1 files changed, 20 insertions, 1 deletions
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 57488121..bf395c2b 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -235,13 +235,17 @@ debug_display!(Expression);
 pub struct Ident(pub String);
 
 impl Ident {
-    fn new(string: String) -> ParseResult<Ident> {
+    pub fn new(string: String) -> ParseResult<Ident> {
         if is_identifier(&string) {
             Ok(Ident(string))
         } else {
             error!("invalid identifier: `{}`", string);
         }
     }
+
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
 }
 
 impl Display for Ident {
@@ -317,3 +321,18 @@ impl<T> ExpressionKind for Spanned<T> where T: ExpressionKind {
             .map(|v| Spanned::new(v, span))
     }
 }
+
+impl<T> ExpressionKind for Option<T> where T: ExpressionKind {
+    const NAME: &'static str = T::NAME;
+
+    fn from_expr(expr: Spanned<Expression>) -> ParseResult<Option<T>> {
+        if let Expression::Ident(ident) = &expr.v {
+            match ident.as_str() {
+                "default" | "none" => return Ok(None),
+                _ => {},
+            }
+        }
+
+        T::from_expr(expr).map(|v| Some(v))
+    }
+}