diff --git a/schala-lang/language/src/typechecking.rs b/schala-lang/language/src/typechecking.rs
index 52b6dba..25fc233 100644
--- a/schala-lang/language/src/typechecking.rs
+++ b/schala-lang/language/src/typechecking.rs
@@ -11,13 +11,25 @@ pub struct TypeContext {
 type InferResult<T> = Result<T, TypeError>;
 
 #[derive(Debug, Clone)]
-struct TypeError { }
+struct TypeError { msg: String }
+
+impl TypeError {
+  fn new<A>(msg: &str) -> InferResult<A> {
+    Err(TypeError { msg: msg.to_string() })
+  }
+}
 
 #[derive(Debug, Clone)]
 enum MonoType {
   Var(Rc<String>),
   Const(TConst),
-  Arrow(Rc<String>)
+  Arrow(Box<MonoType>, Box<MonoType>)
+}
+
+impl TypeIdentifier {
+  fn to_monotype(&self) -> MonoType {
+    unimplemented!()
+  }
 }
 
 #[derive(Debug, Clone)]
@@ -70,7 +82,11 @@ impl TypeContext {
 
   fn infer_expr(&mut self, expr: &Expression) -> InferResult<MonoType> {
     match expr {
-      Expression(expr_type, Some(type_anno)) => unimplemented!(),
+      Expression(expr_type, Some(type_anno)) => {
+        let tx = self.infer_expr_type(expr_type)?;
+        let ty = type_anno.to_monotype();
+        self.unify(&ty, &tx)
+      },
       Expression(expr_type, None) => self.infer_expr_type(expr_type)
     }
   }
@@ -86,11 +102,22 @@ impl TypeContext {
       FloatLiteral(_) => MonoType::Const(TConst::Float),
       StringLiteral(_) => MonoType::Const(TConst::StringT),
       BoolLiteral(_) => MonoType::Const(TConst::Bool),
+      Call { f, arguments } => {
+        let tf: MonoType = self.infer_expr(f)?; //has to be an Arrow MonoType
+        let targ = self.infer_expr(&arguments[0])?; // TODO make this work with functions with more than one arg
+        match tf {
+          MonoType::Arrow(t1, t2) => {
+            self.unify(&t1, &targ)?;
+            *t2.clone()
+          },
+          _ => return TypeError::new("not a function")
+        }
+      },
       _ => MonoType::Const(TConst::user("unimplemented"))
     })
   }
 
-  fn unify(t1: MonoType, t2: MonoType) -> InferResult<MonoType> {
+  fn unify(&mut self, t1: &MonoType, t2: &MonoType) -> InferResult<MonoType> {
     unimplemented!()
   }
 }