From 24fc1cc302a087b2df3d2041b24011a8b65f499e Mon Sep 17 00:00:00 2001
From: Adam Chlipala <adam@chlipala.net>
Date: Thu, 24 Jan 2019 12:10:09 -0500
Subject: Option.mapM

---
 lib/ur/option.ur  | 5 +++++
 lib/ur/option.urs | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/lib/ur/option.ur b/lib/ur/option.ur
index baa08466..dd186161 100644
--- a/lib/ur/option.ur
+++ b/lib/ur/option.ur
@@ -59,3 +59,8 @@ fun unsafeGet [a] (o : option a) =
     case o of
         None   => error <xml>Option.unsafeGet: encountered None</xml>
       | Some v => v
+
+fun mapM [m] (_ : monad m) [a] [b] (f : a -> m b) (x : t a) : m (t b) =
+    case x of
+        None => return None
+      | Some y => z <- f y; return (Some z)
diff --git a/lib/ur/option.urs b/lib/ur/option.urs
index c30c40e7..705c0313 100644
--- a/lib/ur/option.urs
+++ b/lib/ur/option.urs
@@ -14,3 +14,5 @@ val bind : a ::: Type -> b ::: Type -> (a -> option b) -> t a -> t b
 
 val get : a ::: Type -> a -> option a -> a
 val unsafeGet : a ::: Type -> option a -> a
+
+val mapM : m ::: (Type -> Type) -> monad m -> a ::: Type -> b ::: Type -> (a -> m b) -> t a -> m (t b)
-- 
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