- Added function for folding using monoids.

1 files changed, 14 insertions, 7 deletions

diff --git a/stdlib/source/lux/control/monoid.lux b/stdlib/source/lux/control/monoid.lux
index c073bdb0b..09737cb12 100644
--- a/stdlib/source/lux/control/monoid.lux
+++ b/stdlib/source/lux/control/monoid.lux
@@ -1,4 +1,5 @@
-(.module: lux)
+(.module: lux
+  (// [fold #+ Fold]))
 
 (sig: #export (Monoid a)
   {#.doc "A way to compose values.
@@ -11,9 +12,15 @@
 
 (def: #export (compose Monoid<l> Monoid<r>)
   (All [l r] (-> (Monoid l) (Monoid r) (Monoid [l r])))
-  (struct (def: identity
-            [(:: Monoid<l> identity) (:: Monoid<r> identity)])
-          
-          (def: (compose [lL rL] [lR rR])
-            [(:: Monoid<l> compose lL lR)
-             (:: Monoid<r> compose rL rR)])))
+  (struct
+   (def: identity
+     [(:: Monoid<l> identity) (:: Monoid<r> identity)])
+   
+   (def: (compose [lL rL] [lR rR])
+     [(:: Monoid<l> compose lL lR)
+      (:: Monoid<r> compose rL rR)])))
+
+(def: #export (fold Monoid<a> Fold<F> data)
+  (All [a F] (-> (Monoid a) (Fold F) (F a) a))
+  (let [(^open "a/") Monoid<a>]
+    (:: Fold<F> fold a/compose a/identity data)))