1 files changed, 35 insertions, 0 deletions
diff --git a/lib/ur/monad.ur b/lib/ur/monad.ur
index 73001094..356173fd 100644
--- a/lib/ur/monad.ur
+++ b/lib/ur/monad.ur
@@ -7,3 +7,38 @@ fun exec [m ::: Type -> Type] (_ : monad m) [ts ::: {Type}] r (fd : folder ts) =
     (return {}) [ts] fd r
 
 fun ignore [m ::: Type -> Type] (_ : monad m) [t] (v : m t) = x <- v; return ()
+
+fun foldR [K] [m] (_ : monad m) [tf :: K -> Type] [tr :: {K} -> Type]
+          (f : nm :: Name -> t :: K -> rest :: {K}
+               -> [[nm] ~ rest] =>
+           tf t -> tr rest -> m (tr ([nm = t] ++ rest)))
+          (i : tr []) [r :: {K}] (fl : folder r) =
+    Top.fold [fn r :: {K} => $(map tf r) -> m (tr r)]
+             (fn [nm :: Name] [t :: K] [rest :: {K}] [[nm] ~ rest] 
+                              (acc : _ -> m (tr rest)) r =>
+                 acc' <- acc (r -- nm);
+                 f [nm] [t] [rest] ! r.nm acc')
+             (fn _ => return i)
+             [_] fl
+
+fun foldR2 [K] [m] (_ : monad m) [tf1 :: K -> Type] [tf2 :: K -> Type] [tr :: {K} -> Type]
+           (f : nm :: Name -> t :: K -> rest :: {K}
+                -> [[nm] ~ rest] =>
+            tf1 t -> tf2 t -> tr rest -> m (tr ([nm = t] ++ rest)))
+           (i : tr []) [r :: {K}] (fl : folder r) =
+    Top.fold [fn r :: {K} => $(map tf1 r) -> $(map tf2 r) -> m (tr r)]
+       (fn [nm :: Name] [t :: K] [rest :: {K}] [[nm] ~ rest] 
+                        (acc : _ -> _ -> m (tr rest)) r1 r2 =>
+           acc' <- acc (r1 -- nm) (r2 -- nm);
+           f [nm] [t] [rest] ! r1.nm r2.nm acc')
+       (fn _ _ => return i)
+       [_] fl
+
+fun mapR [K] [m] (_ : monad m) [tf :: K -> Type] [tr :: K -> Type]
+         (f : nm :: Name -> t :: K -> tf t -> m (tr t)) =
+    @@foldR [m] _ [tf] [fn r => $(map tr r)]
+    (fn [nm :: Name] [t :: K] [rest :: {K}] [[nm] ~ rest] (v : tf t)
+                     (acc : $(map tr rest)) =>
+        v' <- f [nm] [t] v;
+        return (acc ++ {nm = v'}))
+    {}