&gt; In practicality how difficult&#x2F;easy is it to build cross-runtime libraries and how much reuse can you get out of clojure libs b&#x2F;c of its hosted status?<p>It depends on what you&#x27;re trying to do, and whether the platforms help.<p>Doing something &#x27;pure&#x27; like math or datastructures: trivial. Writing a common interface to two different host libraries (e.g. DateTime in JVM &amp; Browser): manageable. 
Doing something &#x27;host-y&#x27;, like graphics, with no host libraries in common: very hard.<p>Basically, Clojure doesn&#x27;t get in the way of cross-platform concerns, but it might still be a difficult problem.