How referees should respond to illegal plays

Currently, game-level errors (i.e. illegal moves) are handled by RPC 
faults. For example, if you try to play the three of clubs when you're 
not holding it, the referee responds not with a positive RPC response 
packet but with a fault, containing a numeric error code defined by the 
ruleset.

I concluded a while ago (encouraged by developer feedback) that this is 
actually incorrect behavior. For one thing, RPC faults should probably 
be reserved for lower-level errors, such as malformed RPC requests. 
Secondly, fault packets simply can't hold very much data (so long as 
they stick to the XML-RPC standard); the code has to be numeric and 
therefore not immediately human-readable, and it can't contain any 
arguments.

One developer suggested replacing the faults with a standard "illegal 
play" referee-to-client RPC request, whose arguments would be a text 
string identifying the error (from a list of possible strings defined 
by the ruleset) and an optional argument list.

On thinking about this a bit, I propose something simpler: much as 
table configuration is now defined by the ruleset (see 
http://www.volity.org/wiki/index.cgi?Table_Configuration), so is error 
handling. That is, the ruleset would define a set of referee-to-client 
RPC requests that would handle every possible error condition. Rather, 
every possible _class_ of error; the arguments would contain the 
specifics, and it would be up to the receiving UI to pass this 
information to the human player as best it can.

So, the Eights ruleset would add seven RPC requests, perhaps with these 
names (based on the list currently at 
http://www.volity.org/games/eights/#error-codes):

* game.not_your_turn()
* game.dont_have_card(card)
* game.unknown_card(card)
* game.cant_draw()
* game.cant_choose_suit()
* game.unknwon_suit(suit)
* game.no_match(card)

Thoughts?

--
   Jason McIntosh             jmac <at> jmac.org
Somerville, MA, USA       http://www.jmac.org

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