diff --git a/src/language.rs b/src/language.rs
index e5d7ec9..d876d86 100644
--- a/src/language.rs
+++ b/src/language.rs
@@ -45,9 +45,13 @@ pub struct LanguageInterfaceOptions {
 pub trait LanguageInterface {
     fn evaluate_in_repl(&mut self, input: &str, options: LanguageInterfaceOptions) -> String;
     fn get_language_name(&self) -> String;
+    fn set_option(&mut self, option: &str, value: bool) -> bool;
 }
 
 impl<PL, T, A, E> LanguageInterface for (PL, PL::Evaluator) where PL: ProgrammingLanguage<Token=T, AST=A, Evaluator=E>, T: Debug, A: Debug, E: EvaluationMachine {
+    fn set_option(&mut self, option: &str, value: bool) -> bool {
+        self.1.set_option(option, value)
+    }
     fn get_language_name(&self) -> String {
         PL::name()
     }
diff --git a/src/main.rs b/src/main.rs
index 0f3273a..f8cbb77 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -258,7 +258,10 @@ impl Repl {
                 match commands.get(2) {
                     Some(&"tokens") => self.show_tokens = show,
                     Some(&"parse") => self.show_parse = show,
-                    Some(&"eval") => { /*self.evaluator.set_option("trace_evaluation", show);*/ },
+                    Some(&"eval") => {
+                        let ref mut language = self.languages[self.current_language_index];
+                        language.set_option("trace_evaluation", show);
+                    },
                     Some(&"llvm") => self.show_llvm_ir = show,
                     Some(e) => {
                         println!("Bad `show`/`hide` argument: {}", e);