feat(tvix/eval): Implement builtins.foldl' r/5076

Change-Id: Ibc97db4343cb3a1a1677f69fb6c3518c61978aad
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6906
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

3 files changed, 24 insertions, 0 deletions

diff --git a/tvix/eval/src/builtins/mod.rs b/tvix/eval/src/builtins/mod.rs
index 93b04bb37b44..f32a29a20660 100644
--- a/tvix/eval/src/builtins/mod.rs
+++ b/tvix/eval/src/builtins/mod.rs
@@ -222,6 +222,24 @@ fn pure_builtins() -> Vec<Builtin> {
                 .map(|list| Value::List(NixList::from(list)))
                 .map_err(Into::into)
         }),
+        Builtin::new(
+            "foldl'",
+            &[true, false, true],
+            |mut args: Vec<Value>, vm: &mut VM| {
+                let list = args.pop().unwrap().to_list()?;
+                let mut res = args.pop().unwrap();
+                let op = args.pop().unwrap();
+                for val in list {
+                    val.force(vm)?;
+                    vm.push(val);
+                    vm.push(res);
+                    let partial = vm.call_value(&op)?;
+                    res = vm.call_value(&partial)?;
+                }
+
+                Ok(res)
+            },
+        ),
         Builtin::new("genList", &[true, true], |args: Vec<Value>, vm: &mut VM| {
             let len = args[1].as_int()?;
             (0..len)
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldl.exp b/tvix/eval/src/tests/tvix_tests/eval-okay-foldl.exp
new file mode 100644
index 000000000000..8d683a20fab7
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldl.exp
@@ -0,0 +1 @@
+[ 6 [ 0 1 2 3 ] 2 ]
diff --git a/tvix/eval/src/tests/tvix_tests/eval-okay-foldl.nix b/tvix/eval/src/tests/tvix_tests/eval-okay-foldl.nix
new file mode 100644
index 000000000000..44c0349387ff
--- /dev/null
+++ b/tvix/eval/src/tests/tvix_tests/eval-okay-foldl.nix
@@ -0,0 +1,5 @@
+[
+  (builtins.foldl' builtins.add 0 [1 2 3])
+  (builtins.foldl' (l1: l2: l1 ++ l2) [0] [[1] [2 3]])
+  (builtins.foldl' (x: y: if x == 0 then y else x * y) 0 [1 2])
+]