feat(tvix/eval): Support builtins.elemAt r/4882

(Attempt to) index into a list.

Change-Id: I3592e60a79e64d265e34100d4062041b0b410e00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6551
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

1 files changed, 14 insertions, 0 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)?;