From 1b208f7f0a3b7d6b72c2a431d9fcd6e18fe76f5d Mon Sep 17 00:00:00 2001
From: nsfisis <nsfisis@gmail.com>
Date: Sun, 28 Sep 2025 15:24:28 +0900
Subject: feat: support using function as a value

---
 src/ast.h                       |  1 +
 src/codegen.c                   |  7 +++++++
 src/parse.c                     |  9 ++++++++-
 tests/test_function_pointers.sh | 18 +++++++++++++++---
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/src/ast.h b/src/ast.h
index 5f4367d..c5171d7 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -102,6 +102,7 @@ typedef enum {
     AstNodeKind_enum_member,
     AstNodeKind_expr_stmt,
     AstNodeKind_for_stmt,
+    AstNodeKind_func,
     AstNodeKind_func_call,
     AstNodeKind_func_decl,
     AstNodeKind_func_def,
diff --git a/src/codegen.c b/src/codegen.c
index 36f854d..34f6c23 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -439,6 +439,11 @@ static void codegen_gvar(CodeGen* g, AstNode* ast, GenMode gen_mode) {
     }
 }
 
+static void codegen_func_ref(CodeGen* g, AstNode* ast) {
+    fprintf(g->out, "  lea rax, %s[rip]\n", ast->name);
+    fprintf(g->out, "  push rax\n");
+}
+
 static void codegen_composite_expr(CodeGen* g, AstNode* ast) {
     // Standard C does not have composite expression, but ducc internally has.
     for (int i = 0; i < ast->node_len; ++i) {
@@ -478,6 +483,8 @@ static void codegen_expr(CodeGen* g, AstNode* ast, GenMode gen_mode) {
         codegen_lvar(g, ast, gen_mode);
     } else if (ast->kind == AstNodeKind_gvar) {
         codegen_gvar(g, ast, gen_mode);
+    } else if (ast->kind == AstNodeKind_func) {
+        codegen_func_ref(g, ast);
     } else if (ast->kind == AstNodeKind_list) {
         codegen_composite_expr(g, ast);
     } else {
diff --git a/src/parse.c b/src/parse.c
index dbbb61d..80c7aac 100644
--- a/src/parse.c
+++ b/src/parse.c
@@ -406,7 +406,14 @@ static AstNode* parse_primary_expr(Parser* p) {
             if (gvar_idx == -1) {
                 int enum_member_idx = find_enum_member(p, name);
                 if (enum_member_idx == -1) {
-                    fatal_error("undefined variable: %s", name);
+                    int func_idx = find_func(p, name);
+                    if (func_idx == -1) {
+                        fatal_error("undefined variable: %s", name);
+                    }
+                    AstNode* e = ast_new(AstNodeKind_func);
+                    e->name = name;
+                    e->ty = p->funcs.data[func_idx].ty;
+                    return e;
                 }
                 int enum_idx = enum_member_idx / 1000;
                 int n = enum_member_idx % 1000;
diff --git a/tests/test_function_pointers.sh b/tests/test_function_pointers.sh
index c97a087..cddc623 100644
--- a/tests/test_function_pointers.sh
+++ b/tests/test_function_pointers.sh
@@ -1,12 +1,24 @@
 cat <<'EOF' > expected
+a
+h
+g
 EOF
 
 test_diff <<'EOF'
-int* f(int a);
-int (*f)(int a);
+int* f1(int a);
+int (*f2)(int a);
 
 extern int atexit (void (*) (void));
 extern int atexit (void (*fn) (void));
 
-int main() {}
+int printf(const char*, ...);
+
+void g() { printf("g\n"); }
+void h() { printf("h\n"); }
+
+int main() {
+    atexit(g);
+    atexit(h);
+    printf("a\n");
+}
 EOF
-- 
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