List.mem

2 files changed, 12 insertions, 0 deletions

diff --git a/lib/ur/list.ur b/lib/ur/list.ur
index cbb4faf2..11895884 100644
--- a/lib/ur/list.ur
+++ b/lib/ur/list.ur
@@ -216,6 +216,16 @@ fun foldlMap [a] [b] [c] f =
         fold []
     end
 
+fun mem [a] (_ : eq a) (x : a) =
+    let
+        fun mm ls =
+            case ls of
+                [] => False
+              | y :: ls => y = x || mm ls
+    in
+        mm
+    end
+
 fun find [a] f =
     let
         fun find' ls =
diff --git a/lib/ur/list.urs b/lib/ur/list.urs
index 66007a39..55068935 100644
--- a/lib/ur/list.urs
+++ b/lib/ur/list.urs
@@ -54,6 +54,8 @@ val filterM : m ::: (Type -> Type) -> monad m -> a ::: Type
 val foldlMap : a ::: Type -> b ::: Type -> c ::: Type
                -> (a -> b -> c * b) -> b -> t a -> t c * b
 
+val mem : a ::: Type -> eq a -> a -> t a -> bool
+
 val find : a ::: Type -> (a -> bool) -> t a -> option a
 
 val search : a ::: Type -> b ::: Type -> (a -> option b) -> t a -> option b