\version "2.14.2"

#(define (make-cross-stencil coords)
 (ly:stencil-add
   (make-line-stencil 0.15 (- (car coords) 0.2) (- (cdr coords) 0.2)
  (+ (car coords) 0.2) (+ (cdr coords) 0.2))
   (make-line-stencil 0.15 (- (car coords) 0.2) (+ (cdr coords) 0.2)
  (+ (car coords) 0.2) (- (cdr coords) 0.2))))

#(define (display-control-points line)
 (lambda (grob)
   (let ((stencil (ly:slur::print grob))
         (cps (ly:grob-property grob 'control-points)))

   (ly:stencil-add stencil
     (ly:stencil-in-color
         (make-cross-stencil (second cps))
         0 0 1)
     (ly:stencil-in-color
         (make-cross-stencil (third cps))
         0 0 1)
     (if (eq? line #t)
         (make-line-stencil 0.05 
            (car (first cps)) (cdr (first cps))
            (car (second cps))  (cdr (second cps)))
         empty-stencil)
     (if (eq? line #t)
         (make-line-stencil 0.05 
            (car (third cps)) (cdr (third cps))
            (car (fourth cps))  (cdr (fourth cps)))
         empty-stencil)
     )
     )))

displayControlPoints = {
  \override Slur #'stencil = #(display-control-points #f)
  \override PhrasingSlur #'stencil = #(display-control-points #f)
  %\override Tie #'stencil = #(display-control-points #f) % commenting this out gives an error
}

displayControlPointsWithLines = {
  \override Slur #'stencil = #(display-control-points #t)
  \override PhrasingSlur #'stencil = #(display-control-points #t)
  %\override Tie #'stencil = #(display-control-points #t) % commenting this out gives an error
}

hideControlPoints = {
  \revert Slur #'stencil 
  \revert PhrasingSlur #'stencil
}

\paper { indent = 0 }

music = \relative c'' {
  \slurDown
  c2\( d e d( c d~ d f) 
  \break
  g e c d c1\)
}
  
\markup "Show control-points"
\score {
  {
    \displayControlPoints
    \music
  }
}

\markup "Show control-points and 'handle' lines"
\score {
  {
    \displayControlPointsWithLines
    \music
  }
}

\markup "Plain curves"
\score {
  {
    \music
  }
}