Add a new syntax macro, compose-if.

The macro resembles (or x (fn x)) except:
1) The value is only evaluated once, and
2) Multiple (single-argument) functions can be chained.

If the original value or the result of any function is #f, the
final value is #f. Otherwise the result is ((compose ,@fns) x).
Short-circuit evaluation is employed.

src/lib/syntax.rls

@@ -69,4 +69,10 @@
 (define-syntax (let/cc var . body)
   `(call/cc (lambda (,var) ,@body)))
 
+(define-syntax (compose-if x fn . fns)
+  (let ([it (gensym)])
+    (if (pair? fns)
+        `(let ([,it ,x]) (if ,it (compose-if (,fn ,it) ,@fns) #f))
+        `(let ([,it ,x]) (if ,it (,fn ,it) #f)))))
+
 ; vim:set syntax=scheme sw=2 expandtab: