[Haskell-cafe] Re: Fwd: Re: Simple game: a monad for each player

[Limestraël]

Heinrich, I saw you updated your operational package (you considered my
remark about ProgramView, thank you)

I saw you added a liftProgram function, however it is not like the mapMonad
function you were talking about.
mapMonad was:
mapMonad :: (Monad m1, Monad m2) =>
                        (forall a . m1 a -> m2 a)
                   -> ProgramT instr m1 a
                   -> ProgramT instr m2 a

and you turned it into the less generic:
liftProgram :: Monad m => Program instr a -> ProgramT instr m a

Did you change your mind?

2010/4/19 Heinrich Apfelmus <apfelmus at quantentunnel.de>

> Heinrich Apfelmus wrote:
> > Limestraël wrote:
> >> Okay, I start to understand better...
> >>
> >> Just, Heinrich, how would implement the mapMonad function in terms of
> the
> >> operational package?
> >> You just shown the signature.
> >
> > Ah, that has to be implemented by the library, the user cannot implement
> > this. Internally, the code would be as Bertram suggests:
> >
> >     mapMonad :: (Monad m1, Monad m2)
> >              => (forall a . m1 a -> m2 a)
> >              -> ProgramT instr m1 a -> ProgramT instr m2 a
> >     mapMonad f (Lift m1)  = Lift (f m1)
> >     mapMonad f (Bind m k) = Bind (mapMonad f m) (mapMonad f . k)
> >     mapMonad f (Instr i)  = Instr i
>
> Silly me! This can be implement by the user:
>
>    mapMonad f = id' <=< lift . f . viewT
>        where
>        id' :: ProgramViewT instr m1 a -> ProgramT instr m2 a
>        id' (Return a) = return a
>        id' (i :>>= k) = singleton i >>= mapMonad f . k
>
> and it would be a shame for the operational approach if that were not
> possible. :)
>
>
> Regards,
> Heinrich Apfelmus
>
> --
> http://apfelmus.nfshost.com
>
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