add list type check

src/stl/append.rs

@@ -180,6 +180,15 @@ fn reverse_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, Traceback> {
     }
 }
 
+const ISLIST_DOCSTRING: &str = "takes a single argument, returns true if argument is a list";
+fn islist_callback(ast: &Seg, _syms: &mut SymTable) -> Result<Ctr, Traceback> {
+    if let Ctr::Seg(_) = *ast.car {
+        Ok(Ctr::Bool(true))
+    } else {
+        Ok(Ctr::Bool(false))
+    }
+}
+
 pub fn add_list_lib(syms: &mut SymTable) {
     syms.insert(
         "cons".to_string(),
@@ -264,4 +273,16 @@ pub fn add_list_lib(syms: &mut SymTable) {
             ..Default::default()
         },
     );
+
+    syms.insert(
+        "list?".to_string(),
+        Symbol {
+            name: String::from("list?"),
+            args: Args::Lazy(1),
+            conditional_branches: false,
+            docs: ISLIST_DOCSTRING.to_string(),
+            value: ValueType::Internal(Rc::new(islist_callback)),
+            ..Default::default()
+        },
+    );
 }