Support Lookup threading over a list of associations

Lookup[{<|a->1|>, <|a->3|>}, a] now correctly returns {1, 3}.
Previously it errored because a list of associations was not
recognized as a valid first argument.

Author: ad-si

src/functions/association_ast.rs

@@ -206,6 +206,18 @@ pub fn lookup_ast(args: &[Expr]) -> Result<Expr, InterpreterError> {
         args: vec![Expr::String("KeyAbsent".to_string()), args[1].clone()],
       })
     }
+    Expr::List(items) => {
+      // Thread Lookup over a list of associations
+      let results: Result<Vec<Expr>, InterpreterError> = items
+        .iter()
+        .map(|item| {
+          let mut new_args = vec![item.clone()];
+          new_args.extend(args[1..].iter().cloned());
+          lookup_ast(&new_args)
+        })
+        .collect();
+      Ok(Expr::List(results?))
+    }
     _ => Err(InterpreterError::EvaluationError(
       "Lookup expects an association as first argument".into(),
     )),

tests/interpreter_tests/association.rs

@@ -964,3 +964,23 @@ mod association_thread_rule_form {
     );
   }
 }
+
+mod lookup {
+  use super::*;
+
+  #[test]
+  fn lookup_threads_over_list_of_associations() {
+    assert_eq!(
+      interpret("Lookup[{<|a -> 1, b -> 2|>, <|a -> 3, c -> 4|>}, a]").unwrap(),
+      "{1, 3}"
+    );
+  }
+
+  #[test]
+  fn lookup_threads_with_default() {
+    assert_eq!(
+      interpret(r#"Lookup[{<|a -> 1|>, <|b -> 2|>}, a, "miss"]"#).unwrap(),
+      "{1, miss}"
+    );
+  }
+}