title: Amount of available memory in cache initialised with the R function: `memoise::memoise()`
body: <p>I am repeatedly calling a memoised function initialised as so:</p>
  <pre><code>library(memoise)
  memoised_func &lt;- memoise::memoise(func)
  </code></pre>
  <p>I am aware that I can set the size of the cache allocated to <code>memoised_func</code> with the <code>cache</code> argument (<a href="https://www.rdocumentation.org/packages/memoise/versions/2.0.0/topics/memoise" rel="nofollow noreferrer">https://www.rdocumentation.org/packages/memoise/versions/2.0.0/topics/memoise</a>). But is there a way to see how much free memory there is in the cache for at any one point in time?
  I am asking because I want to know if it is filling up and therefore my programme could be sped up by increasing the size of the cache.</p>
  <p>(I'm running <code>R 4.0.2</code> on <code>Ubuntu 20.10</code> in case that's relevant)</p>