diff --git a/src/ast.rs b/src/ast.rs
index 7687bab..e784ea9 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -192,7 +192,8 @@ impl MacroDef {
                     &Expression::Grouped(ref expr) => {
                         stack.push(expr);
                     },
-                    &Expression::Format(_, ref exprs) => {
+                    &Expression::Format(ref def) => {
+                        let exprs = &def.args;
                         for arg_expr in exprs.iter() {
                             stack.push(arg_expr);
                         }
@@ -243,6 +244,12 @@ pub struct CopyDef {
     pub fields: FieldList
 }
 
+#[derive(Debug,PartialEq,Clone)]
+pub struct FormatDef {
+    pub template: String,
+    pub args: Vec<Expression>
+}
+
 /// Expression encodes an expression. Expressions compute a value from operands.
 #[derive(Debug,PartialEq,Clone)]
 pub enum Expression {
@@ -260,7 +267,7 @@ pub enum Expression {
     Copy(CopyDef),
     Grouped(Box<Expression>),
 
-    Format(String, Vec<Expression>),
+    Format(FormatDef),
 
     Call(CallDef),
 
diff --git a/src/build.rs b/src/build.rs
index feeeb59..1327966 100644
--- a/src/build.rs
+++ b/src/build.rs
@@ -545,7 +545,9 @@ impl Builder {
             Expression::Grouped(expr) => {
                 return self.eval_expr(*expr);
             },
-            Expression::Format(tmpl, mut args) => {
+            Expression::Format(def) => {
+                let tmpl = def.template;
+                let mut args = def.args;
                 let mut vals = Vec::new();
                 for v in args.drain(0..) {
                     let rcv = try!(self.eval_expr(v));
diff --git a/src/parse.rs b/src/parse.rs
index 0c9f149..0bc0977 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -356,7 +356,7 @@ named!(select_expression<Expression>,
 );
 
 fn tuple_to_format(t: (String, Vec<Expression>)) -> ParseResult<Expression> {
-    Ok(Expression::Format(t.0, t.1))
+    Ok(Expression::Format(FormatDef{template: t.0, args: t.1}))
 }
 
 named!(format_expression<Expression>,
@@ -712,16 +712,16 @@ mod test {
 
         assert_eq!(format_expression(&b"\"foo @ @\" % (1, 2)"[..]),
                IResult::Done(&b""[..],
-                             Expression::Format("foo @ @".to_string(),
-                                                vec![Expression::Simple(Value::Int(make_value_node(1))),
-                                                     Expression::Simple(Value::Int(make_value_node(2)))])
+                             Expression::Format(FormatDef{template: "foo @ @".to_string(),
+                                                args: vec![Expression::Simple(Value::Int(make_value_node(1))),
+                                                           Expression::Simple(Value::Int(make_value_node(2)))]})
                )
         );
         assert_eq!(format_expression(&b"\"foo @ @\"%(1, 2)"[..]),
                IResult::Done(&b""[..],
-                             Expression::Format("foo @ @".to_string(),
-                                                vec![Expression::Simple(Value::Int(make_value_node(1))),
-                                                     Expression::Simple(Value::Int(make_value_node(2)))])
+                             Expression::Format(FormatDef{template: "foo @ @".to_string(),
+                                                args: vec![Expression::Simple(Value::Int(make_value_node(1))),
+                                                           Expression::Simple(Value::Int(make_value_node(2)))]})
                )
         );
     }