List.foldli

2 files changed, 12 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index 848bbd5f..1eb7626a 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -31,6 +31,16 @@ fun foldl [a] [b] (f : a -> b -> b) =
         foldl'
     end
 
+fun foldli [a] [b] (f : int -> a -> b -> b) =
+    let
+        fun foldli' i acc ls =
+            case ls of
+                [] => acc
+              | x :: ls => foldli' (i + 1) (f i x acc) ls
+    in
+        foldli' 0
+    end
+
 val rev = fn [a] =>
              let
                  fun rev' acc (ls : list a) =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index 6df2d5d9..f81f38a4 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -7,6 +7,8 @@ val foldl : a ::: Type -> b ::: Type -> (a -> b -> b) -> b -> t a -> b
 val foldlAbort : a ::: Type -> b ::: Type -> (a -> b -> option b) -> b -> t a -> option b
 val foldlMapAbort : a ::: Type -> b ::: Type -> c ::: Type
                     -> (a -> b -> option (c * b)) -> b -> t a -> option (t c * b)
+val foldli : a ::: Type -> b ::: Type
+             -> (int -> a -> b -> b) -> b -> t a -> b
 
 val foldr : a ::: Type -> b ::: Type -> (a -> b -> b) -> b -> t a -> b