4 files changed, 22 insertions, 2 deletions
diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 149669015b6a..3f8b73f7d7fd 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -140,6 +140,20 @@ fn pure_builtins() -> Vec<Builtin> {
             let a = args.pop().unwrap();
             arithmetic_op!(a, b, /)
         }),
+        Builtin::new("elemAt", 2, |args, vm| {
+            force!(vm, &args[0], value, {
+                let xs = value.to_list()?;
+                let i = args[1].as_int()?;
+                if i < 0 {
+                    Err(ErrorKind::IndexOutOfBounds { index: i })
+                } else {
+                    match xs.get(i as usize) {
+                        Some(x) => Ok(x.clone()),
+                        None => Err(ErrorKind::IndexOutOfBounds { index: i }),
+                    }
+                }
+            })
+        }),
         Builtin::new("length", 1, |args, vm| {
             if let Value::Thunk(t) = &args[0] {
                 t.force(vm)?;
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 23ac6abbf4de..d3d41cbbcd48 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -26,9 +26,9 @@ pub enum ErrorKind {
         name: String,
     },
 
-    // Attempted to index into a list beyond its boundaries.
+    /// Attempted to index into a list beyond its boundaries.
     IndexOutOfBounds {
-        index: usize,
+        index: i64,
     },
 
     TypeError {
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.exp
new file mode 100644
index 000000000000..3701c9d75f94
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.exp
@@ -0,0 +1 @@
+[ "foo" "bar" "baz" ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.nix
new file mode 100644
index 000000000000..762adeebbf0a
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-elemat.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.elemAt [ "foo" "bar" "baz" ] 0)
+  (builtins.elemAt [ "foo" "bar" "baz" ] 1)
+  (builtins.elemAt [ "foo" "bar" "baz" ] 2)
+]