feat(tvix/eval): implement list concatenation r/4479

Change-Id: Icdf715d116371a9f139bdf95266410bf967bef25
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6144
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

2 files changed, 19 insertions, 0 deletions

diff --git a/tvix/eval/src/value/list.rs b/tvix/eval/src/value/list.rs
index d5f7c8b2ba..f9d8522941 100644
--- a/tvix/eval/src/value/list.rs
+++ b/tvix/eval/src/value/list.rs
@@ -18,3 +18,12 @@ impl Display for NixList {
         f.write_str("]")
     }
 }
+
+impl NixList {
+    pub fn concat(&self, other: &Self) -> Self {
+        let mut lhs = self.clone();
+        let mut rhs = other.clone();
+        lhs.0.append(&mut rhs.0);
+        lhs
+    }
+}
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index 2a89f1c358..0c9042dbfe 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -81,6 +81,16 @@ impl Value {
             }),
         }
     }
+
+    pub fn as_list(self) -> EvalResult<NixList> {
+        match self {
+            Value::List(l) => Ok(l),
+            other => Err(Error::TypeError {
+                expected: "list",
+                actual: other.type_of(),
+            }),
+        }
+    }
 }
 
 impl Display for Value {