[Haskell-cafe] how would I do this properly in Haskell?

[Dennis Raddle]

Thanks very much! I'll look at it in detail.

D

On Thu, Aug 24, 2017 at 10:30 PM, John Wiegley <johnw at newartisans.com>
wrote:

> >>>>> "DR" == Dennis Raddle <dennis.raddle at gmail.com> writes:
>
> DR> I'm an advanced beginner, and I'm finding it hard to comprehend
> DR> Data.Machine.Moore. Is there a way to explain more to me how this would
> DR> look in practice? Or is there another way to organize it that is less
> DR> "computer-sciency" and I could work with more easily?
>
> Thinks of a Moore machine as a packaged up State function, where you have
> an
> initial state, and the type of that state is hidden within the machine.
> For
> example:
>
>     {-# LANGUAGE ExistentialQuantification #-}
>
>     data Moore i m o = forall s. Moore
>         { mooreInit :: s
>         , mooreFunc :: i -> StateT s m o
>         }
>
> This machine is really just a packaged function, yielding outputs from
> inputs,
> while dependent on an internal state that may vary at each call.
>
> Since the state is "existential" (or private to Moore), it can be changed
> freely when you combine machines:
>
>     instance (Monad m, Monoid o) => Monoid (Moore i m o) where
>         mempty = Moore () (const (return mempty))
>         Moore s1 f1 `mappend` Moore s2 f2 = Moore (s1, s2) go
>           where
>             go ev = StateT $ \(st1, st2) -> do
>                 (mres, st1')  <- runStateT (f1 ev) st1
>                 (mres', st2') <- runStateT (f2 ev) st2
>                 return (mres <> mres', (st1', st2'))
>
> Something like this could serve your needs, without needing an external
> package like 'machines'. I've used it to do almost just what you described
> initially.
>
> --
> John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
> http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2
>
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