koka(js)fun catch( hnd : (string) -> e a, action : () -> <raise|e> a ) : e a
with ctl raise(msg) hnd(msg)
action() with abstractionHandler() という感じで使うkoka(js)fun catch-example()
with catch( fn(msg){ println("error: " ++ msg); 42 } )
safe-divide(1,0)