3 files changed, 21 insertions, 0 deletions
diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 45630819e124..94a035c2ece7 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -116,6 +116,20 @@ fn pure_builtins() -> Vec<Builtin> {
                 std::cmp::Ordering::Greater => Ok(Value::Integer(1)),
             }
         }),
+        Builtin::new("concatLists", &[true], |args, vm| {
+            let list = args[0].to_list()?;
+            let lists = list
+                .into_iter()
+                .map(|elem| {
+                    let value = elem.force(vm)?;
+                    value.to_list()
+                })
+                .collect::<Result<Vec<NixList>, ErrorKind>>()?;
+
+            Ok(Value::List(NixList::from(
+                lists.into_iter().flatten().collect::<Vec<Value>>(),
+            )))
+        }),
         Builtin::new(
             "div",
             &[false, false],
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp
new file mode 100644
index 000000000000..64ae529ac257
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.exp
@@ -0,0 +1 @@
+[ [ ] [ 1 2 3 4 5 6 ] [ [ 1 ] [ 2 ] [ 3 ] ] [ 1 2 3 4 5 6 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.nix
new file mode 100644
index 000000000000..19ef5eba1145
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-concat-lists.nix
@@ -0,0 +1,6 @@
+[
+  (builtins.concatLists [ ])
+  (builtins.concatLists [ [ 1 2 ] [ 3 4 ] [ 5 6 ] ])
+  (builtins.concatLists [ [ [ 1 ] [ 2 ] ] [ [ 3 ] ] [ ] ])
+  (builtins.concatLists [ [ 1 2 ] [ ] [ 3 4 ] [ ] [ 5 6 ] ])
+]