1 files changed, 13 insertions, 3 deletions
diff --git a/rust/src/lambda.rs b/rust/src/lambda.rs
index 2ad69f7..d767003 100644
--- a/rust/src/lambda.rs
+++ b/rust/src/lambda.rs
@@ -29,10 +29,11 @@ fn apply(l: &Value, r: &Value) -> Value {
     }
 }
 
-fn subst(var: &str, body: &Value, r: &Value) -> Value {
+fn subst(var: &str, body: &Value, e: &Value) -> Value {
     match body {
-        Value::Sym(x) if x == var => r.clone(),
-        Value::Lam(x, b) => Value::Lam(x.to_string(), Box::new(subst(var, b, r))),
+        Value::Sym(x) if x == var => e.clone(),
+        Value::Lam(x, b) => Value::Lam(x.to_string(), Box::new(subst(var, b, e))),
+        Value::App(l, r) => Value::App(Box::new(subst(var, l, e)), Box::new(subst(var, r, e))),
         other => other.clone(),
     }
 }
@@ -58,4 +59,13 @@ mod lambda_test {
         let value = parse("(((lam x (lam y x)) 13) 12)");
         assert_eq!(Value::Num(13), interpret(&value));
     }
+
+    #[test]
+    fn substitution_occurs_within_application_body() {
+        let value = parse("(((lam x (lam y (y x))) 13) 12)");
+        assert_eq!(
+            Value::App(Box::new(Value::Num(12)), Box::new(Value::Num(13))),
+            interpret(&value)
+        );
+    }
 }