List.mapi

2 files changed, 12 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index bccbc82a..bb814714 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -101,6 +101,16 @@ fun mp [a] [b] f =
         mp' []
     end
 
+fun mapi [a] [b] f =
+    let
+        fun mp' n acc ls =
+            case ls of
+                [] => rev acc
+              | x :: ls => mp' (n + 1) (f n x :: acc) ls
+    in
+        mp' 0 []
+    end
+
 fun mapPartial [a] [b] f =
     let
         fun mp' acc ls =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index 00c95053..7ce3c61f 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -22,6 +22,8 @@ val mp : a ::: Type -> b ::: Type -> (a -> b) -> t a -> t b
 
 val mapPartial : a ::: Type -> b ::: Type -> (a -> option b) -> t a -> t b
 
+val mapi : a ::: Type -> b ::: Type -> (int -> a -> b) -> t a -> t b
+
 val mapX : a ::: Type -> ctx ::: {Unit} -> (a -> xml ctx [] []) -> t a -> xml ctx [] []
 
 val mapM : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type