diff --git a/src/main.rs b/src/main.rs
index e50e2c5..7835ce0 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -167,10 +167,9 @@ pub fn run(src: &'static str) {
 
 pub fn main() {
     let src = "
-let bar = :bar
-match :foo with {
-    :foo -> bar
-}
+let bar = :foo
+fn foo (foo, bar, ...) -> bar
+foo (42)
     ";
     run(src);
     // struct_scalpel::print_dissection_info::<value::Value>()
diff --git a/src/validator.rs b/src/validator.rs
index 9caa326..bf3e6d8 100644
--- a/src/validator.rs
+++ b/src/validator.rs
@@ -37,6 +37,13 @@ pub enum FnInfo {
     Unknown,
 }
 
+fn match_arities(arities: &HashSet<Arity>, num_args: u8) -> bool {
+    arities.iter().any(|arity| match arity {
+        Arity::Fixed(n) => *n == num_args,
+        Arity::Splat(n) => *n <= num_args,
+    })
+}
+
 #[derive(Debug, PartialEq)]
 pub struct Validator<'a> {
     // TODO: add another term here: FnStatus. See Issue #18.
@@ -269,9 +276,6 @@ impl<'a> Validator<'a> {
             }
 
             // TODO!
-            // first check all nodes
-            // then...
-            // check arity is 1 if first term is keyword
             // check arity against fn info if first term is word and second term is args
             Synthetic(first, second, rest) => {
                 match (&first.0, &second.0) {
@@ -290,7 +294,7 @@ impl<'a> Validator<'a> {
                         self.span = *span;
                         self.validate();
                     }
-                    (Ast::Word(_), Ast::Arguments(_)) => {
+                    (Ast::Word(name), Ast::Arguments(args)) => {
                         let (expr, span) = first.as_ref();
                         self.ast = expr;
                         self.span = *span;
@@ -301,7 +305,14 @@ impl<'a> Validator<'a> {
                         self.span = *span;
                         self.validate();
 
-                        //TODO: check arity of call
+                        //TODO: check arities of prelude fns, too
+                        let fn_binding = self.bound(name);
+                        if let Some((_, _, FnInfo::Defined(arities, _))) = fn_binding {
+                            let num_args = args.len();
+                            if !match_arities(arities, num_args as u8) {
+                                self.err(format!("arity mismatch: no clause in function `{name}` with {num_args} argument(s)"))
+                            }
+                        }
                     }
                     _ => unreachable!(),
                 }
@@ -412,6 +423,8 @@ impl<'a> Validator<'a> {
                     }
                 }
 
+                // TODO: devise a placeholder binding for recursive functions
+
                 let from = self.status.used_bindings.len();
                 let mut arities = HashSet::new();
 
@@ -588,8 +601,8 @@ impl<'a> Validator<'a> {
                     self.err("splats in patterns must come last".to_string());
                 }
                 match splatted.as_ref() {
-                    (Ast::Placeholder, _) => (),
-                    (Ast::Word(name), span) => match self.bound(name) {
+                    (PlaceholderPattern, _) => (),
+                    (Word(name), span) => match self.bound(name) {
                         Some(_) => {
                             self.span = *span;
                             self.err(format!("name `{name}` is already bound"))