    {-# LANGUAGE FlexibleContexts #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Diagrams.Trace
    -- Copyright   :  (c) 2013 diagrams-lib team (see LICENSE)
    -- License     :  BSD-style (see LICENSE)
    -- Maintainer  :  diagrams-discuss@googlegroups.com
    --
    -- \"Traces\", aka embedded raytracers, for finding points on the edge
    -- of a diagram.  See "Diagrams.Core.Trace" for internal
    -- implementation details.
    --
    -----------------------------------------------------------------------------

    module Diagrams.Trace
        ( -- * Types
          Trace, Traced

          -- * Diagram traces
        , trace, setTrace, withTrace

          -- * Querying traces
        , traceV, traceP, maxTraceV, maxTraceP

          -- * Subdiagram traces
        , boundaryFrom, boundaryFromMay

        ) where

    import           Diagrams.Core        (OrderedField, Point, Subdiagram, location,
                                           origin, setTrace, trace)
    import           Diagrams.Core.Trace

    import           Data.Maybe
    import           Data.Semigroup
    import           Diagrams.Combinators (withTrace)

    import           Linear.Metric
    import           Linear.Vector

    -- | Compute the furthest point on the boundary of a subdiagram,
    --   beginning from the location (local origin) of the subdiagram and
    --   moving in the direction of the given vector.  If there is no such
    --   point, the origin is returned; see also 'boundaryFromMay'.
    boundaryFrom :: (OrderedField n, Metric v, Semigroup m)
                  => Subdiagram b v n m -> v n -> Point v n
    boundaryFrom s v = fromMaybe origin $ boundaryFromMay s v

    -- | Compute the furthest point on the boundary of a subdiagram,
    --   beginning from the location (local origin) of the subdiagram and
    --   moving in the direction of the given vector, or @Nothing@ if
    --   there is no such point.
    boundaryFromMay :: (Metric v, OrderedField n, Semigroup m)
                    => Subdiagram b v n m -> v n -> Maybe (Point v n)
    boundaryFromMay s v = traceP (location s) (negated v) s