Progressively reveal the map to the player

As the character walks around the map, progressively reveal the entities
on the map to them, using an algorithm based on well known
circle-rasterizing and line-rasterizing algorithms to calculate lines of
sight that are potentially obscured by walls.

src/Xanthous/Data.hs

@@ -12,9 +12,11 @@ module Xanthous.Data
   , y
 
   , Positioned(..)
+  , _Positioned
   , position
   , positioned
   , loc
+  , _Position
   , positionFromPair
 
     -- *
@@ -73,6 +75,12 @@ data Positioned a where
   deriving stock (Show, Eq, Ord, Functor, Foldable, Traversable, Generic)
   deriving anyclass (CoArbitrary, Function)
 
+_Positioned :: Iso (Position, a) (Position, b) (Positioned a) (Positioned b)
+_Positioned = iso hither yon
+  where
+    hither (pos, a) = Positioned pos a
+    yon (Positioned pos b) = (pos, b)
+
 instance Arbitrary a => Arbitrary (Positioned a) where
   arbitrary = Positioned <$> arbitrary <*> arbitrary
 
@@ -92,6 +100,12 @@ loc = iso hither yon
     hither (Position px py) = Location (px, py)
     yon (Location (lx, ly)) = Position lx ly
 
+_Position :: Iso' Position (Int, Int)
+_Position = iso hither yon
+  where
+    hither (Position px py) = (px, py)
+    yon (lx, ly) = Position lx ly
+
 positionFromPair :: (Integral i, Integral j) => (i, j) -> Position
 positionFromPair (i, j) = Position (fromIntegral i) (fromIntegral j)