--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/preprocess_test.c	Sun Sep 01 19:24:03 2019 +0200
@@ -0,0 +1,57 @@
+#include "preprocess.h"
+
+#include "eval.h"
+#include "expr.h"
+#include "parse.h"
+#include "value.h"
+
+#include <stdio.h>
+
+// The very first successfully executable program!
+void test_preprocess_defn(void) {
+    fprintf(stderr, "test_preprocess_defn ===========\n");
+
+    // my-func adds the two arguments plus 42.
+    ystr_t input = ystr_new(
+        "(defn my-func (arg1 arg2) (+ arg1 arg2) (undef) (+ arg1 arg2 41))"
+        "(my-func 42 (my-func 43 (my-func 44 0)))");  // (((44 + 0 + 41) + 43 +
+                                                      // 41) + 42 + 41) == 252
+    yexpr_t program = yexpr_new();
+    ystr_t error = ystr_new(NULL);
+    assert(yparse_str(&input, &program, &error));
+
+    ypreprocess_resolve_builtins(&program);
+    yexpr_debug(&program);
+    ypreprocess_defn(&program);
+    yexpr_debug(&program);
+
+    yvalue_t* func = yvalue_get(yref_new_c("my-func"));
+    assert(func != NULL);
+    fprintf(stderr, "arg ref 0: %lu\n",
+            YVEC_AT(&func->value.func.args, 0, yarg_desc_t)->argref.ref.id);
+    fprintf(stderr, "arg ref 1: %lu\n",
+            YVEC_AT(&func->value.func.args, 1, yarg_desc_t)->argref.ref.id);
+    fprintf(
+        stderr, "arg name 0: %s\n",
+        ystr_str(&YVEC_AT(&func->value.func.args, 0, yarg_desc_t)->argname));
+    fprintf(
+        stderr, "arg name 1: %s\n",
+        ystr_str(&YVEC_AT(&func->value.func.args, 1, yarg_desc_t)->argname));
+
+    yeval_state_t state;
+    state.call_stack = YVEC_NEW(NULL, 0, yexpr_t);
+    yexpr_t result = yeval_list_return_last(&state, &program.value.list, 0);
+    assert(result.typ == YEXPR_NUMBER);
+    yexpr_debug(&result);
+
+    ystr_destroy(&input);
+    yexpr_destroy(&program);
+    yvec_destroy(&state.call_stack);
+    yexpr_destroy(&result);
+}
+
+int main(void) {
+    test_preprocess_defn();
+    yvalue_free_all();
+    return 0;
+}