It looks like at this time Spawn doesn't support the closures yet. http://www.gocircuit.org/lang.html#worker-functions: "The need for boilerplate will be removed in an upcoming Circuit Compiler"

From https://github.com/tumblr/gocircuit/blob/master/src/circuit/...: "The worker function fn, whose type must have previously been registered with RegisterFunc"

And it looks like only function type id is communicated to worker: https://github.com/tumblr/gocircuit/blob/master/src/circuit/...