feat(tvix/eval): Implement builtins.elem

Change-Id: Id99c1d33f87ad9866990d3483d3531e9e48f861f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6916
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-elem.exp

@@ -0,0 +1 @@
+[ true false 30 ]

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) ]
+

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);
+
+}