implement builtin function append()

src/builtin/list.rs

@@ -107,6 +107,36 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
                 list[len - n.abs().to_integer().to_usize().unwrap()] = val;
             }
 
+            Ok(Value::List(list, sep))
+        }),
+    );
+    f.insert(
+        "append".to_owned(),
+        Box::new(|args, _| {
+            max_args!(args, 3);
+            let (mut list, sep) = match arg!(args, 0, "list") {
+                Value::List(v, sep) => (v, sep),
+                v => (vec![v], ListSeparator::Space),
+            };
+            let val = arg!(args, 1, "val");
+            let sep = match arg!(
+                args,
+                2,
+                "separator" = Value::Ident("auto".to_owned(), QuoteKind::None)
+            ) {
+                Value::Ident(s, ..) => match s.as_str() {
+                    "auto" => sep,
+                    "comma" => ListSeparator::Comma,
+                    "space" => ListSeparator::Space,
+                    _ => {
+                        return Err("$separator: Must be \"space\", \"comma\", or \"auto\".".into())
+                    }
+                },
+                _ => return Err("$separator: Must be \"space\", \"comma\", or \"auto\".".into()),
+            };
+
+            list.push(val);
+
             Ok(Value::List(list, sep))
         }),
     );

tests/list.rs

@@ -80,3 +80,28 @@ test!(
     "a {\n  color: set-nth((a, b, c), 1, e);\n}\n",
     "a {\n  color: e, b, c;\n}\n"
 );
+test!(
+    append_space_separated,
+    "a {\n  color: append(a b, c);\n}\n",
+    "a {\n  color: a b c;\n}\n"
+);
+test!(
+    append_comma_separated,
+    "a {\n  color: append((a, b), c);\n}\n",
+    "a {\n  color: a, b, c;\n}\n"
+);
+test!(
+    append_list,
+    "a {\n  color: append(a b, c d);\n}\n",
+    "a {\n  color: a b c d;\n}\n"
+);
+test!(
+    append_list_separator_comma,
+    "a {\n  color: append(a, b, comma);\n}\n",
+    "a {\n  color: a, b;\n}\n"
+);
+test!(
+    append_list_separator_space,
+    "a {\n  color: append((a, b), c, space);\n}\n",
+    "a {\n  color: a b c;\n}\n"
+);