make empty BorderMaps even emptier (issue 370)

The BorderMap module provides a notion of an empty map. Previously, this
was accidentally conflating two kinds of emptiness: an map that doesn't
track points, and a map that tracks a point (or multiple points) but
doesn't associate it with anything. This meant that the empty map was
not a unit for the union operation, which seems like a natural
expectation.

This has been resolved by revising the empty map to be the one that
tracks no points. Callers that expected that behavior were left alone,
while callers that expected the other behavior were fixed to use
`emptyCoordinates`.

src/Brick/BorderMap.hs

@@ -7,7 +7,7 @@ module Brick.BorderMap
     ( BorderMap
     , Edges(..)
     , eTopL, eBottomL, eRightL, eLeftL
-    , empty, emptyCoordinates, singleton
+    , empty, clear, emptyCoordinates, singleton
     , insertH, insertV, insert
     , unsafeUnion
     , coordinates, bounds
@@ -54,14 +54,23 @@ data BorderMap a = BorderMap
 emptyCoordinates :: Edges Int -> BorderMap a
 emptyCoordinates cs = BorderMap { _coordinates = cs, _values = pure IM.empty }
 
--- | An empty 'BorderMap' that only tracks the point (0,0).
+-- | An empty 'BorderMap' that tracks the same points as the input.
+clear :: BorderMap a -> BorderMap b
+clear = emptyCoordinates . coordinates
+
+-- | An empty 'BorderMap' that does not track any points.
 empty :: BorderMap a
-empty = emptyCoordinates (pure 0)
+empty = emptyCoordinates Edges
+    { eTop = 0
+    , eBottom = -1
+    , eLeft = 0
+    , eRight = -1
+    }
 
 -- | A 'BorderMap' that tracks only the given the point (and initially maps it
 -- to the given value).
 singleton :: Location -> a -> BorderMap a
-singleton l v = translate l . insert origin v $ empty
+singleton l v = translate l . insert origin v . emptyCoordinates $ pure 0
 
 {-# INLINE coordinates #-}
 -- | The positions of the edges of the rectangle whose border is retained in a

src/Brick/Widgets/Core.hs

@@ -194,7 +194,7 @@ separateBorders p = Widget (hSize p) (vSize p) $
 --
 -- Frozen borders cannot be thawed.
 freezeBorders :: Widget n -> Widget n
-freezeBorders p = Widget (hSize p) (vSize p) $ (bordersL .~ BM.empty) <$> render p
+freezeBorders p = Widget (hSize p) (vSize p) $ (bordersL %~ BM.clear) <$> render p
 
 -- | The empty widget.
 emptyWidget :: Widget n