dsl: split ns trait

dsl/src/dsl_expr.rs

@@ -68,3 +68,18 @@ dsl_type!(DslExpr {
         ($name, get)
     }),* )? ];
 });
+
+pub trait DslNsExprs<'a, T: 'a>: 'a {
+    /// Known expressions.
+    const EXPRS: DslExprs<'a, Self, T> = &[];
+    /// Resolve an expression if known.
+    fn from_expr (&'a self, dsl: impl DslExpr + 'a) -> Perhaps<T> {
+        let head = dsl.head()?;
+        for (key, get) in Self::EXPRS.iter() {
+            if Some(*key) == head {
+                return get(self, dsl.tail()?.unwrap_or(""))
+            }
+        }
+        return Ok(None)
+    }
+}