List.mapMi

2 files changed, 13 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index d28d2868..0da9316e 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -153,6 +153,16 @@ fun mapM [m ::: (Type -> Type)] (_ : monad m) [a] [b] f =
         mapM' []
     end
 
+fun mapMi [m ::: (Type -> Type)] (_ : monad m) [a] [b] f =
+    let
+        fun mapM' i acc ls =
+            case ls of
+                [] => return (rev acc)
+              | x :: ls => x' <- f i x; mapM' (i + 1) (x' :: acc) ls
+    in
+        mapM' 0 []
+    end
+
 fun mapPartialM [m ::: (Type -> Type)] (_ : monad m) [a] [b] f =
     let
         fun mapPartialM' acc ls =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index f4593dda..c5c279b7 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -31,6 +31,9 @@ val mapXi : a ::: Type -> ctx ::: {Unit} -> (int -> a -> xml ctx [] []) -> t a -
 val mapM : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type
            -> (a -> m b) -> t a -> m (t b)
 
+val mapMi : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type
+            -> (int -> a -> m b) -> t a -> m (t b)
+
 val mapPartialM : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type -> (a -> m (option b)) -> t a -> m (t b)
                                                                         
 val mapXM : m ::: (Type -> Type) -> monad m -> a ::: Type -> ctx ::: {Unit}