Fix a bug in C list unurlification

2 files changed, 16 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index 71d8fa98..354ef132 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -308,6 +308,20 @@ fun sort [a] (gt : a -> a -> bool) (ls : t a) : t a =
         sort' ls
     end
 
+val nth [a] =
+    let
+        fun nth (ls : list a) (n : int) : option a =
+            case ls of
+                [] => None
+              | x :: ls' =>
+                if n <= 0 then
+                    Some x
+                else
+                    nth ls' (n-1)
+    in
+        nth
+    end
+
 fun assoc [a] [b] (_ : eq a) (x : a) =
     let
         fun assoc' (ls : list (a * b)) =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index a0cec8fb..c23bf840 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -71,6 +71,8 @@ val mapQueryPartialM : tables ::: {{Type}} -> exps ::: {Type} -> t ::: Type
 
 val sort : a ::: Type -> (a -> a -> bool) (* > predicate *) -> t a -> t a
 
+val nth : a ::: Type -> list a -> int -> option a
+
 (** Association lists *)
 
 val assoc : a ::: Type -> b ::: Type -> eq a -> a -> t (a * b) -> option b