feat(tvix/eval): Support builtins.tail r/4917

TL;DR:
- support `builtins.tail`
- define `ErrorKind::TailEmptyList` and canonical error code
- support basic unit tests

Unsure whether or not the error should be a dedicated `ErrorKind`...

Change-Id: Iae90fda1bb21ce7bdb1aaa2aeb2b8c1e6dcb0f05
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6545
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 10 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 4bcaaf7a00..527a8f8530 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -221,6 +221,16 @@ fn pure_builtins() -> Vec<Builtin> {
                 x.as_str()[(beg as usize)..(end as usize)].into(),
             ))
         }),
+        Builtin::new("tail", 1, |args, vm| {
+            let xs = args[0].force(vm)?.to_list()?;
+
+            if xs.len() == 0 {
+                Err(ErrorKind::TailEmptyList)
+            } else {
+                let output = xs.into_iter().skip(1).collect::<Vec<_>>();
+                Ok(Value::List(NixList::construct(output.len(), output)))
+            }
+        }),
         Builtin::new("throw", 1, |mut args, _| {
             return Err(ErrorKind::Throw(
                 args.pop().unwrap().to_str()?.as_str().to_owned(),