[Haskell-cafe] Implementing a MUD server in haskell

Jack Kelly endgame.dos at gmail.com
Sun Dec 16 10:48:03 EST 2007

Jules Bean wrote:
> Jack Kelly wrote: -snip-
> Essentially you have to choose some form of identity (relational DB fans 
> might call it a "key") for your players and rooms. My gut feeling is 
> that String will do fine; it's nice for debugging, gives rise to a 
> natural way to write your admin commands and so on. However, it may not 
> be quite good enough (100 mobs all called "an orc" need individual keys) 
> and you may end up using an Int or Int64 for your key.

Perhaps doing the keying on an Int and then having a [String] of names 
that entities can be called is the way to go. List comprehensions will 
still work:

let ents = [ ent | ent <- allents, name `elem` (Entity.aliases ent) ] in
-- ...

but so will addressing an object uniquely when needed:

let target = listToMaybe $ [ ent | ent <- allents, id == (Entity.id ent) 
] in
-- ...
> Once you've chosen a key, you can do something very like the C++ 
> structure: -snip-

I'm thinking of doing the following (based on some suggestions from 

data Room = Room {...}
type Zone = Data.Map String Room
type World = Data.Map String Zone
type ZoneID = String
type RoomID = String
data Location = Location { zone :: ZoneID, room :: RoomID}
data Player = Player { location :: Location, ... }

Aside: When should I use newtype vs type? From what I understand, both 
create type aliases that are only relevant at compile time, but newtype 
disallows things like:

-- Bad
newtype Fred = Fred String
putStrLn $ Fred "this won't work"

as a rule of thumb, is it `better' to newtype things so that the type 
system can trip you up?

> I should mention, actually, that there is also the naive solution:
That does seem like a bad idea.

> There are always two techniques for maintaining invariants.
> In solution 1, you would simply not store the player list in the room 
> structure at all. Instead, you would only store the room in the player, 
> and whenever you wanted to answer the question "Which players are in 
> this room" you would do a quick search through all players to find them
> playersInRoom :: Room -> [Player]
> playersInRoom r = [p | p <- allplayers, (room p) == r]
> -- Just like a database! In SQL we'd say:
> -- SELECT p FROM allplayers WHERE p.room = r
> Of course the disadvantage to this is the linear search through all the 
> players. Depending on how often this is performed, this may actually 
> cost you less that explicitly "caching" the value inside the room.

I'd not thought of doing it that way. Maybe because keeping a couple of 
pointers in sync in C is less painful then coding up a search to 
generate an appropriate list. Learning to stop looking through the 
"imperative lens" is hard.

> In solution 2, of course, whenever you  move a player from room A to 
> room B you do it via a function which is explicitly designed to keep 
> everythign up to date:
> movePlayer PlayerID -> RoomID -> RoomID -> Game ()
> movePlayer pid raid rbid = do
>   p <- getPlayer pid
>   setPlayer pid (p {room = rbid})
>   ra <- getRoom raid
>   rb <- getRoom rbid
>   setRoom raid (ra {players = (players ra)\\[pid]} )
>   setRoom rbid (rb {players = pid : (players rb)} )
> Which I've written in an imaginary Game monad, and using rather ugly 
> low-level combinators get/set Room/Player. You could make it look much 
> more pleasant in practice with some better chosen combinators.

Game looks like it'd behave like some kind of specialisation of State, 
but since there'd be responses sent out to the sockets, I'm thinking of 
something like StateT GameState IO (). Is it worth having the StateT 
because IO already handles some concept of state? If so, will running 
something like:

forkIO $ evalStateT gameLoop initialstate

work or will the IO actions be queued up until the end of gameLoop? The 
idea of building a Game monad is attractive regardless because I'll 
still need functions like movePlayer regardless of which path is chosen.

> Still I think solution 1 (don't store redundant data which can get out 
> of sync) has a lot to recommend it and solution 2 may be premature 
> optimization; i.e. implement solution 2, which is a kind of caching of 
> computed data, only once you've convinced yourself that recalculating 
> that data every time really is slowing you down.

I think I'll go with solution 1. It's conceptually simpler and if it's 
slow I can profile and optimise later, right?

> The simplest way to do this is to bundle all your big shared mutable 
> world into a single MVar. What this amounts to is perfect brute force 
> serialisation of the actual modification part: i.e. all world 
> modifications share a global lock. This is easy to implement and easy to 
> reason about.

I think that my fear of doing this was again instinctive premature 

> If that turns out to be too restrictive, then you split up the MVars 
> into smaller pieces, but then you have to think a bit harder to convince 
> yourself it is safe.

Another idea that I picked up from the "Simple TCP server" 
(http://sequence.complete.org/node/258) is to have a single thread 
that's dedicated to manipulating the world and have each socket reading 
thread send messages to this game logic thread through (TChan String)s 
or similar. Is this a better or worse idea than having clients mutate 
the world directly? Both amount to doing one thing at a time (if the 
global MVar route is chosen).

Thanks Jules. It's been a great help.

-- Jack

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