Refactor builtin function declaration

src/builtin/meta.rs

@@ -4,9 +4,7 @@ use super::Builtin;
 use crate::value::Value;
 
 pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
-    f.insert(
-        "if".to_owned(),
-        Box::new(|args| {
+    decl!(f "if", |args| {
         let cond: &Value = arg!(args, 0, "condition");
         let if_true = arg!(args, 1, "if-true").clone();
         let if_false = arg!(args, 2, "if-false").clone();
@@ -15,6 +13,5 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         } else {
             Some(if_false)
         }
-        }),
-    );
+    });
 }

src/builtin/mod.rs

@@ -4,26 +4,8 @@ use std::collections::BTreeMap;
 use crate::args::CallArgs;
 use crate::value::Value;
 
-macro_rules! arg {
-    ($args:ident, $idx:literal, $name:literal) => {
-        match $args.get(stringify!($idx)) {
-            Some(v) => v,
-            None => match $args.get($name) {
-                Some(v) => v,
-                None => panic!("missing variable"),
-            },
-        };
-    };
-    ($args:ident, $idx:literal, $name:literal, $default:literal) => {
-        match $args.get(stringify!($idx)) {
-            Some(v) => v,
-            None => match $args.get($name) {
-                Some(v) => v,
-                None => $default,
-            },
-        };
-    };
-}
+#[macro_use]
+mod macros;
 
 
 mod color;

tests/meta.rs

@@ -0,0 +1,20 @@
+#![cfg(test)]
+
+#[macro_use]
+mod macros;
+
+test!(
+    if_true,
+    "a {\n  color: if(true, 1, 2)\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    if_named_args,
+    "a {\n  color: if($condition: true, $if-true: 1, $if-false: 2)\n}\n",
+    "a {\n  color: 1;\n}\n"
+);
+test!(
+    if_false,
+    "a {\n  color: if(false, 1, 2);\n}\n",
+    "a {\n  color: 2;\n}\n"
+);