diff --git a/src/common.rs b/src/common.rs
index 882793c..c52a593 100644
--- a/src/common.rs
+++ b/src/common.rs
@@ -110,7 +110,7 @@ impl ListSeparator {
 ///
 /// This struct protects that invariant by normalizing all
 /// underscores into hypens.
-#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+#[derive(Debug, Clone, Eq, PartialEq, Hash, PartialOrd, Ord)]
 pub(crate) struct Identifier(String);
 
 impl From<String> for Identifier {
diff --git a/src/scope.rs b/src/scope.rs
index 90e30f7..733b9cd 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 
 use codemap::Spanned;
 
@@ -12,18 +12,18 @@ use crate::{
 
 #[derive(Debug, Default)]
 pub(crate) struct Scope {
-    vars: HashMap<Identifier, Spanned<Value>>,
-    mixins: HashMap<Identifier, Mixin>,
-    functions: HashMap<Identifier, Function>,
+    vars: BTreeMap<Identifier, Spanned<Value>>,
+    mixins: BTreeMap<Identifier, Mixin>,
+    functions: BTreeMap<Identifier, Function>,
 }
 
 impl Scope {
     #[must_use]
     pub fn new() -> Self {
         Self {
-            vars: HashMap::new(),
-            mixins: HashMap::new(),
-            functions: HashMap::new(),
+            vars: BTreeMap::new(),
+            mixins: BTreeMap::new(),
+            functions: BTreeMap::new(),
         }
     }