handle zero case, add documentation

src/Diagrams/TwoD/Arrow.hs

@@ -180,13 +180,20 @@ headSize :: Lens' ArrowOpts Double
 -- | Radius of a circumcircle around the tail.
 tailSize :: Lens' ArrowOpts Double
 
+-- | Set the size of both the head and tail. The @headSize@ is set to the
+--   given value and the @tailSize@ is set so that it is the same width as
+--   the head. Both @arrowHead@ and @arrowTail@ should be set before using
+--   bothSize.
 bothSize :: Traversal' ArrowOpts Double
 bothSize f opts =
   (\h t -> opts & headSize .~ h & tailSize .~ toTailSize opts t)
     <$> f (opts ^. headSize) <*> f (opts ^. tailSize)
 
+-- Calculate the tailSize needed so that the head and tail are the same width.
+-- If either is zero, revert to the default size. This is needed for example
+-- in the noHead arrow head case.
 toTailSize :: ArrowOpts -> Double -> Double
-toTailSize opts s = hw / tw
+toTailSize opts s = if (hw > 0) && (tw > 0) then hw / tw else 0.3
   where
     (h, j) = (opts^.arrowHead) s (widthOfJoint $ shaftSty opts)
     (t, k) = (opts^.arrowTail) 1 (widthOfJoint $ shaftSty opts)
@@ -199,6 +206,7 @@ headGap :: Lens' ArrowOpts Double
 -- | Distance to leave between the starting point and the tail.
 tailGap :: Lens' ArrowOpts Double
 
+-- | Set both the @headGap@ and @tailGap@ simultaneously.
 gap :: Traversal' ArrowOpts Double
 gap f opts = (\h t -> opts & headGap .~ h & tailGap .~ t) <$> f (opts ^. headGap) <*> f (opts ^. tailGap)
 

src/Diagrams/TwoD/Arrowheads.hs

@@ -210,6 +210,7 @@ arrowheadMissile :: Angle -> ArrowHT
 arrowheadMissile theta = smoothArrowhead $ arrowheadDart theta
 
 -- Standard heads ---------------------------------------------------------
+-- | A line the same width as the shaft.
 lineHead :: ArrowHT
 lineHead l w = (square 1 # scaleX l # scaleY w # alignL, mempty)
 
@@ -297,7 +298,7 @@ arrowtailQuill theta =aTail
                 [ v0, n1, n2, v0, n3, n4, v0 ])
 
 -- Standard tails ---------------------------------------------------------
-
+-- | A line the same width as the shaft.
 lineTail :: ArrowHT
 lineTail l w = (square 1 # scaleX l # scaleY w # alignR, mempty)