Note that:
call(Goal) is logically the same as Goal and !/0 does not cut through call/1.
Success:
[eclipse]: [user].
or(A -> B, C) :- call(A), !, call(B).
or(_ -> _, C) :- call(C).
user compiled 412 bytes in 0.02 seconds
[eclipse]: or(write(a)->fail, write(k)).
a
no.
[eclipse]: or(fail->write(here),true).
yes.
Fail:
[eclipse]: call(fail),write(here).
no.
Error:
call(G). (Error 4).
call("write(a)"). (Error 5).