lib/Quox/Thin/View.idr

@@ -3,6 +3,7 @@ module Quox.Thin.View
 import public Quox.Thin.Base
 import Quox.NatExtra
 import Data.Singleton
+import Data.SnocVect
 
 %default total
 
@@ -90,3 +91,13 @@ export
              view (Keep {mask} ope eq) = KeepV mask ope
 viewKeep ope eq with (view (Keep ope eq))
   viewKeep ope Refl | KeepV _ ope = Refl
+
+
+namespace SnocVect
+  export
+  select : {n, mask : Nat} -> (0 ope : OPE m n mask) ->
+           SnocVect n a -> SnocVect m a
+  select ope sx with (view ope)
+    select _ [<]       | StopV       = [<]
+    select _ (sx :< x) | DropV _ ope = select ope sx
+    select _ (sx :< x) | KeepV _ ope = select ope sx :< x