title: Arbitrary precision floating types in Julia
body: <p>I have a constant value:</p>
  
  <pre><code>M_LN2 = 0.693147180559945309417
  </code></pre>
  
  <p>I tried storing it as:</p>
  
  <pre><code>M_LN2 = BigFloat(0.693147180559945309417)
  </code></pre>
  
  <p>The result seems to be adding too many extra digits:</p>
  
  <blockquote>
    <p>6.931471805599452862267639829951804131269454956054687500000000000000000000000000e-01</p>
  </blockquote>
  
  <p>I tried:</p>
  
  <pre><code>M_LN2 = Float64(0.693147180559945309417)
  </code></pre>
  
  <p>But it is truncating the value:</p>
  
  <blockquote>
    <p>0.6931471805599453</p>
  </blockquote>
  
  <p>Could you suggest what would be the correct way to initialize the constant?
  Thanks!</p>