5 files changed, 82 insertions, 0 deletions
diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 5819796236cd..8929d91cc9c6 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -174,6 +174,14 @@ fn pure_builtins() -> Vec<Builtin> {
             &[false, false],
             |args: Vec<Value>, vm: &mut VM| arithmetic_op!(&*args[0].force(vm)?, &*args[1].force(vm)?, /),
         ),
+        Builtin::new("elem", &[true, true], |args: Vec<Value>, vm: &mut VM| {
+            for val in args[1].to_list()? {
+                if val.nix_eq(&args[0], vm)? {
+                    return Ok(true.into());
+                }
+            }
+            Ok(false.into())
+        }),
         Builtin::new("elemAt", &[true, true], |args: Vec<Value>, _: &mut VM| {
             let xs = args[0].to_list()?;
             let i = args[1].as_int()?;
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp
new file mode 100644
index 000000000000..3cf6c0e962f0
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp
@@ -0,0 +1 @@
+[ true false 30 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-elem.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.nix
new file mode 100644
index 000000000000..71ea7a4ed03d
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-elem.nix
@@ -0,0 +1,6 @@
+with import ./lib.nix;
+
+let xs = range 10 40; in
+
+[ (builtins.elem 23 xs) (builtins.elem 42 xs) (builtins.elemAt xs 20) ]
+
diff --git a/tvix/eval/src/tests/tvix_tests/lib.nix b/tvix/eval/src/tests/tvix_tests/lib.nix
new file mode 100644
index 000000000000..028a538314b7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/lib.nix
@@ -0,0 +1,61 @@
+with builtins;
+
+rec {
+
+  fold = op: nul: list:
+    if list == []
+    then nul
+    else op (head list) (fold op nul (tail list));
+
+  concat =
+    fold (x: y: x + y) "";
+
+  and = fold (x: y: x && y) true;
+
+  flatten = x:
+    if isList x
+    then fold (x: y: (flatten x) ++ y) [] x
+    else [x];
+
+  sum = foldl' (x: y: add x y) 0;
+
+  hasSuffix = ext: fileName:
+    let lenFileName = stringLength fileName;
+        lenExt = stringLength ext;
+    in !(lessThan lenFileName lenExt) &&
+       substring (sub lenFileName lenExt) lenFileName fileName == ext;
+
+  # Split a list at the given position.
+  splitAt = pos: list:
+    if pos == 0 then {first = []; second = list;} else
+    if list == [] then {first = []; second = [];} else
+    let res = splitAt (sub pos 1) (tail list);
+    in {first = [(head list)] ++ res.first; second = res.second;};
+
+  # Stable merge sort.
+  sortBy = comp: list:
+    if lessThan 1 (length list)
+    then
+      let
+        split = splitAt (div (length list) 2) list;
+        first = sortBy comp split.first;
+        second = sortBy comp split.second;
+      in mergeLists comp first second
+    else list;
+
+  mergeLists = comp: list1: list2:
+    if list1 == [] then list2 else
+    if list2 == [] then list1 else
+    if comp (head list2) (head list1) then [(head list2)] ++ mergeLists comp list1 (tail list2) else
+    [(head list1)] ++ mergeLists comp (tail list1) list2;
+
+  id = x: x;
+
+  const = x: y: x;
+
+  range = first: last:
+    if first > last
+      then []
+      else genList (n: first + n) (last - first + 1);
+
+}
diff --git a/tvix/eval/src/value/mod.rs b/tvix/eval/src/value/mod.rs
index ea9e173ebc40..d3b981b78cba 100644
--- a/tvix/eval/src/value/mod.rs
+++ b/tvix/eval/src/value/mod.rs
@@ -359,6 +359,12 @@ impl Display for Value {
     }
 }
 
+impl From<bool> for Value {
+    fn from(b: bool) -> Self {
+        Value::Bool(b)
+    }
+}
+
 impl From<i64> for Value {
     fn from(i: i64) -> Self {
         Self::Integer(i)