 default                           ocaml-base-compiler.4.09.1  default

[WARNING] The environment is not in sync with the current switch.
You should run: eval $(opam env)
</code></pre>

<h3>Install the right compiler</h3>

<p>Real World OCaml requires OCaml 4.13.1.  You can use <code>opam
    switch</code> to see which version of OCaml you have installed.
  If, as shown in the above invocation of switch, you have an
  older version installed, you can use opam to install a more
  up-to-date version:</p>
<pre