From fefa8c55c45d82fc1d81e02a96e126812e1e1223 Mon Sep 17 00:00:00 2001
From: William Carroll <wpcarro@gmail.com>
Date: Mon, 5 Sep 2022 11:42:53 -0700
Subject: [PATCH] feat(tvix/eval): Support builtins.tail

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
---
 tvix/eval/src/builtins/mod.rs                          | 10 ++++++++++
 tvix/eval/src/errors.rs                                |  6 ++++++
 .../src/tests/tvix_tests/eval-okay-builtins-tail.exp   |  1 +
 .../src/tests/tvix_tests/eval-okay-builtins-tail.nix   |  4 ++++
 4 files changed, 21 insertions(+)
 create mode 100644 tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp
 create mode 100644 tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 4bcaaf7a0..527a8f853 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(),
diff --git a/tvix/eval/src/errors.rs b/tvix/eval/src/errors.rs
index 6331ee64b..10bc9276a 100644
--- a/tvix/eval/src/errors.rs
+++ b/tvix/eval/src/errors.rs
@@ -31,6 +31,9 @@ pub enum ErrorKind {
         index: i64,
     },
 
+    /// Attempted to call `builtins.tail` on an empty list.
+    TailEmptyList,
+
     TypeError {
         expected: &'static str,
         actual: &'static str,
@@ -150,6 +153,8 @@ impl Error {
                 format!("list index '{}' is out of bounds", index)
             }
 
+            ErrorKind::TailEmptyList => format!("'tail' called on an empty list"),
+
             ErrorKind::TypeError { expected, actual } => format!(
                 "expected value of type '{}', but found a '{}'",
                 expected, actual
@@ -257,6 +262,7 @@ to a missing value in the attribute set(s) included via `with`."#,
             ErrorKind::NotAnAbsolutePath(_) => "E020",
             ErrorKind::ParseIntError(_) => "E021",
             ErrorKind::NegativeLength { .. } => "E022",
+            ErrorKind::TailEmptyList { .. } => "E023",
             ErrorKind::NotImplemented(_) => "E999",
         }
     }
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp
new file mode 100644
index 000000000..b9e3aa1ef
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.exp
@@ -0,0 +1 @@
+[ [ ] [ 2 3 ] ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix
new file mode 100644
index 000000000..2be9496a9
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tail.nix
@@ -0,0 +1,4 @@
+[
+  (builtins.tail [ "foo" ])
+  (builtins.tail [ 1 2 3 ])
+]