\version "2.19.52"

#(define l-arc (quote '(rcurveto 0 -0.5 0.5 -1 1 -1)))

#(define my-path
   `(append
    '(rmoveto 0.5 -3.5)
    '(rcurveto 0 -0.5 0.5 -1 1 -1)
    ; I want to replace the previous list with an expression
    ,l-arc
    '(rlineto 6 0)
    '(rcurveto 3 0 2 -4 2 2)
    ))

#(display   
  `(path
    0.15
    ,my-path))


curveStencil =
#(ly:make-stencil
  `(path
    0.15
    ,my-path))

{
  \override Slur.stencil = #curveStencil
  d' ( e' f' g' )
}
%