[AGNETS.md] guide control flow

youknowone · youknowone · commit 890fa5dbad27 · 2026-02-06T09:27:36.000+09:00
diff --git a/AGENTS.md b/AGENTS.md
@@ -125,6 +125,31 @@ Run `./scripts/whats_left.py` to get a list of unimplemented methods, which is h
 - Follow Rust best practices for error handling and memory management
 - Use the macro system (`pyclass`, `pymodule`, `pyfunction`, etc.) when implementing Python functionality in Rust
 
+#### Avoid Duplicate Code in Branches
+
+When branches differ only in a value but share common logic, extract the differing value first, then call the common logic once.
+
+**Bad:**
+```rust
+let result = if condition {
+    let msg = format!("message A: {x}");
+    some_function(msg, shared_arg)
+} else {
+    let msg = format!("message B");
+    some_function(msg, shared_arg)
+};
+```
+
+**Good:**
+```rust
+let msg = if condition {
+    format!("message A: {x}")
+} else {
+    format!("message B")
+};
+let result = some_function(msg, shared_arg);
+```
+
 ### Python Code
 
 - **IMPORTANT**: In most cases, Python code should not be edited. Bug fixes should be made through Rust code modifications only
