diff --git a/src/main.rs b/src/main.rs
index b30d8fa..85a51d6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -167,7 +167,9 @@ pub fn run(src: &'static str) {
 
 pub fn main() {
     let src = "
-do 42 > recur ()
+loop () with {
+    () -> :baz
+}
     ";
     run(src);
     // struct_scalpel::print_dissection_info::<value::Value>()
diff --git a/src/validator.rs b/src/validator.rs
index 64b77e9..f73a5e4 100644
--- a/src/validator.rs
+++ b/src/validator.rs
@@ -18,6 +18,7 @@ impl VErr {
 struct VStatus {
     tail_position: bool,
     in_loop: bool,
+    loop_arity: u8,
     last_term: bool,
     has_placeholder: bool,
     used_bindings: Vec<String>,
@@ -60,6 +61,7 @@ impl<'a> Validator<'a> {
             status: VStatus {
                 tail_position: false,
                 in_loop: false,
+                loop_arity: 0,
                 last_term: false,
                 has_placeholder: false,
                 used_bindings: vec![],
@@ -429,15 +431,19 @@ impl<'a> Validator<'a> {
                 self.ast = expr;
                 self.validate();
 
-                let in_loop = self.status.in_loop;
-                self.status.in_loop = true;
+                let Ast::Tuple(input) = expr else {
+                    unreachable!()
+                };
 
-                // let (expr, span) = body;
-                // self.span = span;
-                // self.expr = expr;
-                // self.validate();
+                let in_loop = self.status.in_loop;
+                let arity = self.status.loop_arity;
+                self.status.in_loop = true;
+                self.status.loop_arity = input.len() as u8;
+
+                // for clause in body {}
 
                 self.status.in_loop = in_loop;
+                self.status.loop_arity = arity;
             }
             Ast::Recur(args) => {
                 if !self.status.in_loop {
@@ -448,6 +454,12 @@ impl<'a> Validator<'a> {
                     self.err("you may only use `recur` in tail position".to_string());
                 }
 
+                let num_args = args.len() as u8;
+                let loop_arity = self.status.loop_arity;
+                if num_args != loop_arity {
+                    self.err(format!("loop arity mismatch: loop has arity of {loop_arity}; `recur` called with {num_args} arguments"))
+                }
+
                 self.status.tail_position = false;
                 for arg in args {
                     let (expr, span) = arg;