From 534241d59a44193f80e31ae3a8341580d9cb57f3 Mon Sep 17 00:00:00 2001
From: Lucas Kramer <lucas@matx.com>
Date: Tue, 30 Jun 2026 18:43:43 -0700
Subject: [PATCH] Add Functor/Traversable/Foldable instances for UAList (#984)

---
 src/Libraries/Base3-Misc/UnitAppendList.bsv | 52 ++++++++++++++++-----
 1 file changed, 41 insertions(+), 11 deletions(-)

diff --git a/src/Libraries/Base3-Misc/UnitAppendList.bsv b/src/Libraries/Base3-Misc/UnitAppendList.bsv
index 63895f452..b7ca36b0e 100644
--- a/src/Libraries/Base3-Misc/UnitAppendList.bsv
+++ b/src/Libraries/Base3-Misc/UnitAppendList.bsv
@@ -1,5 +1,8 @@
 package UnitAppendList;
 
+import Foldable::*;
+import Traversable::*;
+
 typedef union tagged {
     void NoItems;
     a One;
@@ -22,18 +25,19 @@ endfunction: flatten0
 function List#(a) flatten(UAList#(a) c) = flatten0(c, Nil);
 
 
+// Append two lists, treating NoItems as the identity so the result never
+// contains a redundant empty branch.
+function UAList#(a) uaAppend(UAList#(a) c1, UAList#(a) c2) =
+   c1 matches tagged NoItems ? c2 :
+   c2 matches tagged NoItems ? c1 :
+   tagged Append tuple2(c1, c2);
+
+
 function UAList#(b) uaMap(function b f(a x), UAList#(a) c);
    case (c) matches
       tagged NoItems:          return tagged NoItems;
       tagged One .x:           return tagged One (f(x));
-      tagged Append {.c1,.c2}:
-	 begin
-	    let r1 = uaMap(f, c1);
-	    let r2 = uaMap(f, c2);
-	    return (r1 matches tagged NoItems ? r2 :
-		    r2 matches tagged NoItems ? r1 :
-		    tagged Append tuple2(r1, r2));
-	 end
+      tagged Append {.c1,.c2}: return uaAppend(uaMap(f, c1), uaMap(f, c2));
    endcase
 endfunction: uaMap
 
@@ -50,11 +54,37 @@ module uaMapM#(function module#(b) f(a x), UAList#(a) c)(UAList#(b));
 	 begin
 	    let r1 <- uaMapM(f, c1);
 	    let r2 <- uaMapM(f, c2);
-	    return (r1 matches tagged NoItems ? r2 :
-		    r2 matches tagged NoItems ? r1 :
-		    tagged Append tuple2(r1, r2));
+	    return uaAppend(r1, r2);
 	 end
    endcase
 endmodule: uaMapM
 
+
+instance Functor#(UAList);
+   function fmap = uaMap;
+endinstance
+
+instance Foldable#(UAList);
+   function b foldr(function b f(a x, b z), b z, UAList#(a) c);
+      case (c) matches
+         tagged NoItems:          return z;
+         tagged One .x:           return f(x, z);
+         tagged Append {.c1,.c2}: return foldr(f, foldr(f, z, c2), c1);
+      endcase
+   endfunction
+   function toList = flatten;
+endinstance
+
+instance Traversable#(UAList);
+   function f#(UAList#(b)) traverse(function f#(b) func(a x), UAList#(a) c)
+      provisos (Applicative#(f));
+      case (c) matches
+         tagged NoItems:  return \pure (tagged NoItems);
+         tagged One .x:   return fmap(One, func(x));
+         tagged Append {.c1,.c2}:
+            return liftA2(uaAppend, traverse(func, c1), traverse(func, c2));
+      endcase
+   endfunction
+endinstance
+
 endpackage