diff --git a/src/frontend.rs b/src/frontend.rs
index 170ca2c..ea77959 100644
--- a/src/frontend.rs
+++ b/src/frontend.rs
@@ -282,7 +282,7 @@ impl Context {
         )))
     }
 
-    pub fn infer_expr_types(&self, expr: &typed_ir::Expr) -> typed_ir::Expr {
+    pub fn infer_expr_types(&mut self, expr: &typed_ir::Expr) -> typed_ir::Expr {
         if self.types.is_fully_typed(expr.typ) {
             // there is nothing more to infer
             return expr.clone();
@@ -295,18 +295,17 @@ impl Context {
                 args,
                 genargs,
             } => {
+                let args_typed: Vec<_> = args.iter().map(|ex| self.infer_expr_types(ex)).collect();
                 let callee_def = self.callables.get(*called);
                 if self.types.is_fully_typed(callee_def.ret_type) {
                     expr.clone().with_type(callee_def.ret_type)
                 } else {
-                    let args_typed: Vec<_> =
-                        args.iter().map(|ex| self.infer_expr_types(ex)).collect();
                     let param_types: Vec<_> = callee_def.args.iter().map(|param| param.1).collect();
                     let mut genargs = callee_def.genargs.clone();
                     let inferred_args: Vec<_> = param_types
                         .iter()
                         .zip(args_typed)
-                        .map(|(param, arg)| {})
+                        .map(|(param, arg)| self.types.infer_type(*param, arg.typ))
                         .collect();
                     expr.clone().with_type(callee_def.ret_type)
                 }
@@ -314,7 +313,7 @@ impl Context {
         }
     }
 
-    pub fn infer_types(&self, mut block: typed_ir::Block) -> typed_ir::Block {
+    pub fn infer_types(&mut self, mut block: typed_ir::Block) -> typed_ir::Block {
         let new_root = self.infer_expr_types(&block.expr);
         block.expr = new_root;
         block
diff --git a/src/frontend/types.rs b/src/frontend/types.rs
index 9a794a2..b791c4a 100644
--- a/src/frontend/types.rs
+++ b/src/frontend/types.rs
@@ -172,6 +172,13 @@ impl TypingContext {
         }
     }
 
+    /// Given the type of a variable in two locations, infer what the true type should be
+    pub fn infer_type(&mut self, typ_a: Type, typ_b: Type) -> Type {
+        match (&self.get(typ_a).kind, &self.get(typ_b).kind) {
+            (a, b) => panic!("cannot infer between: {:?}, {:?}", a, b),
+        }
+    }
+
     /// return whether the type has no unfilled parameters
     pub fn is_fully_typed(&self, typ: Type) -> bool {
         match &self.get(typ).kind {