Fiddly tweaks

2 files changed, 14 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index 354ef132..3153cc32 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -322,6 +322,19 @@ val nth [a] =
         nth
     end
 
+fun replaceNth [a] (ls : list a) (n : int) (v : a) : list a =
+    let
+        fun repNth (ls : list a) (n : int) (acc : list a) =
+            case ls of
+                [] => rev acc
+              | x :: ls' => if n <= 0 then
+                                revAppend acc (v :: ls')
+                            else
+                                repNth ls' (n-1) (x :: acc)
+    in
+        repNth ls n []
+    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 c23bf840..9ad738f1 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -72,6 +72,7 @@ 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
+val replaceNth : a ::: Type -> list a -> int -> a -> list a
 
 (** Association lists *)