The way I understand it, when you type in (eval '(- 3 4)) at the shell, eval passes its argument to ac. Ac in turn calls ac-call for things that are function calls. Ac-call converts an arc function call into an equivalent scheme form. During this process, it finds the underlying scheme name of an arc function by calling ac-var-ref. If you just pass in the actual function, it can't find the scheme name of the function and so can't evaluate it.
