diff --git a/src/ast/val.rs b/src/ast/val.rs
index daf5586..8a03729 100644
--- a/src/ast/val.rs
+++ b/src/ast/val.rs
@@ -10,7 +10,7 @@ pub enum Type {
 }
 
 /// The (immutable) result of evaluating an [`Expr`](super::expr::Expr).
-pub trait Val {
+pub trait Val : fmt::Display {
     fn clone(&self) -> Box<dyn Val>;
 
     fn typ(&self) -> Type;
@@ -26,6 +26,14 @@ pub trait Val {
     fn string(&self) -> Result<&str, TypeError> {
         Err(TypeError::new(self.typ(), Type::String))
     }
+
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Ok(s) = self.string() {
+            write!(f, "[{} {}]", self.typ(), s)
+        } else {
+            write!(f, "[{} ??]", self.typ())
+        }
+    }
 }
 
 /// The actual data structure behind [`Val`].
@@ -72,6 +80,12 @@ impl Val for Data<bool> {
     }
 }
 
+impl fmt::Display for Data<bool> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "[bool {}]", self.data)
+    }
+}
+
 impl Val for Data<i64> {
     fn clone(&self) -> Box<dyn Val> {
         Box::new(Data {
@@ -88,6 +102,12 @@ impl Val for Data<i64> {
     }
 }
 
+impl fmt::Display for Data<i64> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "[int {}]", self.data)
+    }
+}
+
 impl Val for Data<String> {
     fn clone(&self) -> Box<dyn Val> {
         Box::new(Data {
@@ -104,6 +124,12 @@ impl Val for Data<String> {
     }
 }
 
+impl fmt::Display for Data<String> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "[string \"{}\"", self.data)
+    }
+}
+
 pub struct TypeError {
     pub msg: String,
     pub actual: Type,