List.existsM

2 files changed, 17 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index cc533676..a7296552 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -204,6 +204,21 @@ fun exists [a] f =
         ex
     end
 
+fun existsM [m] (_ : monad m) [a] f =
+    let
+        fun ex ls =
+            case ls of
+                [] => return False
+              | x :: ls =>
+                b <- f x;
+                if b then
+                    return True
+                else
+                    ex ls
+    in
+        ex
+    end
+
 fun foldlMap [a] [b] [c] f =
     let
         fun fold ls' st ls =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index fd56679d..37cbe442 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -42,6 +42,8 @@ val filter : a ::: Type -> (a -> bool) -> t a -> t a
 
 val exists : a ::: Type -> (a -> bool) -> t a -> bool
 
+val existsM : m ::: (Type -> Type) -> monad m -> a ::: Type -> (a -> m bool) -> t a -> m bool
+
 val foldlM : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type
              -> (a -> b -> m b) -> b -> t a -> m b