.
 
diff --git a/main.c b/main.c
index 8d71f2a..8a4278f 100644
--- a/main.c
+++ b/main.c
@@ -36,8 +36,8 @@ static void usage(void) {
 	exit(EXIT_FAILURE);
 }
 
-static int call_cb(flamingo_t* flamingo, char* name, void* data) {
-	printf("function call: %s\n", name);
+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);
 	return 0;
 }
 
@@ -95,7 +95,7 @@ int main(int argc, char* argv[]) {
 		goto err_flamingo_create;
 	}
 
-	flamingo_register_cb_call(&flamingo, call_cb, NULL);
+	flamingo_register_external_fn_cb(&flamingo, external_fn_cb, NULL);
 
 	// run program