&gt; of course it is not possible to write a Swift compiler in Swift before the language exists<p>No, but you can do a self-hosting port after you have a first rudimentary compiler. Many compilers have been built like this. Some did it early in language development with a purpouse built one-off bootstrapping compiler, some did it after the language was somewhat stable (eg Go).<p>ObWP: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Bootstrapping_%28compilers%29" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Bootstrapping_%28compilers%29</a>