1 files changed, 5 insertions, 1 deletions

diff --git a/src/lisp/error.rs b/src/lisp/error.rs
index 53681d8..f905e6d 100644
--- a/src/lisp/error.rs
+++ b/src/lisp/error.rs
@@ -1,5 +1,5 @@
 use crate::lisp::{
-    expr::Arity,
+    expr::{Arity, LispExpr},
     lex::{Span, SpanDisplay},
 };
 
@@ -98,6 +98,7 @@ pub enum EvalError {
     TypeMismatch,
     NoFileName,
     AccessEmptyList,
+    AssertionError { expected: LispExpr, got: LispExpr },
     ScriptLoadError(io::Error),
     CustomInternal(&'static str),
     Custom(String),
@@ -124,6 +125,9 @@ impl fmt::Display for EvalError {
             Self::DivByZero => write!(f, "attempt to divide by zero"),
             Self::NoFileName => write!(f, "no file name specified"),
             Self::AccessEmptyList => write!(f, "attempted to access empty list"),
+            Self::AssertionError { expected, got } => {
+                write!(f, "assertion error: expected `{}` got `{}`", expected, got)
+            }
             Self::ScriptLoadError(s) => write!(f, "error while loading script: {}", s),
             Self::CustomInternal(s) => write!(f, "{}", s),
             Self::Custom(s) => write!(f, "error: {}", s),