Add optics for common types

src/Control/Lens.idr

@@ -1,5 +1,6 @@
 module Control.Lens
 
+import public Control.Lens.At
 import public Control.Lens.Equality
 import public Control.Lens.Fold
 import public Control.Lens.Getter

src/Control/Lens/At.idr

@@ -0,0 +1,36 @@
+module Control.Lens.At
+
+import Control.Lens.Optic
+import Control.Lens.Lens
+import Control.Lens.Optional
+import Control.Lens.Traversal
+import Control.Lens.Setter
+
+%default total
+
+
+public export
+interface Ixed i v a | a where
+  ix : i -> Optional' a v
+
+public export
+[Function] Eq e => Ixed e a (e -> a) where
+  ix k = optional' (Just . ($ k)) (\f,x,k' => if k == k' then x else f k')
+
+
+public export
+interface Ixed i v a => Ixed' i i' v a | a where
+  ix' : i' -> Lens' a v
+
+public export
+[Function'] Eq e => Ixed' e e a (e -> a) using Function where
+  ix' k = lens ($ k) (\f,x,k' => if k == k' then x else f k')
+
+
+public export
+interface Ixed i v a => At i v a | a where
+  at : i -> Lens' a (Maybe v)
+
+public export
+sans : At i v a => i -> a -> a
+sans k = at k .~ Nothing