title: How to define a constant in Julia struct
body: <p>I want to define a struct:</p>
  
  <pre><code>struct unit_SI_gen
      x::Float32
      const c = 2.99792458e8
      speed(x)=c*x
  end
  </code></pre>
  
  <p>However, it raise an error :</p>
  
  <pre><code>syntax: "c = 2.99792e+08" inside type definition is reserved
  </code></pre>
  
  <p>I know I cant use struct as class in python, but I can not find how to solve this problem.</p>
  
  <p>How to define a constant in struct ? </p>