From 1eb34989d46bfe93cbb0b105ce904046d8748433 Mon Sep 17 00:00:00 2001
From: Michael Camilleri <mike@inqk.net>
Date: Fri, 21 Jan 2022 13:07:11 +0900
Subject: [PATCH] Support looking up missing symbols during compilation

---
 src/boot/boot.janet  |  2 ++
 src/core/util.c      | 10 ++++++++--
 test/suite0011.janet | 13 +++++++++++++
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/src/boot/boot.janet b/src/boot/boot.janet
index 7da63078..566d3dad 100644
--- a/src/boot/boot.janet
+++ b/src/boot/boot.janet
@@ -2746,6 +2746,8 @@
                 (and as (string as "/"))
                 prefix
                 (string (last (string/split "/" path)) "/")))
+  (unless (zero? (length prefix))
+    (put-in env [:modules prefix] newenv))
   (merge-module env newenv prefix ep))
 
 (defmacro import
diff --git a/src/core/util.c b/src/core/util.c
index 8a1fcad6..3f5b9a90 100644
--- a/src/core/util.c
+++ b/src/core/util.c
@@ -607,8 +607,14 @@ JanetBinding janet_resolve_ext(JanetTable *env, const uint8_t *sym) {
     };
 
     /* Check environment for entry */
-    if (!janet_checktype(entry, JANET_TABLE))
-        return binding;
+    if (!janet_checktype(entry, JANET_TABLE)) {
+        Janet fn_entry = janet_table_get(env, janet_ckeywordv("missing-symbol"));
+        if (janet_checktype(fn_entry, JANET_FUNCTION)) {
+            Janet args[2] = { janet_wrap_symbol(sym), janet_wrap_table(env) };
+            entry = janet_call(janet_unwrap_function(fn_entry), 2, args);
+        }
+        if (!janet_checktype(entry, JANET_TABLE)) return binding;
+    }
     entry_table = janet_unwrap_table(entry);
 
     /* deprecation check */
diff --git a/test/suite0011.janet b/test/suite0011.janet
index 6e6d1a23..a67ac787 100644
--- a/test/suite0011.janet
+++ b/test/suite0011.janet
@@ -26,5 +26,18 @@
 (assert (< 11899423.08 (math/gamma 11.5) 11899423.085) "math/gamma")
 (assert (< 2605.1158 (math/log-gamma 500) 2605.1159) "math/log-gamma")
 
+# missing symbols
+
+(def replacement 10)
+(defn lookup-symbol [sym env] (dyn 'replacement))
+
+(setdyn :missing-symbol lookup-symbol)
+
+(assert (= (eval-string "(+ a 5)") 15) "lookup missing symbol")
+
+(setdyn :missing-symbol nil)
+
+(assert-error "compile error" (eval-string "(+ a 5)"))
+
 (end-suite)