List.findM

2 files changed, 17 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index a7296552..95d6fbc8 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -255,6 +255,21 @@ fun find [a] f =
         find'
     end
 
+fun findM [m] (_ : monad m) [a] f =
+    let
+        fun find' ls =
+            case ls of
+                [] => return None
+              | x :: ls =>
+                b <- f x;
+                if b then
+                    return (Some x)
+                else
+                    find' ls
+    in
+        find'
+    end
+
 fun search [a] [b] f =
     let
         fun search' ls =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index 37cbe442..fe730152 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -60,6 +60,8 @@ val mem : a ::: Type -> eq a -> a -> t a -> bool
 
 val find : a ::: Type -> (a -> bool) -> t a -> option a
 
+val findM : m ::: (Type -> Type) -> monad m -> a ::: Type -> (a -> m bool) -> t a -> m (option a)
+
 val search : a ::: Type -> b ::: Type -> (a -> option b) -> t a -> option b
 
 val all : a ::: Type -> (a -> bool) -> t a -> bool