Merge pull request #133 from cscheid/Alignable-b-to-a-instance

add b->a instance for Alignable

src/Diagrams/Align.hs

@@ -112,6 +112,15 @@ instance ( HasLinearMap v, InnerSpace v, OrderedField (Scalar v)
          ) => Alignable (QDiagram b v m) where
   defaultBoundary = envelopeBoundary
 
+-- | Although the alignBy method for the (b -> a) instance is
+--   sensible, there is no good implementation for
+--   defaultBoundary. Instead, we provide a total method, but one that
+--   is not sensible. This should not present a serious problem as long
+--   as your use of Alignable happens through alignBy.
+instance (HasOrigin a, Alignable a) => Alignable (b -> a) where
+  alignBy v d f b = alignBy v d (f b)
+  defaultBoundary _ _ = origin
+
 -- | @align v@ aligns an enveloped object along the edge in the
 --   direction of @v@.  That is, it moves the local origin in the
 --   direction of @v@ until it is on the edge of the envelope.  (Note