diff --git a/src/optimize.rs b/src/optimize.rs
index eaa8aeb3..738ea300 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -456,7 +456,7 @@ fn optimize_expr(expr: Expr, state: &mut State) -> Expr {
             // "xxx" in "xxxxx"
             (Expr::StringConstant(a), Expr::StringConstant(b)) => {
                 state.set_dirty();
-                if b.0.contains(a.0.as_ref()) { Expr::True(a.1) } else { Expr::False(a.1) }
+                if b.0.contains(a.0.as_str()) { Expr::True(a.1) } else { Expr::False(a.1) }
             }
             // 'x' in "xxxxx"
             (Expr::CharConstant(a), Expr::StringConstant(b)) => {
@@ -560,7 +560,7 @@ fn optimize_expr(expr: Expr, state: &mut State) -> Expr {
             let has_script_fn = state.lib.iter_fn().find(|(_, _, _, f)| {
                 if !f.is_script() { return false; }
                 let fn_def = f.get_fn_def();
-                &fn_def.name == name && (args.len()..=args.len() + 1).contains(&fn_def.params.len())
+                fn_def.name.as_str() == name && (args.len()..=args.len() + 1).contains(&fn_def.params.len())
             }).is_some();
 
             #[cfg(feature = "no_function")]
diff --git a/src/parser.rs b/src/parser.rs
index b6a3fa08..0655c903 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -346,7 +346,7 @@ impl fmt::Display for FnAccess {
 #[derive(Debug, Clone, Hash)]
 pub struct ScriptFnDef {
     /// Function name.
-    pub name: String,
+    pub name: ImmutableString,
     /// Function access mode.
     pub access: FnAccess,
     /// Names of function parameters.
@@ -2852,7 +2852,7 @@ fn parse_fn(
     let params = params.into_iter().map(|(p, _)| p).collect();
 
     Ok(ScriptFnDef {
-        name,
+        name: name.into(),
         access,
         params,
         body,
@@ -2940,7 +2940,7 @@ fn parse_anon_fn(
     let hash = s.finish();
 
     // Create unique function name
-    let fn_name = format!("{}{}", FN_ANONYMOUS, hash);
+    let fn_name: ImmutableString = format!("{}{:16x}", FN_ANONYMOUS, hash).into();
 
     let script = ScriptFnDef {
         name: fn_name.clone(),
@@ -2950,7 +2950,7 @@ fn parse_anon_fn(
         pos: settings.pos,
     };
 
-    let expr = Expr::FnPointer(Box::new((fn_name.into(), settings.pos)));
+    let expr = Expr::FnPointer(Box::new((fn_name, settings.pos)));
 
     Ok((expr, script))
 }