[Haskell-cafe] Re: Map constructor in a DSL

steffen steffen.siering at googlemail.com
Wed Oct 27 00:12:40 EDT 2010


I think you may want to over think your types again.
Especially your Evaluator-Monad, and maybe your Map constructor.

The Problem is, due to your use of Either and the need for evalObs to
finally transform from "Obs [a]" type to "Evaluator [a]" you will end
up in another Monad for Either:

    instance Monad (Either Actions) where
      return = Right
      (Left x) >>= _ = Left x
      (Right a) >>= f = f a

Then one solution may be:

    evalObs (Map f obs) = evalMap (f.Konst) (evalObs obs)

    evalMap :: (a -> Obs b) -> Evaluator [a] -> Evaluator [b]
    evalMap f o = liftE (map evalObs) (liftE (map f) o) >>= \x ->
                 case x of
                   Left actions -> return $ Left actions
                   Right evals  -> sequence evals >>= return .
    -- first "sequence evals" creates [Either Actions a]
    -- second "sequence" create Either Actions [a]

After building up the "Evaluator [a]" construct inside your Evaluator-
Monad, you have to join the construct "evals" back into your real
Monad and since you pass around results using Either inside your
Evaluator-Monad, you have to treat the Either-type just like another

If you get stuck on your types, define new toplevel functions (as
undefined) each taking one argument less  and play with the types in
your files and in ghci until it begins to make sense.

On 26 Okt., 19:42, Dupont Corentin <corentin.dup... at gmail.com> wrote:
> Hey Chris!
> Values for PlayerNumber are acquired at evaluation time, from the state of
> the system.
> I have not included the evaluation of AllPlayers.
> Here how it looks:
> evalObs AllPlayers  = return . pure  =<< gets players
> But when you build your Obs, you have yet no idea how much players it will
> be.
> This is just symbolic at this stage.
> To give you a better insight, here is want I want to do with Map:
> everybodyVote :: Obs [Bool]
> everybodyVote = Map (Vote (Konst "Please vote")) AllPlayers
> In memory, everybodyVote is just a tree.
> This rule can be executed latter whenever I want to perform this democratic
> vote ;)
> Hope this answer to your question.
> Corentin
> On Tue, Oct 26, 2010 at 7:17 PM, Christopher Done
> <chrisd... at googlemail.com>wrote:
> > On 26 October 2010 18:07, Dupont Corentin <corentin.dup... at gmail.com>
> > wrote:
> > > But how can I write the evaluator for Map?
> > Where do values for PlayerNumber come from? Unless I'm mistaken, the
> > only thing that Map can be used with is Obs [PlayerNumber], a list of
> > values PlayerNumber which we have no means of acquiring in order to
> > provide to the Map function.
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