diff --git a/src/builtin/mod.rs b/src/builtin/mod.rs
new file mode 100644
index 0000000..c8bafb9
--- /dev/null
+++ b/src/builtin/mod.rs
@@ -0,0 +1,15 @@
+use lazy_static::lazy_static;
+use std::collections::BTreeMap;
+
+use crate::common::Scope;
+use crate::function::Function;
+use crate::value::Value;
+
+pub(crate) type Builtin = dyn Fn(&Scope) -> Value + Send + Sync;
+
+lazy_static! {
+    pub(crate) static ref GLOBAL_FUNCTIONS: BTreeMap<String, Function> = {
+        let m = BTreeMap::new();
+        m
+    };
+}
diff --git a/src/lib.rs b/src/lib.rs
index 1cb2e44..024b647 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -62,6 +62,7 @@ use crate::utils::{devour_whitespace, eat_variable_value, IsComment, IsWhitespac
 
 mod args;
 mod atrule;
+mod builtin;
 mod color;
 mod common;
 mod css;