[Haskell-cafe] Re: An interesting monad: "Prompt"
apfelmus
apfelmus at quantentunnel.de
Wed Nov 21 07:03:25 EST 2007
Ryan Ingram wrote:
> I've been trying to implement a few rules-driven board/card games in Haskell
> and I always run into the ugly problem of "how do I get user input"?
>
> The usual technique is to embed the game in the IO Monad:
>
> The problem with this approach is that now arbitrary IO computations are
> expressible as part of a game action, which makes it much harder to
> implement things like replay, undo, and especially testing!
>
> The goal was to be able to write code like this:
>
> ] takeTurn :: Player -> Game ()
> ] takeTurn player = do
> ] piece <- action (ChoosePiece player)
> ] attack <- action (ChooseAttack player piece)
> ] bonusTurn <- executeAttack piece attack
> ] when bonusTurn $ takeTurn player
>
> but be able to script the code for testing, allow undo, automatically
> be able to save replays, etc.
>
> While thinking about this problem earlier this week, I came up with the
> following solution:
>
>> class Monad m => MonadPrompt p m | m -> p where
>> prompt :: p a -> m a
>
> "prompt" is an action that takes a prompt type and gives you a result.
>
> A simple example:
> ] prompt [1,3,5] :: MonadPrompt [] m => m Int
>
> This prompt would ask for someone to pick a value from the list and return
> it.
> This would be somewhat useful on its own; you could implement a "choose"
> function that picked randomly from a list of options and gave
> non-deterministic (or even exhaustive) testing, but on its own this wouldn't
> be much better than the list monad.
> [...]
>> data Prompt (p :: * -> *) :: (* -> *) where
>> PromptDone :: result -> Prompt p result
>> -- a is the type needed to continue the computation
>> Prompt :: p a -> (a -> Prompt p result) -> Prompt p result
>
> Intuitively, a (Prompt p result) either gives you an immediate result
> (PromptDone), or gives you a prompt which you need to reply to in order to
> continue the computation.
>
> This type is a MonadPrompt:
>
>> instance Functor (Prompt p) where
>> fmap f (PromptDone r) = PromptDone (f r)
>> fmap f (Prompt p cont) = Prompt p (fmap f . cont)
>>
>> instance Monad (Prompt p) where
>> return = PromptDone
>> PromptDone r >>= f = f r
>> Prompt p cont >>= f = Prompt p ((>>= f) . cont)
>>
>> instance MonadPrompt p (Prompt p) where
>> prompt p = Prompt p return
Marvelous!
Basically, by making the continuation (a -> Prompt p result) explicit,
we have the flexibility to acquire the value a differently, like
through user input or a replay script. The popular continuations for
implementing web applications in Lisp/Scheme do the same thing.
A slightly different point of view is that you use a term implementation
for your monad, at least for the interesting primitive effects like
choosePiece :: Player -> Game Piece
chooseAttack :: Player -> Piece -> Game Attack
By using constructors for them, you have the flexibility to write
different interpreters for Game a , like
play :: Game a -> IO a
replay :: Game a -> GameScript -> a
with the semantics
play (choosePiece pl >>= f) = do
putStrLn "Player " ++ show pl ++ ", choose your piece:"
play f . read =<< getLine
replay (choosePiece pl >>= f) (Piece pl' piece:xs)
| pl == pl' = replay (f piece) xs
Just for the record, the most general term implementation is presented here
Chuan-kai Lin. Programming Monads Operationally with Unimo.
http://web.cecs.pdx.edu/~cklin/papers/unimo-143.pdf
Btw, the web framework WASH/CGI for Haskell uses some kind of prompt
monad, too.
Peter Thiemann. An Embedded Domain-Specific Language for
Type-Safe Server-Side Web-Scripting.
http://www.informatik.uni-freiburg.de/~thiemann/WASH/draft.pdf
Here, the server replays parts of the CGI monad when the user submits a
form i.e. answers to a prompt.
Regards,
apfelmus
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